IEKE W.A., WIYONO, S. PRIYONO, dan SOEMARNO 2012
What is Soil Moisture? Lengas Tanah?
Soil moisture is difficult to define because it means
different things in different disciplines.
For example, a farmer's concept of soil moisture is
different from that of a water resource manager or a
weather forecaster.
Secara umum, lengas tanah adalah air yang ditahan dalam
ruang pori tanah.
Surface soil moisture is the water that is in the upper 10
cm of soil, whereas root zone soil moisture is the water
that is available to plants, which is generally considered to
be in the upper 200 cm of soil.
Diunduh dari: http://wwwghcc.msfc.nasa.gov/landprocess/lp_home.html …… 11/11/2012
Soil moisture – Lengas Tanah
Lengas tanah merupakan air yang idtahan dalam pori tanah dalam
zone perakaran tanaman, biasanya dalam profil tanah hingga
kedalaman 200 cm.
Water storage in the soil profile is extremely important for
agriculture, especially in locations that rely on rainfall for
cultivating plants. For example, in Africa rain-fed agriculture
accounts for 95% of farmed land.
Water storage is a term used within agriculture to define locations
where water is stored for later use. These range from natural water
stores, such as groundwater aquifers, soil water and natural
wetlands to small artificial ponds, tanks and reservoirs behind
major dams.
Diunduh dari:
http://en.wikipedia.org/wiki/Water_storage…… 11/11/2012
SOIL WATER CONTENT – Kadar Air (Lengas) Tanah
Kadar air tanah (lengas tanah) adalah jumlah air
yang ada di dalam tanah.
Water content is used in a wide range of scientific
and technical areas, and is expressed as a ratio,
which can range from 0 (completely dry) to the value
of the materials' porosity at saturation. It can be
given on a volumetric or mass (gravimetric) basis.
Diunduh dari: http://en.wikipedia.org/wiki/Water_content …… 11/11/2012
KADAR LENGAS TANAH
The water content in soil is also known as moisture content and
can be expressed as
w = 100 Mw/Ms
Where:
w = moisture content (%)
Mw = mass of water in soil (kg, lb)
Ms = dry mass of soil (kg, lb)
The water content test according ASTM D 2216-92 consists of
determining the mass of the wet soil specimen and then drying the
soil in an oven 12 - 16 hours at a temperature of 110oC.
Diunduh dari: http://www.engineeringtoolbox.com/soil-water-content-d_1643.html …… 11/11/2012
NERACA AIR – NERACA LENGAS
The water balance is an accounting of the inputs and outputs of
water.
The water balance of a place, whether it be an agricultural field,
watershed, or continent, can be determined by calculating the
input, output, and storage changes of water at the Earth's surface.
The major input of water is from precipitation and output is
evapotranspiration.
The geographer C. W. Thornthwaite (1899-1963) pioneered the
water balance approach to water resource analysis.
He and his team used the water-balance methodology to assess
water needs for irrigation and other water-related issues.
Diunduh dari:
http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html……
11/11/2012
NERACA AIR
The soil water
balance
(After Strahler & Strahler,
2006)
Precipitation (P).
Precipitation in the form of
rain, snow, sleet, hail,
etc. makes up the
primarily supply of water
to the surface. In some
very dry locations, water
can be supplied by dew
and fog.
Diunduh dari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html ……
11/11/2012
NERACA AIR
Actual Evapotranspiration (AE).
Evaporation is the phase change from a liquid to a gas releasing water from a
wet surface into the air above. Similarly, transpiration is represents a phase
change when water is released into the air by plants.
Evapotranspiration is the combined transfer of water into the air by evaporation
and transpiration. Actual evapotranspiration is the amount of water delivered to
the air from these two processes. Actual evapotranspiration is an output of
water that is dependent on moisture availability, temperature and humidity.
Think of actual evapotranspiration as "water use", that is, water that is actually
evaporating and transpiring given the environmental conditions of a place.
Actual evapotranspiration increases as temperature increases, so long as there
is water to evaporate and for plants to transpire.
The amount of evapotranspiration also depends on how much water is
available, which depends on the field capacity of soils. In other words, if there is
no water, no evaporation or transpiration can occur
Diunduh dari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html ……
11/11/2012
NERACA AIR
Potential evapotranspiration (PE).
The environmental conditions at a place create a demand for water. Especially in
the case for plants, as as energy input increases, so does the demand for water
to maintain life processes. If this demand is not met, serious consequences can
occur. If the demand for water far exceeds that which is actual present, dry soil
moisture conditions prevail. Natural ecosystems have adapted to the demands
placed on water.
Potential evapotranspiration is the amount of water that would be evaporated
under an optimal set of conditions, among which is an unlimited supply of
water. Think of potential evapotranspiration of "water need". In other words, it
would be the water needed for evaporation and transpiration given the local
environmental conditions. One of the most important factors that determines
water demand is solar radiation. As energy input increases the demand for
water, especially from plants increases. Regardless if there is, or isn't, any water
in the soil, a plant still demands water. If it doesn't have access to water, the
plant will likely wither and die.
Diunduh dari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html ……
11/11/2012
NERACA AIR
Soil Moisture Storage (ST).
Soil moisture storage refers to the amount of water held in the soil at any
particular time. The amount of water in the soil depends soil properties like soil
texture and organic matter content. The maximum amount of water the soil can
hold is called the field capacity. Fine grain soils have larger field capacities than
coarse grain (sandy) soils. Thus, more water is available for actual
evapotranspiration from fine soils than coarse soils. The upper limit of soil
moisture storage is the field capacity, the lower limit is 0 when the soil has dried
out.
Change in Soil Moisture Storage (ΔST).
The change in soil moisture storage is the amount of water that is being added
to or removed from what is stored. The change in soil moisture storage falls
between 0 and the field capacity.
Diunduh dari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html ……
11/11/2012
NERACA AIR
Deficit (D)
A soil moisture deficit occurs when the demand for water exceeds that which is
actually available .
In other words, deficits occur when potential evapotranspiration exceeds actual
evapotranspiration (PE>AE). Recalling that PE is water demand and AE is actual
water use (which depends on how much water is really available), if we demand
more than we have available we will experience a deficit. But, deficits only occur
when the soil is completely dried out. That is, soil moisture storage (ST) must be
0. By knowing the amount of deficit, one can determine how much water is
needed from irrigation sources.
Diunduh dari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html ……
11/11/2012
NERACA AIR
Surplus (S)
Surplus water occurs when P exceeds PE and the soil is at its field
capacity (saturated). That is, we have more water than we actually
need to use given the environmental conditions at a place.
The surplus water cannot be added to the soil because the soil is
at its field capacity so it runs off the surface. Surplus runoff often
ends up in nearby streams causing stream discharge to increase.
A knowledge of surplus runoff can help forecast potential flooding
of nearby streams.
Diunduh dari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html ……
11/11/2012
NERACA AIR
Computing a Soil - Moisture Budget
The best way to understand how the water balance works is to actually calculate
a soil water budget.
A knowledge of soil moisture status is important to the agricultural economy of
this region that produces mostly corn and soy beans.
To work through the budget, we'll take each month (column) one at a time. It's
important to work column by column as we're assessing the moisture status in
a given month and one month's value may be determined by what happened in
the previous month.
Diunduh dari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html ……
11/11/2012
Water Budget - Rockford, IL
NERACA AIR
Field Capacity = 90 mm
Water Budget (location of Rockford, Illinois). Field Capacity = 90 mm
J
F
M
A
M
J
J
A
S
O
N
D
Year
P
50
49
66
78
100
106
88
84
86
73
56
45
881
PE
0
0
5
40
84
123
145
126
85
44
8
0
531
P-PE
50
49
61
38
16
-17
-57
-42
1
29
48
45
ΔST
0
0
0
0
0
17
57
16
1
29
48
12
ST
90
90
90
90
90
73
16
0
1
30
78
90
AE
0
0
5
40
84
123
145
100
85
44
8
0
634
D
0
0
0
0
0
0
0
26
0
0
0
0
26
S
50
49
61
38
16
0
0
0
0
0
0
33
258
Diunduh dari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html ……
11/11/2012
Soil Moisture Recharge - Rockford, IL
NERACA AIR
Field Capacity = 90 mm
Soil Moisture Recharge . Field Capacity = 90 mm
J
F
M
A
M
J
J
A
S
O
N
D
Year
P
50
49
66
78
100
106
88
84
86
73
56
45
881
PE
0
0
5
40
84
123
145
126
85
44
8
0
531
P-PE
50
49
61
38
16
-17
-57
-42
1
29
48
45
ΔST
0
0
0
0
0
-17
-57
-16
1
29
48
12
ST
90
90
90
90
90
73
16
0
1
30
78
90
AE
0
0
5
40
84
123
145
100
85
44
8
0
634
D
0
0
0
0
0
0
0
26
0
0
0
0
26
S
50
49
61
38
16
0
0
0
0
0
0
33
258
Diunduh dari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html ……
11/11/2012
Soil Moisture Recharge - Rockford, IL
Field Capacity = 90 mm
NERACA AIR
We'll start the budget process at the end of the dry season when precipitation begins to
replenish the soil moisture, called soil moisture recharge, in September. At the beginning
of the month the soil is considered dry as the storage in August is equal to zero. During
September, 86 mm of water falls on the surface as precipitation. Potential
evapotranspiration requires 85 mm. Precipitation therefore satisfies the need for water
with one millimeter of water left over (P-PE=1). The excess one millimeter of water is put
into storage (ΔST=1) bringing the amount in storage to one millimeter (August ST =0 so 0
plus the one millimeter in September equals one millimeter). Actual evapotranspiration is
equal to potential evapotranspiration as September is a wet month (P>PE). There is no
deficit during this month as the soil now has some water in it and no surplus as it has
not reached its water holding capacity.
During the month of October, precipitation far exceeds potential evapotranspiration (PPE=29). All of the excess water is added to the existing soil moisture (ST (September) +
29 mm = 30 mm). Being a wet month, AE is again equal to PE.
Calculating the budget for November is very similar to that of September and October.
The difference between P and PE is all allocated to storage (ST now equal to 78 mm) and
AE is equal to PE.
Diunduh dari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html ……
11/11/2012
NERACA AIR
Soil Moisture Surplus
During December, potential evapotranspiration has dropped to zero as plants
have gone into a dormant period thus reducing their need for water and cold
temperatures inhibit evaporation. Notice that P-PE is equal to 45 but not all is
placed into storage.
Why? At the end of November the soil is within 12 mm of being at its field
capacity. Therefore, only 12 millimeters of the 45 available is put in the soil and
the remainder runs off as surplus (S=33).
Given that the soil has reached its field capacity in December, any excess water
that falls on the surface in January will likely generate surplus runoff. According
to the water budget table this is indeed true. Note that P-PE is 50 mm and ΔST is
0 mm. What this indicates is that we cannot change the amount in storage as
the soil is at its capacity to hold water. As a result the amount is storage (ST)
remains at 90 mm. Being a wet month (P>PE) actual evapotranspiration is equal
to potential evapotranspiration. Note that all excess water (P-PE) shows up as
surplus (S=50 mm).
Diunduh dari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html ……
11/11/2012
NERACA AIR
Soil Moisture Surplus . Field Capacity = 90 mm
J
F
M
A
M
J
J
A
S
O
N
D
Year
P
50
49
66
78
100
106
88
84
86
73
56
45
881
PE
0
0
5
40
84
123
145
126
85
44
8
0
531
P-PE
50
49
61
38
16
-17
-57
-42
1
29
48
45
ΔST
0
0
0
0
0
-17
-57
-16
1
29
48
12
ST
90
90
90
90
90
73
16
0
1
30
78
90
AE
0
0
5
40
84
123
145
100
85
44
8
0
634
D
0
0
0
0
0
0
0
26
0
0
0
0
26
S
50
49
61
38
16
0
0
0
0
0
0
33
258
Diunduh dari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html ……
11/11/2012
Surplus Lengas Tanah
Given that the soil has reached its field capacity in December, any excess water that falls
on the surface in January will likely generate surplus runoff. According to the water
budget table this is indeed true. Note that P-PE is 50 mm and ΔST is 0 mm. What this
indicates is that we cannot change the amount in storage as the soil is at its capacity to
hold water. As a result the amount is storage (ST) remains at 90 mm. Being a wet month
(P>PE) actual evapotranspiration is equal to potential evapotranspiration. Note that all
excess water (P-PE) shows up as surplus (S=50 mm).
Similar conditions occur for the months of February, March, April, and May. These are all
wet months and the soil remains at its field capacity so all excess water becomes
surplus. Note too that the values of PE are increasing through these months. This
indicates that plants are springing to life and transpiring water. Evaporation is also
increasing as insolation and air temperatures are increasing. Notice how the difference
between precipitation and potential evapotranspiration decreases through these months.
As the demand on water increases, precipitation is having a harder time satisfying it. As
a result, there is a smaller amount of surplus water for the month.
Surplus runoff can increase stream discharge to the point where flooding occurs. The
flood duration period lasts from December to May (6 months), with the most intense
flooding is likely to occur in March when surplus is the highest (61 mm).
….
Diunduh dari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html ……
11/11/2012
NERACA AIR
Soil Moisture Utilization. Field Capacity = 90 mm
J
F
M
A
M
J
J
A
S
O
N
D
Year
P
50
49
66
78
100
106
88
84
86
73
56
45
881
PE
0
0
5
40
84
123
145
126
85
44
8
0
531
P-PE
50
49
61
38
16
-17
-57
-42
1
29
48
45
ΔST
0
0
0
0
0
-17
-57
-16
1
29
48
12
ST
90
90
90
90
90
73
16
0
1
30
78
90
AE
0
0
5
40
84
123
145
100
85
44
8
0
634
D
0
0
0
0
0
0
0
26
0
0
0
0
26
S
50
49
61
38
16
0
0
0
0
0
0
33
258
Diunduh dari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html ……
11/11/2012
Soil Moisture Utilization
By the time June rolls around, temperatures have increased to the point where
evaporation is proceeding quite rapidly and plants are requiring more water to keep them
healthy. As potential evapotranspiration is approaching its maximum value during these
warmer months, precipitation is falling off. During June P-PE is -17 mm. What this means
is precipitation no longer is able to meet the demands of potential evapotranspiration. In
order to meet their needs, plants must extract water that is stored in the soil from the
previous months. This is shown in the table by a value of 17 in the cell for ΔST (change in
soil storage). Once the 17 m is taken out of storage (ST) it reduces its value to 73.
The month of June is considered a dry month (P<PE) so AE is equal to precipitation plus
the absolute value of ΔST (P + |ΔST|). When we complete this calculation (106 mm + 17
mm = 123 mm) we see that AE is equal to PE. What this means is precipitation and what
was extracted from storage was able to meet the needs demanded by potential
evapotranspiration. Note that there is no surplus in June as the soil moisture storage has
dropped below its field capacity. There is still no deficit as water remains in storage. The
calculations for July is similar to June, just different values. Note that by the time July
ends, water held in storage is down to a mere 16 mm.
Diunduh dari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html ……
11/11/2012
NERACA AIR
Soil Moisture Deficit
J
F
M
A
M
J
J
A
S
O
N
D
Year
P
50
49
66
78
100
106
88
84
86
73
56
45
881
PE
0
0
5
40
84
123
145
126
85
44
8
0
531
P-PE
50
49
61
38
16
-17
-57
-42
1
29
48
45
ΔST
0
0
0
0
0
-17
-57
-16
1
29
48
12
ST
90
90
90
90
90
73
16
0
1
30
78
90
AE
0
0
5
40
84
123
145
100
85
44
8
0
634
D
0
0
0
0
0
0
0
26
0
0
0
0
26
S
50
49
61
38
16
0
0
0
0
0
0
33
258
Diunduh dari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html ……
11/11/2012
Soil Moisture Deficit
August, like June and July, is a dry month.
Potential evapotranspiration still exceeds precipitation and the difference is a 42 mm.
Up until this month there has been enough water from precipitation and what is
in storage to meet the demands of potential evapotranspiration. However,
August begins with only 16 mm of water in storage (ST of July). Thus we'll only
be able to extract 16 mm of the 42 mm of water needed to meet the demands of
potential evapotranspiration So, of the 42 mm of water we would need (P-PE) to
extract from the soil. In so doing, the amount in storage (ST) falls to zero and the
soil is dried out.
What happens to the remaining 26 mm of the original P-PE of 42? The unmet
need for water shows up as soil moisture deficit. In other words, we have not
been able to meet our need for water from both precipitation and what we can
extract from storage. AE is therefore equal to 100 mm (84 mm of precipitation
plus 16 mm of ΔST).….
Diunduh dari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html ……
11/11/2012
Soil Moisture Seasons
Four soil moisture seasons can be defined by the soil moisture conditions.
Recharge
The recharge season is a time when water is
added to soil moisture storage (+ΔST). The
recharge period occurs when precipitation
exceeds potential evapotranspiration but
the soil has yet to reach its field capacity.
Surplus
The surplus season occurs when
precipitation exceeds potential
evapotranspiration and the soil has reached
its field capacity. Any additional water
applied to the soil runs off. If this water runs
off into nearby streams and rivers it could
cause flooding. Thus, the intensity (amount)
and duration (length of season) of surplus
can be used to predict the severity of
potential flooding.
Diunduh dari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html ……
11/11/2012
NERACA AIR
Utilization
The utilization season is a time when water is withdrawn from soil moisture storage (ΔST). The utilization period occurs when potential evapotranspiration exceeds
precipitation but soil storage has yet to reach 0 (dry soil).
Deficit
The deficit season occurs when occurs when potential evapotranspiration exceeds
precipitation and soil storage has reached 0. This is a time when there is essentially
no water for plants. Farmers then tap ground water reserves or water in nearby
streams and lakes to irrigate their crops. Thus, the intensity (amount) and duration
(length of season) of deficit can be used to predict the need for irrigation water.
Whether a place experiences all four seasons depends on the climate and soil
properties. Wet climate and those places with soils having high field capacities are
less likely to experience a deficit period. Likewise the duration and intensity of any
season will be determined by the climate and soil properties. Given equal amounts of
precipitation, coarse textured soils will generate runoff faster than fine textured soils
and may experience more intense surplus
Diunduh dari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html ……
11/11/2012
AIR
DALAM TANAH
STRUKTUR &
CIRI
H2O
Molekul air terdiri atas atom oksigen dan dua atom
hidrogen, yang berikatan secara kovalen
Atom-atom tidak terikat secara linear (H-O-H), tetapi
atom hidrogen melekat pada atom oksigen seperti huruf
V dengan sudut 105o.
Molekul air bersifat dipolar:
Zone elektro positif
+
H
H
105o
Zone elektro negatif
-
Lingkaran
Tanah-AirTanaman
LTAT mrpk sistem dinamik dan terpadu dimana air mengalir
dari tempat dengan tegangan rendah menuju tempat dengan
tegangan air tinggi.
Hilang melalui stomata
daun (transpirasi)
Air kembali ke
atmosfer
(evapo-transpirasi)
Air dikembalikan ke
tanah melalui hujan
dan irigasi
Penguapan
Serapan bulu akar
SISTEM TANAH-TANAMAN
Structure of water transport model for the soil-leaf continuum, with the
inputs outlined in boxes.
Root and shoot components are represented by a resistance network, each
component of which varies according to the inputted K(y) function from
vulnerability curves of xylem.
Layers of roots reach to different soil depths according to an inputted root
area profile. Canopy layers reflect an inputted leaf area and Y profile.
Soil is modeled as a rhizosphere resistance connecting roots to bulk soil of
an inputted y and K(y).
The model predicts transpiration (E) as a function of the inputs.
Kekuatan ikatan antara molekul air dengan partikel tanah
dinyatakan dengan TEGANGAN AIR TANAH. Ini merupakan fungsi
dari gaya-gaya adesi dan kohesi di antara molekul - molekul air dan
partikel tanah
Adesi
Kohesi
H2O
Partikel tanah
Air terikat
Air bebas
30
Air Tersedia untuk pertumbuhan tanaman
).
Fine textured soils with small
pores can hold the greatest
amounts of PAW.
Coarse textured sandy soils with
large pores can hold the least
amounts of PAW.
32
Status Air
Tanah
Perubahan status air dalam tanah, mulai dari
kondisi jenuh hingga titik layu
Jenuh
Kap. Lapang
Padatan
Titik layu
Pori
100g
air
40g
tanah jenuh air
100g
20g
udara
kapasitas lapang
100g
10 g
udara
koefisien layu
100g
8g
udara
koefisien higroskopis
TEGANGAN
&
KADAR AIR
PERHATIKANLAH proses yang terjadi kalau tanah basah
dibiarkan mengering.
Bagan berikut melukiskan hubungan antara tebal lapisan air di
sekeliling partikel tanah dengan tegangan air
Bidang singgung tanah dan air
Koef.
Koef.
padatan tanah
higroskopis layu
10.000
atm
31 atm
10.000 atm
15 atm
Kapasitas
lapang
1/3 atm
Mengalir krn gravitasi
Tegangan air
1/3 atm
tebal lapisan air
34
Representasi bola air yang menyelubungi partikel padatan
tanah
JUMLAH AIR DALAM TANAH
The amount of soil water is usually measured in terms of water content as
percentage by volume or mass, or as soil water potential. Water content
does not necessarily describe the availability of the water to the plants, nor
indicates, how the water moves within the soil profile. The only
information provided by water content is the relative amount of water in
the soil.
Soil water potential, which is defined as the energy required to remove
water from the soil, does not directly give the amount of water present in
the root zone either. Therefore, soil water content and soil water potential
should both be considered when dealing with plant growth and irrigation.
The soil water content and soil water potential are related to each other,
and the soil water characteristic curve provides a graphical
representation of this relationship.
TEGANGAN
vs
kadar air
Air
higroskopis
Kurva tegangan - kadar air tanah bertekstur
lempung
Air kapiler
Air tersedia
Lambat tersedia
Cepat tersedia
Air gravitasi
Zone optimum
Tegangan air, bar
31
Koefisien higroskopis
Koefisien layu
0.1
Kapasitas lapang
Kap. Lapang maksimum
37
persen air tanah
Tegangan air tanah (bar / atm
Air tersedia
bagi
tanaman
Air Gravitasi
Titik
Layu
Kapasitas lapang
Kadar air volumetrik, %
Hubungan antara kadar air tanah dan tegangan air tanah
untuk tekstur lempung
STRUKTUR
&
CIRI
POLARITAS
Molekul air mempunyai dua ujung, yaitu ujung oksigen yg
elektronegatif dan ujung hidrogen yang elektro-positif.
Dalam kondisi cair, molekul-molekul air saling bergandengan
membentuk kelompok-kelompok kecil tdk teratur.
Ciri polaritas ini menyebabkan plekul air tertarik pada ion-ion
elektrostatis.
Kation-kation K+, Na+, Ca++ menjadi berhidrasi kalau ada
molekul air, membentuk selimut air, ujung negatif melekat
kation.
Permukaan liat yang bermuatan negatif, menarik ujung positif
molekul air.
Kation hidrasi
Selubung air
Tebalnya selubung air tgt
pd rapat muatan pd permukaan kation.
Rapat muatan =
muatan kation / luas permukaan
39
STRUKTUR
&
CIRI
IKATAN HIDROGEN
Atom hidrogen berfungsi sebagai titik penyambung (jembatan)
antar molekul air.
Ikatan hidrogen inilah yg menyebabkan titik didih dan viskositas
air relatif tinggi
KOHESI vs. ADHESI
Kohesi: ikatan hidrogen antar molekul air
Adhesi: ikatan antara molekul air dengan permukaan padatan
lainnya
Melalui kedua gaya-gaya ini partikel tanah mampu menahan air dan
mengendalikan gerakannya dalam tanah
TEGANGAN PERMUKAAN
Terjadinya pada bidang persentuhan air dan udara, gaya kohesi antar
molekul air lebih besra daripada adhesi antara air dan udara.
Udara
Permukaan air-udara
air
40
ENERGI AIR
TANAH
Retensi dan pergerakan air tanah melibatkan energi, yaitu:
Energi Potensial, Energi Kinetik dan Energi Elektrik.
Selanjutnya status energi dari air disebut ENERGI BEBAS,
yang merupakan PENJUMLAHAN dari SEMUA BENTUK
ENERGI yang ada.
Air bergerak dari zone air berenergi bebas tinggi (tanah basah)
menuju zone air berenergi bebas rendah (tanah kering).
Gaya-gaya yg berpengaruh
Gaya matrik: tarikan padatan tanah (matrik) thd molekul air;
Gaya osmotik: tarikan kation-kation terlarut thd molekul air
Gaya gravitasi: tarikan bumi terhadap molekul air tanah.
Potensial air tanah
Ketiga gaya tersebut di atas bekerja bersama mempengaruhi energi bebas air tanah,
dan selanjutnya menentukan perilaku air tanah, ….. POTENSIAL TOTAL AIR
TANAH (PTAT)
PTAT adalah jumlah kerja yg harus dilakukan untuk memindahkan secara
berlawanan arah sejumlah air murni bebas dari ketinggian tertentu secara isotermik
ke posisi tertentu air tanah.
PTAT = Pt = perbedaan antara status energi air tanah dan air murni bebas
Pt = Pg + Pm + Po + …………………………
41
( t = total; g = gravitasi; m = matrik; o = osmotik)
Hubungan potensial air tanah dengan energi bebas
Potensial
positif
+
Energi bebas naik bila air tanah berada pada
letak ketinggian yg lebih tinggi dari titik
baku pengenal (referensi)
Energi bebas dari air murni
Potensial tarikan bumi
0
Menurun karena pengaruh osmotik
Potensial
negatif
-
Menurun karena pengaruh matrik
Energi bebas dari air tanah
Potensial osmotik
(hisapan)
Potensial matrik
(hisapan)
POTENSIAL
AIR TANAH
POTENSIAL TARIKAN BUMI = Potensial gravitasi
Pg = G.h
dimana G = percepatan gravitasi, h = tinggi air tanah di atas posisi
ketinggian referensi.
Potensial gravitasi berperanan penting dalam menghilangkan kelebihan
air dari bagian atas zone perakaran setelah hujan lebat atau irigasi
Potensial matrik dan Osmotik
Potensial matrik merupakan hasil dari gaya-gaya jerapan dan kapilaritas.
Gaya jerapan ditentukan oleh tarikan air oleh padatan tanah dan kation jerapan
Gaya kapilaritas disebabkan oleh adanya tegangan permukaan air.
Potensial matriks selalu negatif
Potensial osmotik terdapat pd larutan tanah, disebabkan oleh adanya bahan-bahan terlarut
(ionik dan non-ionik).
Pengaruh utama potensial osmotik adalah pada serapan air oleh tanaman
Hisapan dan Tegangan
Potensial matrik dan osmotik adalah negatif, keduanya bersifat menurunkan energi bebas air tanah. Oleh
karena itu seringkali potensial negatif itu disebut HISAPAN atau TEGANGAN.
Hisapan atau Tegangan dapat dinyatakan dengan satuan-satuan positif.
Jadi padatan-tanah bertanggung jawab atas munculnya HISAPAN atau TEGANGAN.
43
Cara
Menyatakan
Tegangan
Energi
Tinggi unit
kolom air (cm)
10
100
346
1000
10000
15849
31623
100.000
Tegangan: dinyatakan dengan “tinggi (cm) dari satuan
kolom air yang bobotnya sama dengan tegangan tsb”.
Tinggi kolom air (cm) tersebut lazimnya dikonversi
menjadi logaritma dari sentimeter tinggi kolom air,
selanjutnya disebut pF.
Logaritma
tinggi kolom air (pF)
1
2
2.53
3
4
4.18
4.5
5
Bar
Atmosfer
0.01
0.1
0.346
1
10
15.8
31.6
100
0.0097
0.0967
1.3
9.6749
15
31
96.7492
KANDUNGAN
AIR DAN
TEGANGAN
KURVA ENERGI - LENGAS TANAH
Tegangan air menurun secara gradual dengan meningkatnya
kadar air tanah.
Tanah liat menahan air lebih banyak dibanding tanah pasir pada
nilai tegangan air yang sama
Tanah yang Strukturnya baik mempunyai total pori lebih
banyak, shg mampu menahan air lebih banyak
Pori medium dan mikro lebih kuat menahan air dp pori makro
Tegangan air tanah, Bar
10.000
Liat
Lempung
Pasir
0.01
10
Kadar air tanah, %
70
Tekstur tanah dan air tersedia
Hubungan antara kadar air tanah dengan tegangan air tanah
47
Kapasitas air tersedia dalam tanah yang teksturnya berbedabeda
Gerakan
Air Tanah
Tidak Jenuh
Gerakan tidak jenuh = gejala kapilaritas = air bergerak dari
muka air tanah ke atas melalui pori mikro.
Gaya adhesi dan kohesi bekerja aktif pada kolom air (dalam pri
mikro), ujung kolom air berbentuk cekung.
Perbedaan tegangan air tanah akan menentukan arah gerakan
air tanah secara tidak jenuh.
Air bergerak dari daerah dengan tegangan rendah (kadar air tinggi)
ke daerah yang tegangannya tinggi (kadar air rendah, kering).
Gerakan air ini dapat terjadi ke segala arah dan berlangsung secara
terus-menerus.
Pelapisan tanah berpengaruh terhadap gerakan air tanah.
Lapisan keras atau lapisan kedap air memperlambat gerakan air
Lapisan berpasir menjadi penghalang bagi gerakan air dari lapisan
yg bertekstur halus.
Gerakan air dlm lapisan berpasir sgt lambat pd tegangan
49
Air hujan dan irigasi memasuki tanah, menggantikan udara
dalam pori makro - medium - mikro. Selanjutnya air bergerak
ke bawah melalui proses gerakan jenuh dibawah pengaruh gaya
gravitasi dan kapiler.
Gerakan air jenuh ke arah bawah ini berlangsung terus selama
cukup air dan tidak ada lapisan penghalang
Gerakan Jenuh
(Perkolasi)
Lempung berpasir
cm
Lempung berliat
0
15 mnt
4 jam
30
60
90
1 jam
24 jam
120
24 jam
48 jam
150
30 cm
60 cm
Jarak dari tengah-tengah saluran, cm
50
Pola Penetrasi dan Pergerakan Air pada tanah Berpasir dan
tanah Lempung-liat
52
Pola pergerakan air gravitasi dalam tanah
Pengaruh struktur tanah terhadap pergerakan air tanah ke
arah bawah
PERKOLASI
Jumlah air perkolasi
Faktor yg berpengaruh:
1. Jumlah air yang ditambahkan
2. Kemampuan infiltrasi permukaan tanah
3. Daya hantar air horison tanah
4. Jumlah air yg ditahan profil tanah pd kondisi
kapasitas lapang
Keempat faktor di atas ditentukan oleh struktur dan tekstur tanah
Tanah berpasir punya kapasitas ilfiltrasi dan daya hantar air sangat
tinggi, kemampuan menahan air rendah, shg perkolasinya mudah
dan cepat
Tanah tekstur halus, umumnya perkolasinya rendah dan sangat
beragam; faktor lain yg berpengaruh:
1. Bahan liat koloidal dpt menyumbat pori mikro & medium
2. Liat tipe 2:1 yang mengembang-mengkerut sangat berperan
54
LAJU
GERAKAN
AIR TANAH
Kecepatan gerakan air dlm tanah dipengaruhi oleh dua faktor:
1. Daya dari air yang bergerak
2. Hantaran hidraulik = Hantaran kapiler = daya hantar
i = k.f
dimana i = volume air yang bergerak; f = daya air yg bergerak dan k =
konstante.
Daya air yg bergerak = daya penggerak, ditentukan oleh dua faktor:
1. Gaya gravitasi, berpengaruh thd gerak ke bawah
2. Selisih tegangan air tanah, ke semua arah
Gerakan air semakin cepat kalau perbedaan tegangan semakin tinggi.
Hantaran hidraulik ditentukan oleh bbrp faktor:
1. Ukuran pori tanah
2. Besarnya tegangan untuk menahan air
Pada gerakan jenuh, tegangan airnya rendah, shg hantaran hidraulik berbanding
lurus dengan ukuran pori
Pd tanah pasir, penurunan daya hantar lebih jelas kalau terjadi penurunan kandungan
air tanah
Lapisan pasir dlm profil tanah akan menjadi penghalang gerakan air tidak jenuh
55
Gerakan
air tanah
Gerakan air tanah dipengaruhi oleh kandungan
air tanah
Penetrasi air dari tnh basah ke tnh kering
(cm)
18
Tanah lembab, kadar air awal 29%
Tanah lembab, kadar air awal 20.2%
Tanah lembab, kadar air awal 15.9%
0
26
156
Jumlah hari kontak, hari
Sumber: Gardner & Widtsoe, 1921.
GERAKAN
UAP AIR
Penguapan air tanah terjadi internal (dalam pori tanah) dan
eksternal (di permukaan tanah)
Udara tanah selalu jenus uap air, selama kadar air tanah
tidak lebih rendah dari koefisien higroskopis (tegangan 31
atm).
Mekanisme Gerakan uap air
Difusi uap air terjadi dlm udara tanah, penggeraknya adalah perbedaan tekanan uap
air.
Arah gerapan menuju ke daerah dg tekanan uap rendah
Pengaruh suhu dan lengas tanah terhadap gerapan uap air dalam tanah
Lembab Dingin
Kering
Dingin
Kering Panas
Lembab Panas
57
RETENSI
AIR TANAH
KAPASITAS RETENSI MAKSIMUM adalah:
Kondisi tanah pada saat semua pori terisi penuh air, tanah jenuh
air, dan tegangan matrik adalah nol.
KAPASITAS LAPANG: air telah meninggalkan pori makro, mori
makro berisi udara, pori mikro masih berisi air; tegangan matrik
0.1 - 0.2 bar; pergerakan air terjadi pd pori mikro/ kapiler
KOEFISIEN LAYU: siang hari tanaman layu dan malam hari segar kembali,
lama-lama tanaman layu siang dan malam; tegangan matrik 15 bar.
Air tanah hanya mengisi pori mikro yang terkecil saja, sebagian besar air
tidak tersedia bagi tanaman.
Titik layu permanen, bila tanaman tidak dapat segar kembali
KOEFISIEN HIGROSKOPIS
Molekul air terikat pada permukaan partikel koloid tanah, terikat kuat
sehingga tidak berupa cairan, dan hanya dapat bergerak dlm bentuk uap air,
tegangan matrik-nya sekitar 31 bar.
Tanah yg kaya bahan koloid akan mampu menahan air higroskopis lebih
banyak dp tanah yg miskin bahan koloidal.
58
Klasifikasi
Air Tanah
Klasifikasi Fisik:
1. Air Bebas (drainase)
2. Air Kapiler
3. Air Higroskopis
Air Bebas (Drainase):
a. Air yang berada di atas kapasitas lapang
b. Air yang ditahan tanah dg tegangan kurang dari 0.1-0.5 atm
c. Tidak diinginkan, hilang dengan drainase
d. Bergerak sebagai respon thd tegangan dan tarika gravitasi bumi
e. Hara tercuci bersamanya
AIR KAPILER:
a. Air antara kapasitas lapang dan koefisien higroskopis
b. Tegangan lapisan air berkisar 0.1 - 31 atm
c. Tidak semuanya tersedia bagi tanaman
d. Bergerak dari lapisan tebal ke lapisan tipis
e. Berfungsi sebagai larutan tanah
AIR HIGROSKOPIS :
a. Air diikat pd koefisien higroskopis
b. Tegangan berkisar antara 31 - 10.000 atm
c. Diikat oleh koloid tanah
d. Sebagian besar bersifat non-cairan
e. Bergerak sebagai uap air
59
Agihan air
dalam tanah
Berdasarkan tegangan air tanah dapat dibedakan menjadi
tiga bagian: Air bebas, kapiler dan higroskopis
Koef. Higroskopis
kurang lebih 31 atm
Kap. Lapang
kurang lebih 1/3 atm
Jml ruang pori
Lapisan olah
Air higroskopik
Hampir tdk
menunjukkan
sifat cairan
Air Kapiler
Peka thd gerakan
kapiler, laju penyesuaian meningkat dg meningkatnya kelembaban tanah
Ruang diisi udara
Biasanya jenuh uap air
Setelah hujan lebat
sebagian diisi air,
tetapi air cepat hilang krn gravitasi
bumi
Lapisan bawah tanah
Karena pemadatan ruang
pori berkurang
Strata bawah (jenuh air)
Kolom tanah
Jumlah ruang pori
Klasifikasi
Biologi
Air tanah
Klasifikasi berdasarkan ketersediaannya bagi tanaman:
1. AIR BERLEBIHAN: air bebas yg kurang tersedia bagi tanaman.
Kalau jumlahnya banyak berdampak buruk bagi tanaman, aerasi
buruk, akar kekurangan oksigen, anaerobik, pencucian air
2. AIR TERSEDIA: air yg terdapat antara kap. Lapang dan koef. Layu.
Air perlu ditambahkan untuk mencapai pertumbuhan tanaman yang
optimum apabila 50 - 85% air yg tersedia telah habis terpakai.
Kalau air tanah mendekati koefisien layu, penyerapan air oleh akar tanaman
tdk begitu cepat dan tidak mampu mengimbangi pertumbuhan tanaman
3. AIR TIDAK TERSEDIA: AIR yg diikat oleh tanah pd TITIK LAYU permanen,
yaitu air higroskopis dan sebagian kecil air kapiler.
KH
31 atm
KL
KP
15 atm
1/3 atm
Air
Higroskopis
Air
Kapiler
Tdk tersedia
Tersedia
100 % pori
Ruang udara dan
air drainase
Berlebihan
Daerah Optimum
Faktor yg
mempengaruhi
Air Tersedia
Faktor yg berpengaruh:
1. Hubungan tegangan dengan kelengasan
2. Kedalaman tanah
3. Pelapisan Tanah
TEGANGAN MATRIK : tekstur, struktur dan kandungan bahan organik
mempengaruhi jumlah air yg dapat disediakan tanah bagi tanaman
TEGANGAN OSMOTIK: adanya garam dalam tanah meningkatkan
tegangan osmotik dan menurunkan jumlah air tersedia, yaitu menaikkan
koefisien layu.
Persen air
Sentimeter air setiap 30 cm tanah
10
18
Kap. Lapang
Air tersedia
Koef. Layu
6
5
Air tidak tersedia
Pasir Sandy loam
Loam Silty-loam Clay-loam
Liat
Tekstur semakin halus
KEHILANGAN
UAP AIR
DARI TANAH
HADANGAN HUJAN OLEH TUMBUHAN
Tajuk tumbuhan mampu menangkap sejumlah air hujan, sebagian air ini
diuapkan kembali ke atmosfer.
Vegetasi hutan di daerah iklim basah mampu menguapkan kembali air
hujan yg ditangkapnya hingga 25%, dan hanya 5% yg mencapai tanah
melalui cabang dan batangnya.
Awan hujan
presipitasi
Pembentukan Awan
transpirasi
evaporasi
Run off
infiltrasi
Tanah permukaan
perkolasi
Batuan
Groundwater
Sungai - laut
Pengendalian
Penguapan
MULSA & PENGELOLAAN
Mulsa adalah bahan yg dipakai pd permukaan tanah untuk
mengurangi penguapan air atau untuk menekan pertumbuhan
gulma.
Lazimnya mulsa spt itu digunakan untuk tanaman yang tidak
memerlukan pengolahan tanah tambahan
MULSA KERTAS & PLASTIK
Bahan mulsa dihamparkan di permukaan tanah, diikat spy tdk terbang, dan tanaman
tumbuh melalui lubang-lubang yg telah disiapkan
Selama tanah tertutup mulsa, air tanah dapat diawetkan dan pertumbuhan gulma
dikendalikan
MULSA SISA TANAMAN
Bahan mulsa berasal dari sisa tanaman yg ditanam sebelumnya, misalnya jerami padi,
jagung, dan lainnya
Bahan mulsa dipotong-potong dan disebarkan di permukaan tanah
Cara WALIK DAMI sebelum penanaman kedelai gadu setelah padi sawah
MULSA TANAH  Pengolahan tanah
Efektivitas mulsa tanah dalam konservasi air-tanah (mengendalikan evaporasi) masih
diperdebatkan, hasil-hasil penelitian masih snagat beragam
Olah Tanah vs
Penguapan
Air Tanah
Alasan pengolahan tanah:
1. Mempertahankan kondisi fisika tanah yg memuaskan
2. Membunuh gulma
3. Mengawetkan air tanah.
Pengendalian Penguapan vs Pemberantasan Gulma
Perlakuan
Hasil jagung (t/ha)
Tanah dibajak dg persiapan yg baik
1. Dibebaskan dari gulma
2. Gulma dibiarkan tumbuh
3. Tiga kali pengolahan dangkal
Persiapan Buruk
4. Dibebaskan dari gulma
Kadar air tanah (%)
hingga kedalaman 1 m
2.9
0.4
2.5
22.3
21.8
21.9
2.0
23.1
Sumber: Mosier dan Gutafson, 1915.
Pengolahan tanah yg dapat mengendalikan gulma dan memperbaiki kondisi fisik
tanah akan berdampak positif thd produksi tanaman
Pengolahan tanah yg berlebihan dapat merusak akar tanaman dan merangsang
evaporasi, shg merugikan tanaman
KAPASITAS SIMPANAN AIR TANAH
Soil "holds" water available for crop
use, retaining it against the pull of
gravity.
This is one of the most important
physical facts for agriculture.
If the soil did not hold water, if water
was free to flow downward with the
pull of gravity as in a river or canal,
we would have to constantly irrigate,
or hope that it rained every two or
three days.
There would be no reason to preirrigate. And there would be no such
thing as dryland farming.
www.icsu-scope.org/.../scope56/Chapter05.html
Hubungan antara Potensial Air Tanah dnegan Air Tersedia pada tiga
macam tekstur tanah
Kapasitas lapang
Titik layu permanen
Air tersedia (%)
Potensial air tanah (-bar)
The soil's ability to hold water depends
on both the soil texture and structure.
Texture describes the relative
percentages of sand, silt, and clay
particles.
The finer the soil texture (higher
percentage of silt and clay), the more
water soil can hold.
Gravity is always working to pull
water downwards below the plant's
root zone.
To counteract the pull of gravity, soil is
able to generate its own forces,
commonly called "matric forces"
("matric" because of the soil "matrix"
structure that forms the basis for the
forces).
www.ctahr.hawaii.edu/MauiSoil/a_comp03.aspx
An important fact about the soil's
water-holding forces is that as the level
of soil moisture goes down, the soil
generates more force.
This is the reason that some water will
move up into the root zone from a
shallow ground water table. As the plant
extracts water in the root zone, the soil
pulls water up from the area with more
water to the area with less.
As you would expect, the rate at which
the water-holding forces go up with
decreasing soil moisture is different for
different soils. In a coarse soil, they will
go up slowly.
This means that plants can extract a
great amount of water from coarse soils
before they stress. In contrast, these
forces rise quickly in finer soils.
69
www.fao.org/docrep/R4082E/r4082e03.htm
HUBUNGAN TANAH – AIR - TANAMAN
Lapisan olah
Lapisan olah dalam
Lapisan subsoil
Lapisan bahan induk
www.fao.org/docrep/R4082E/r4082e03.htm
HUBUNGAN TANAH - AIR
The role of soil in the soil-plant-atmosphere continuum is unique.
It has been demonstrated that soil is not essential for plant growth and
indeed plants can be grown hydroponically (in a liquid culture).
However, usually plants are grown in the soil and soil properties directly
affect the availability of water and nutrients to plants.
Soil water affects plant growth directly through its controlling effect on
plant water status and indirectly through its effect on aeration,
temperature, and nutrient transport, uptake and transformation.
The understanding of these properties is helpful in good irrigation design
and management.
The soil system is composed of
three major components: solid
particles (minerals and organic
matter), water with various
dissolved chemicals, and air.
The percentage of these
components varies greatly with
soil texture and structure.
An active root system requires a
delicate balance between the
three soil components; but the
balance between the liquid and
gas phases is most critical, since
it regulates root activity and
plant growth process.
73
Jumlah air tersedia dipengaruhi tekstur tanah
The only information provided
by water content is the relative
amount of water in the soil.
Kapasitas
Lapang
Air
Tersedia
Titik
Laytu
Air
Tidak Tersedia
Inchi Air per foot tanah
Water content does not
necessarily describe the
availability of the water to the
plants, nor indicates, how the
water moves within the soil
profile.
Persen Air
The amount of soil water is
usually measured in terms of
water content as percentage by
volume or mass, or as soil water
potential.
Therefore, soil water content
and soil water potential should
both be considered when
dealing with plant growth and
irrigation.
The soil water content and soil
water potential are related to
each other, and the soil water
characteristic curve provides a
graphical representation of this
relationship.
Potensial air tanah (MPa)
Soil water potential, which is
defined as the energy required
to remove water from the soil,
does not directly give the
amount of water present in the
root zone either.
Kapasitas
lapang
Titik layu
permanen
-1.5MPa
Air dalam tanah (% berat kering)
The nature of the soil characteristic curve
depends on the physical properties of the
soil namely, texture and structure. Soil
texture refers to the distribution of the soil
particle sizes.
The mineral particles of soil have a wide
range of sizes classified as sand, silt, and
clay.
The proportion of each of these particles
in the soil determines its texture.
All mineral soils are classified depending
on their texture. Every soil can be placed
in a particular soil group using a soil
textural triangle .
For example a soil with 60% sand and
10% clay separates is classified as a Sandy
loam
76
KAPASITAS LAPANG
There are limits on the amount of water that soil holds for crop use.
The upper limit is termed "field capacity".
During an irrigation, or whenever excess water is added to soil, water
drains down through the soil due to the pull of gravity.
At first, this internal drainage is relatively rapid.
However, it soon slows to almost nothing.
(The increasing soil water-holding forces finally start to counteract
gravity.) At this point we would say the soil is at field capacity.
You can demonstrate field capacity
using a visualization of a sponge (like
soil, a porous material that will hold
water).
Using a pan of water, hold a sponge
under water until it is saturated. Now,
pull the sponge out of the water.
It will immediately start to drip water,
quickly at first, then slower and
slower.
At some point it will essentially stop
dripping.
The internal drainage has stopped and
the sponge is at field capacity.
It is very important to note that you
can soak more water into soil that is
already at field capacity.
There will be open soil pores that will
take the water. However, the excess
water will not be held.
It will just drain down until the soil
moisture returns to field capacity.
KAPASITAS LAPANG
Field capacity
is a soil-based concept.
That is, it depends on the
texture and structure of the
soil as well as the physical
conditions in the field.
Coarse soils have lower field
capacities than fine soils.
If there is a high water table
or severe stratification that
would restrict drainage, the
field capacity would be higher
than normal.
79
AIR TERSEDIA & ZONE AKAR
The water held by the soil between field capacity and permanent
wilting point is termed the "available water holding capacity" of the
soil.
It is water that is "available" for the plant to use. Water added to the
soil in excess of field capacity will drain down, below the active root
system.
Water held by the soil that is below the permanent wilting point is of
no use, the plant has died.
As a crop manager you are concerned with the soil moisture
throughout the depth of the plant's active root system, the "effective
root zone".
The effective root zone is that depth
of soil where you want to control soil
moisture (just as you control fertility
and weed/pest pressures).
The effective root zone may or may
not be the actual depth of all active
roots. It may be shallower because of
concerns for crop quality or
development (as with many vegetable
crops).
For example, with cotton you may
estimate the effective root zone as 6
feet for a preirrigation, 2 feet for the
first seasonal irrigation, 4 feet for the
second seasonal, and 6 feet
thereafter. For an almond orchard,
you may estimate the effective root
zone as four feet for the entire
season. With onions, the major
concern is with the top 2 feet.
HUBUNGAN AIR & TANAH
The soil is composed of three major parts: air, water, and solids . The solid
component forms the framework of the soil and consists of mineral and
organic matter.
The mineral fraction is made up of sand, silt, and clay particles. The
proportion of the soil occupied by water and air is referred to as the pore
volume.
The pore volume is generally constant for a given soil layer but may be
altered by tillage and compaction. The ratio of air to water stored in the
pores changes as water is added to or lost from the soil. Water is added by
rainfall or irrigation, as shown in Figure 2.
Water is lost through surface runoff, evaporation (direct loss from the soil
to the atmosphere), transpiration (losses from plant tissue), and either
percolation (seepage into lower layers) or drainage.
Saturated (wet) soil. All pores (light areas) are filled with water. The dark areas
represent soil solids.
83
Water distribution in a soil at field capacity. Capillary water (lightly shaded areas
) in soil pores is available to plants. Field capacity represents the upper limit of
plant-available water.
Water distribution in a soil at thw wilting point. This water is held tightly in thin
films around soil particles and is unavailable to plants. The wilting point
represents the lower limit of plant-available water.
HUBUNGAN ANTARA AIR-TERSEDIA DAN DISTRIBUSI AIR DALAM
TANAH .
Kapasitas tanah menyimpan air
Air dalam tanah (in/ft)
Jumlah air tanah pada tiga macam tekstur tanah
Jumlah air tersedia dalam tanah yang teksturnya
berbeda-beda
89
AIR TANAH & STRES TANAMAN
Kalau tanaman menyerap air dari tanah , jumlah air tersedia yang tersisa
dalam tanah menjadi berkurang.
The amount of PAW removed since the last irrigation or rainfall is the
depletion volume.
Irrigation scheduling decisions are often based on the assumption that crop
yield or quality will not be reduced as long as the amount of water used by
the crop does not exceed the allowable depletion volume.
The allowable depletion of PAW depends on the soil and the crop. For
example, consider corn growing in a sandy loam soil three days after a
soaking rain.
Even though enough PAW may be avai1able for good plant growth, the plant
may wilt during the day when potential evapotranspiration (PET) is high.
AIR TANAH & STRES TANAMAN
Evapotranspiration merupakan proses hilangnya air tanah ke atmosfer,
melalui evaporasi dari permukaan tanah dan proses transpirasi dari
tanaman yang tumbuh di tanah .
Potential evapotranspiration is the maximum amount of water that could
be lost through this process under a given set of atmospheric conditions,
assuming that the crop covers the entire soil sur- face and that the
amount of water present in the soil does not limit the process.
Potential evapotranspiration is controlled by atmospheric conditions and
is higher during the day. Plants must extract water from the soil that is
next to the roots.
As the zone around the root begins to dry, water must move through the
soil toward the root (Figure 7). Daytime wilting occurs because PET is
high and the plant takes up water faster than the water can be replaced.
Hubungan antara distribusi air dalam tanah dan konsep jadwal
irigasi ketika 50 percent air tersedia telah habis
Ketersediaan air tanah bagi tanaman
Jumlah air tanah tersedia dalam berbagai tipe tanah
Konduktivitas hidraulik tanah
Efek potensial air tanah thd konduktivitas
hidraulik
Tititk layu
permanen
Kapasitas lapang
Potensial air tanah
Pengaruh Potensial Air tanah thd konduktivitas hidraulik
tanah
Pola penyerapan air oleh tanaman yang tumbuh pada profil tanah yang
tidak mempunyai lapisan penghambat dan suplai air tersedia cukup di
seluruh zone perakaran tanaman
AIR DALAM TANAH
Soil is made up of soil particles in crumb-form (peds), and pore
spaces around the soil crumbs.
In a well-structured soil, these crumbs are nice and stable....but in a
poorly structured soil, the crumbs are unstable which often limits
pore-space.
The pore-spaces are necessary for holding water, and for the free
gaseous exchange of oxygen and carbon dioxide between the plant
roots and the soil surface (respiration process).
There are three types of soil water (ie. water in the soil).
AIR -TANAH
AIR GRAVITASI
This is the water which is susceptible to the forces of gravity. It exists after
significant rainfall, and after substantial irrigation. This is the water which fills
all the pore-space, and leaves no room for oxygen and gaseous exchange. In
"light" soils, this tends to drain away quickly. In heavy soils, this can take time.
AIR KAPILER
This is the water which is held with the force of SURFACE TENSION by the soil
particles, and is resistent to the forces of gravity. This is the water which is
present after the gravitational water has drained away, leaving spaces free for
gaseous exchange. When the soil is holding it's MAXIMUM capillary water (after
the gravitational water has drained), this is called FIELD CAPACITY. At this
point, the plant is able to take up water easily, and has the oxygen that it needs in
the root zone.
98
AIR HIGROSKOPIS
Adalah air yang diikat sedemikian kuat (oleh tegangan permukaan) ke
partikel tanah sehingga akar tanaman tidak dapat menyerapnya.
Sehingga air ini tidak tersedia bagi tanaman.
At this stage there's generally sufficient oxygen, but there just isn't
enough available water.
The plant wilts, and will eventually die if it doesn't get water.
When the plant wilts and is unable to recover, this is called the TITIK
LAYU PERMANEN
TITIK LAYU PERMANEN
The closer to the soil particle the water is held, the tighter it's held. And the further from
the particle, the looser it's held. It takes little energy for the plant roots to take up the
water that's far from the particle and is present at the field capacity point. By contrast, as
the water is used up (or evaporates), it takes more and more energy for the plant to take
up water.
I often use the analogy of drinking through a straw. A short straw, ie. when a cup is 15 cm
away from you, is easy to use. A one-metre long straw takes a lot of energy to suck up a
drink. A twenty-metre straw is impossible to use. It works much the same with plants.
The more the soil dries out, the more energy the plant needs to output in order to get a
decent drink.
The effect of increased soil salinity (due to high soil salinity, high soil-water salinity, or
both) has basically the same effect as a soil drying out. Salt in the soil has as osmotic
effect, and causes the water to be held more tightly around the soil particles.
Semakin tinggi tingkat salinitas tanahnya, tanaman semakin sulit
menyerap air, meskipun air itu ada dalam tanah.
Kadar air tanah (%; mm/100mm)
Tegangan air tanah (kPa, sekala log)
Representasi ketersediaan air dalam tanah bagi
pertumbuhan tanaman
AIR –TANAH TERSEDIA
In other words, Plant Available Water (PAW) is the amount of water
held in a soil between the limits of Field Capacity and Permanent
Wilting Point.
However, only the water near to Field Capacity may be Readily
Available Water (RAW).
This is particularly so for fine textured, clayey soils because a high
proportion of PAW is held in small pores and as thin films and
plants need to 'do more work' to extract this fraction of water from
soils.
AIR –TANAH MUDAH TERSEDIA
Not all PAW is equally available to plants.
As soils dry out and PAW approaches PWP, plants will come under
water-stress and wilt. It is the objective of irrigators to avoid this
situation.
They prefer to irrigate when the soil water content is about 50% of
FC or about 100kPa.
These limits, however, are set by the irrigator to suit the business
enterprise. For example, if growth rates are to be restricted then
the trigger for an irrigation event may be 300kPa.
As the name suggests, Readily Available Water or RAW is the
amount and availability of water in soils that is readily available to
plants.
AIR –TANAH TERSEDIA
Following rainfall, or irrigation, all the pores in soil will be filled with water; this
is the Saturation Water Content (SWC). With time the water in the largest pores
will drain to depth due to gravitational forces.
In coarser textured, sandy and loamy soils this drainage will take place in less
than a day and will, therefore, be unavailable to plants.
Fine-textured, clayey soils, however, may be somewhat poorly drained and all
pores may remain filled with water for several days.
In these cases some of the SWC may be available for EvapoTranspiration and
would need to be considered in calculations of soil water balances and
irrigation scheduling.
Poorly drained soils, however, are less suitable for irrigation.
They are difficult to manage and may be waterlogged for times that can cause
damage to plants for reasons of anaerobic root environments.
PERGERAKAN AIR TANAH
During long-continued heavy rains, infiltration of soil water continues
under the force of gravity, carrying the water down to successively greater
depths. Soil pores become filled with water, with only a small amount of
free air remaining entrapped in bubbles.
The soil may, for a time, become almost completely saturated with water.
Downward percolation continues beyond the soil water belt into the
intermediate belt, a zone too deep to be reached by plat roots. Water may
ultimately reach the ground-water zone below .
After the rain has ceased, water continues to drain downward under the
influence of gravity, but some remains held in the soil, clinging to the soil
grains in thin films, by the force of capillary tension.
This is the same force that causes ink to be drawn upward in a piece of
blotting paper and which permits small water droplets to cling to the side
of a vertical pane of glass. Films of capillary water in the soil remain held
in place until gradually dissipated by evaporation or drawn into root
systems.
PERGERAKAN AIR TANAH
After soil has been saturated by prolonged rains and then drains until no
more water moves downward under the force of gravity, the soil is said to
be holding its field capacity of water. Most excess water drains out in a
day’s time; usually not more that two or three days are required for
gravity drainage to cease.
Soil-moisture content can be stated in terms of the equivalent depth in
inches of water in a given thickness of soil. At field capacity, soil-moisture
content ranges from 1 to 4 inches per foot of soil, depending upon soil
texture .
Sandy soils have low field capacity, which is rapidly reached because of the
ease with which the water penetrates the large openings (macro pores).
Clay soils, on the other hand, have a high field capacity, but require much
longer periods to attain it because of the slow rate of water penetration
due to the much smaller openings (micro pores).
A comparable, but lower value of soil moisture is the wilting point, below
which foliage wilts because of the inability of the plants to extract the
remaining moisture .
Air tanah pada berbagai kondisi kelengasan (kadar air)
WATER STORAGE
IN
SOIL
Proses Simpanan lengas dalam tanah.
Rain water can also be stored in the ground. Soils consist of particles and
pores. Those pores can be filled with air but also with water. The amount of
pores is a soil is different for different types of soil. The pores in a clay soil
account for 40% to 60% of the volume. In fine sand this can be 20%–45%
The soil particles have small pores in them where water can enter (soil water)
and between the particles are larger pores that can be filled. The soil is filled
with water up a certain level. This level goes up and down with changing
weather conditions. This water level is the ground water level.
The process of water entering the soil is called infiltration. When the soil has
taken up all the water it can, we say that it is saturated. If you walk over a
saturated soil, you feel that it is wet and soggy, like biscuits dipped in tea.
Part of the water that infiltrates, will move on. It will go to underground storage
reservoirs or to underground rivers and may, through ground water flows,
eventually reach a river or a lake. Another part will be used by plants or will
evaporate.
Diunduh dari: http://www.floodsite.net/juniorfloodsite/html/en/student/thingstoknow/hydrology/waterstorage2.html ……
Proses Simpanan lengas dalam tanah.
Diunduh dari: http://www.floodsite.net/juniorfloodsite/html/en/student/thingstoknow/hydrology/waterstorage2.html ……
11/11/2012
KAPASITAS SIMPANAN LENGAS TANAH
For irrigation the soil water storage (SWS) capacity is defined as the total
amount of water that is stored in the soil within the plant’s root zone. The soil
texture and the crop rooting depth determine this. A deeper rooting depth
means there is a larger volume of water stored in the soil and therefore a larger
reservoir of water for the crop to draw upon between irrigations.
Only a portion of the total soil water is readily available for plant use. Plants can
only extract a portion of the stored water without being stressed. An availability
coefficient is used to calculate the percentage of water that is readily available
to the plant. The maximum soil water deficit (MSWD) (also referred to as the
management allowable deficit) is the amount of water stored in the soil that is
readily available to the plant.
The crop should be irrigated once this amount of moisture has been removed
from the soil. Once depleted this is the amount that must be replenished by
irrigation. It is also the maximum amount that can be applied at one time, before
the risk of deep percolation occurs. However, in some cases leaching of salts is
desirable and extra irrigation would be desired.
Diunduh dari: http://www.agf.gov.bc.ca/resmgmt/publist/600Series/619000-1.pdf …… 11/11/2012
HOW TO DETERMINE THE SOIL WATER STORAGE AND THE
MAXIMUM SOIL WATER DEFICIET
Step 1 Determine the crop rooting depth, RD (m)
Step 2 Determine the available water storage capacity of the soil, AWSC (mm/m),
Table 2
Step 3 Calculate the total soil water storage, SWS (mm)
SWS (mm) = RD (m) x AWSC (mm/m) …………… (Equation 1)
Step 4 Determine the availability coefficient of the water to the crop, AC (%)
Step 5 Calculate the maximum soil water Deficit, MSWD (mm)
MSWD = SWS (mm) x AC (%) …………….. (Equation 2)
Diunduh dari: http://www.agf.gov.bc.ca/resmgmt/publist/600Series/619000-1.pdf …… 11/11/2012
Effective Rooting Depth of Mature Crops
for Irrigation System Design
Diunduh dari: http://www.agf.gov.bc.ca/resmgmt/publist/600Series/619000-1.pdf …… 11/11/2012
A Guide to Available Water Storage Capacities of Soils
Diunduh dari: http://www.agf.gov.bc.ca/resmgmt/publist/600Series/619000-1.pdf …… 11/11/2012
SOIL WATER STORAGE CAPACITY.
Availability Coefficients
Diunduh dari: http://www.agf.gov.bc.ca/resmgmt/publist/600Series/619000-1.pdf …… 11/11/2012
Infiltration and Soil Water Storage
Pidwirny, M. (2006). "Infiltration and Soil Water Storage". Fundamentals of Physical Geography, 2nd Edition.
Infiltration
Infiltration refers to the movement of water into the soil layer. The rate of this
movement is called the infiltration rate. If rainfall intensity is greater than the
infiltration rate, water will accumulate on the surface and runoff will begin.
Movement of water into the soil is controlled by gravity, capillary action, and soil
porosity. Of these factors soil porosity is most important. A soil's porosity is
controlled by its texture, structure, and organic content. Coarse textured soils
have larger pores and fissures than fine-grained soils and therefore allow for
more water flow. Pores and fissures found in soils can be made larger through a
number of factors that enhance internal soil structure. For example, the
burrowing of worms and other organisms and penetration of plant roots can
increase the size and number of macro and micro-channels within the soil.
The amount of decayed organic matter found at the soil surface can also
enhance infiltration. Organic matter is generally more porous than mineral soil
particles and can hold much greater quantities of water.
Diunduh dari:
http://www.physicalgeography.net/fundamentals/8l.html…… 11/11/2012
Infiltration and Soil Water Storage
Infiltration
The rate of infiltration normally declines rapidly during the early part of a
rainstorm event and reaches a constant value after several hours of rainfall.
A number of factors are responsible for this phenomena, including:
1. The filling of small pores on the soil surface with water reduces the ability of
capillary forces to actively move water into the soil.
2. As the soil moistens, the micelle structure of the clay particles absorb water
causing them to expand. This expansion reduces the size of soil pores.
3. Raindrop impact breaks large soil clumps into smaller particles. These
particles then clog soil surface pores reducing the movement of water into
the soil.
Diunduh dari:
http://www.physicalgeography.net/fundamentals/8l.html…… 11/11/2012
SIMPANAN LENGAS TANAH
Within the soil system, the storage of water is influenced by several different forces.
The strongest force is the molecular force of elements and compounds found on the
surface of soil minerals.
The water retained by this force is called hygroscopic water and it consists of the
water held within 0.0002 millimeters of the surface of soil particles. The maximum
limit of this water around a soil particle is known as the hygroscopic coefficient.
Hygroscopic water is essentially non-mobile and can only be removed from the soil
through heating.
Matric force holds soil water from 0.0002 to 0.06 millimeters from the surface of soil
particles. This force is due to two processes: soil particle surface molecular
attraction (adhesion and absorption) to water and the cohesion that water molecules
have to each other. This force declines in strength with distance from the soil
particle. The force becomes nonexistent past 0.06 millimeters.
Diunduh dari:
http://www.physicalgeography.net/fundamentals/8l.html…… 11/11/2012
SIMPANAN LENGAS TANAH
Capillary action moves this water from areas where the matric force is low
to areas where it is high. Because this water is primarily moved by capillary
action, scientists commonly refer to it as capillary water.
Plants can use most of this water by way of capillary action until the soil
wilting point is reached.
Water in excess of capillary and hygroscopic water is called gravitational
water.
Gravitational water is found beyond 0.06 millimeters from the surface of soil
particles and it moves freely under the effect of gravity.
When gravitational water has drained away the amount of water that
remains is called the soil's field capacity.
Diunduh dari:
http://www.physicalgeography.net/fundamentals/8l.html…… 11/11/2012
TEGANGAN AIR TANAH
The relationship between the
thickness of water film around
soil particles and the strength
of the force that holds this
water. Force is measured in
units called bars. One bar is
equal to a 1000 millibars.
The graph also displays the
location of hygroscopic water,
the hygroscopic coefficient,
the wilting point, capillary
water, field capacity, and
gravitational water along this
line.
Diunduh dari:
http://www.physicalgeography.net/fundamentals/8l.html…… 11/11/2012
Soil Water Dynamics
O'Geen, A. T. (2012) Soil Water Dynamics. Nature Education Knowledge 3(6):12.
Stored water in soil is a dynamic
property that changes spatially in
response to climate, topography
and soil properties, and temporally
as a result of differences between
utilization and redistribution via
subsurface flow.
Changes in soil moisture storage
can be generalized with a mass
balance equation , as a result of the
difference between the amount of
water added and that which is lost
(Hillel 1982).
Conceptual diagram of a soil profile
illustrating the multiple flow paths
through which water moves through
soil (Modified from O’Geen et al.
2010)
Diunduh dari:
http://www.nature.com/scitable/knowledge/library/soil-water-dynamics-59718900…… 11/11/2012
Change in soil moisture storage = inputs – outputs.
Water content increases (positive change in storage) when inputs including
precipitation or irrigation exceed outputs. Water content decreases (negative
change in storage) when outputs such as deep percolation, surface runoff,
subsurface lateral flow, and evapotranspiration (ET) exceed inputs.
Water storage and redistribution are a function of soil pore space and pore-size
distribution, which are governed by texture and structure. Generally speaking,
clay-rich soils have the largest pore space, hence the greatest total water
holding capacity. However, total water holding capacity does not describe how
much water is available to plants, or how freely water drains in soil. These
processes are governed by potential energy. Water is stored and redistributed
within soil in response to differences in potential energy. A potential energy
gradient dictates soil moisture redistribution and losses, where water moves
from areas of high- to low-potential energy (Hillel 1982).
When at or near saturation, soils typically display water potentials near 0 MPa.
Negative water potentials arise as soil dries resulting in suction or tension on
water allowing the soil to retain water like a sponge.
Diunduh dari:
…… 11/11/2012
.
Water storage and redistribution are a function of soil pore space and pore-size
distribution, which are governed by texture and structure.
Water content and water
potential at saturation,
field capacity and
permanent wilting point.
The difference in water
content between field
capacity and permanent
wilting point is plant
available water.
Drainable porosity is the
amount of water that
drains from macropores
by gravity between
saturation to field
capacity typically
representing three days
of drainage in the field.
Diunduh dari:
…… 11/11/2012
Influence of Texture and Structure.
Texture and structure determine pore size
distribution in soil, and therefore, the amount of
PAW.
Coarse textured soils (sands and loamy sands)
have low PAW because the pore size distribution
consists mainly of large pores with limited ability
to retain water. Although fine textured soils have
the highest total water storage capacity due to
large porosity values, a significant fraction of
water is held too strongly (strong matric
forces/low, negative water potentials) for plant
uptake.
Fine textured soils (clays, sandy clays and silty
clays) have moderate PAW because their pore
size distribution consists mainly of micropores.
Loamy textured soils (loams, sandy loams, silt
loams, silts, clay loams, sandy clay loams and
silty clay loams) have the highest PAW, because
these textural classes give rise to a wide range
in pore size distribution that results in an ideal
combination of meso- and micro-porosity.
Soil structure can increase PAW by increasing
porosity.
Diunduh dari: http://www.nature.com/scitable/knowledge/library/soil-water-dynamics-59718900 …… 11/11/2012
PERMEABILITAS TANAH
Soil structure is highly relevant to water management in soils because it is subject to change either
through deterioration by improper management, or to improvement through additions of soil
organic matter. In contrast, it is usually infeasible to change texture.
Permeability Class
Permeability
(cm/hr)
Textural class
Very slow
<0.13
clay
Slow
0.13–0.5
Moderately slow
Moderate
Moderately rapid
Rapid
Very Rapid
sandy clay, silty clay
0.05–2.0
clay loam, sandy clay
loam, silty clay loam
2.0–6.3
very fine sandy loam,
loam, silt loam, silty clay
loam, silt
6.3–12.7
sandy loam, fine sandy
loam
12.7–25.4
>25.4
sand, loamy sand
coarse sand
Diunduh dari: http://www.nature.com/scitable/knowledge/library/soil-water-dynamics-59718900 …… 11/11/2012
Total Soil Water Storage Capacity
The total soil water storage capacity refers to when all the soil pores or voids are filled
with water. This occurs when the soil is saturated or flooded.
A peat soil usually has the highest total soil water storage capacity of around 70 to 85%
by volume. Sands and gravels will have the lowest total porosity of around 30 to 40% by
volume.
Total porosity for silt soils ranges from 35 to 50%, and clay soils typically range from 40
to 60%.
Restricted drainage conditions can cause the soil to attain its total porosity water
content, at which time free water is observed and perched water tables develop (in
layered soils) or the apparent water table is found near the surface.
When the total soil water storage capacity is reached, air is pushed out of the pores or
void spaces and oxygen and other gaseous diffusion in the soil is severely restricted.
Most agricultural plants cannot tolerate this condition very long (usually no more than a
day or two) as plant root respiration requires some oxygen diffusion to the roots.
Without air-filled pores, the concentration of carbon dioxide and other gases like
ethylene increase, producing toxic conditions and limiting plant growth.
Diunduh dari:
http://nrcca.cals.cornell.edu/soil/CA2/CA0212.4.php…… 11/11/2012
Soil Water Storage.
Soil water storage is a function of the surface area of soil particles (i.e., particle-size) and the
amount of porosity occurring between these particles (i.e., soil structure). Soil pores occur across a
wide range of diameters and are often categorized as macropores (>60 µm) and micropores (<60
µm).
Water is present in macropores following precipitation events and is drained by the force of gravity.
after water has freely drained due to the force of gravity, the soil is at field capacity and has a soil
water potential generally between 0.01 to 0.03 mpa. water in macropores is not available for plant
use because it freely drains from the soil profile and is lost from the rooting zone. water held in very
small micropores (<0.2 µm) is held so tightly that plants are not able to extract if for use.
The permanent wilting point is the soil water
potential to which plants can effectively
utilize water and corresponds to a soil water
potential of approximately 1.5 mpa. thus, the
pores in the diameter range 0.2 to 60 µm are
the primary storage pores for plant available
water (i.e., water held between approximately
0.01 and 1.5 mpa). the distribution of pore
sizes is primarily a function of the soil texture
and structure. the amount of water storage as
a function of soil texture is illustrated in this
figure.
Diunduh dari:
http://lawr.ucdavis.edu/classes/ssc219/biogeo/sws.htm…… 11/11/2012
Soil Water and its Availability.
Figure 2.33 indicates the availability of soil
water. A soil is at saturation or near
saturation following a heavy irrigation or
rainfall in which most or all of the spaces
between soil particles are filled by water.
The force of gravity is greater than the
force with which soil particles hold water,
so between saturation and field capacity
(see below), water is free to drain through
the soil by the force of gravity.
Field capacity (FC) is the amount of
water that a soil can hold against drainage
by gravity.
Permanent wilting point (PWP) is the
moisture content in a soil at which plants
permanently wilt and will not recover.
Available water (AW) is the water content
that the soil can hold between field
capacity and wilting point.
Diunduh dari:
…… 11/11/2012
SIMPANAN LENGAS TANAH
The soil water storage or soil water content can be quantified on the basis of its
volumetric or gravimetric water content.
The volumetric water content is the volume of water per unit volume of soil,
expressed as a percentage of the volume.
The gravimetric water content is the mass of water per unit mass of dry (or wet)
soil.
The volumetric water content is equal to the gravimetric water content times the
soil's bulk density (on a dry soil basis).
Factors that affect the soil water storage are:
1. Total Porosity or Void Space
2. Pore-size and Distribution and Connectivity
3. Soil Water Pressure Potential or Energy Status of the Soil Water
Diunduh dari: http://nrcca.cals.cornell.edu/soil/CA2/CA0211.5.php …… 13/11/2012
SIMPANAN LENGAS TANAH
The total porosity or void space ultimately establishes
the upper limit of how much water can be stored in a
given volume of soil. When all the pores are filled with
water the soil is saturated, and cannot store any more
water. The total porosity is a function of the soil's
particle size, particle uniformity and packing or
structure because the void space that remains
between the solid particles determines the extent and
distribution of pore sizes and their connectivity.
If one fills the same volume with sand and clay sized
particles, the total porosity of the clay is somewhat
higher, about 50-55% of the volume compared to about
35-40% for sand. The spaces between the sand
particles will have larger voids, but there will be fewer
of them. The total porosity of medium textured loamy
soils is generally around 50% because the smaller silt
and clay particles fill some of the voids between the
larger sand particles. Soils with good structure will
have somewhat higher total porosity than soil that has
been compacted (i.e., where the soil particles are
forced closer together).
The important influence of pore-size
and distribution on soil water storage
is in regards to how different pore
sizes respond to energy forces or the
soil water pressure potential. Under
saturated conditions, large pores
drain more easily in response to
gravity potential. Also, when the soil
is unsaturated, large pores are less
subject to capillary (or matric
potential) forces.
In unsaturated soil conditions, the
soil water pressure potential
becomes negative (suction), and the
degree to which this occurs greatly
influences the soil water storage
(retention) or water content in
different sized pores.
Diunduh dari: http://nrcca.cals.cornell.edu/soil/CA2/CA0211.5.php …… 13/11/2012
SIMPANAN LENGAS TANAH.
The soil water characteristic
(retention) curve defines the
relationship between the soil water
pressure potential or energy status
(matric or suction potential) and the
soil water content.
It's important to note that soil water
moves in direct response to the
energy or pressure potential forces
acting upon it (i.e., moving from a
higher to lower energy status), and
not necessarily in response to
different soil moisture contents (i.e.,
from higher to lower soil moisture
content).
Diunduh dari: http://nrcca.cals.cornell.edu/soil/CA2/CA0211.5.php …… 13/11/2012
SIMPANAN LENGAS TANAH.
Sumber: http://www.tutorvista.com/content/biology/biology-i/naturalresources/water.php#
The soil water
characteristic curve(s) and
definitions are used to
establish and further refine
and quantify the general
availability of soil water
which is often referred to
as :
1. Gravitational water
(water subject to
drainage),
2. Capillary water (water
available to plants), and
3. Hygroscopic water
(water that is not
available to plants).
Diunduh dari: http://nrcca.cals.cornell.edu/soil/CA2/CA0211.5.php …… 13/11/2012
PERGERAKAN LENGAS TANAH.
Water movement is directly
related to the size of pores in
the soil. In the small pores of
clayey soils, water slowly
moves in all directions by
capillary action. The lack of
large pore space leads to
drainage problems and low soil
oxygen levels.
On sandy soils with large
pores, water readily drains
downwards by gravitational
pull. Excessive irrigation
and/or precipitation can leach
water-soluble nutrients, like
nitrogen, out of the root zone
and into ground water.
Diunduh dari: http://www.cmg.colostate.edu/gardennotes/213.html …… 13/11/2012
SIMPANAN LENGAS TANAH
The water table is defined as the
upper surface of groundwater
(saturated zone) or that level in the
ground below the soil surface where
the water is at (and in equilibrium
with) atmospheric pressure. At the
water table reference, the pressure
potential is set equal to zero.
Water infiltration through the soil-water unsaturated zone
and into the water table
Sumber:
http://iowacedarbasin.org/runoff/showMan.php?c1=2E-1
Thus, below the water table, the
pressure potential becomes positive,
and above the water table the
pressure potential becomes
negative. This negative pressure in
unsaturated soil is termed matric,
tension or suction pressure potential
so as not to confuse it with positive
pressures.
Diunduh dari:
http://nrcca.cals.cornell.edu/soil/CA2/CA0211.5.php
…… 13/11/2012
SOIL WATER STORAGE.
Available Water Capacity
The total available water (holding) capacity is the portion of water that can be absorbed
by plant roots. By definition it is the amount of water available, stored, or released
between field capacity and the permanent wilting point water contents.
The soil types with higher total available water content are generally more conducive to
high biomass productivity because they can supply adequate moisture to plants during
times when rainfall does not occur. Sandy soils are more prone to drought and will
quickly (within a few days) be depleted of their available water when evapotranspiration
rates are high. For example, for a plant growing on fine sand with most of its roots in the
top foot of soil, there is less than one inch of readily available water.
A plant transpiring at the rate of 0.25 inches per day will thus start showing stress
symptoms within four days if no rainfall occurs. Shallow rooted crops have limited
access to the available soil water, and so shallow rooted crops on sandy soils are
particularly vulnerable to drought periods. Irrigation may be needed and is generally
quite beneficial on soils with low available water capacity.
Diunduh dari: http://nrcca.cals.cornell.edu/soil/CA2/CA0211.5.php …… 13/11/2012
SOIL WATER STORAGE.
Soil Type
Total Available Water, %
Total Available Water, in/ft
coarse sand
5
0.6
fine sand
15
1.8
loamy sand
17
2.0
sandy loam
20
2.4
sandy clay loam
16
1.9
loam
32
3.8
silt loam
35
4.2
silty clay loam
20
2.4
clay loam
18
2.2
silty clay
22
2.6
clay
20
2.4
peat
50
6.0
Diunduh dari: http://nrcca.cals.cornell.edu/soil/CA2/CA0211.5.php …… 13/11/2012
SOIL WATER STORAGE.
Total Soil Water Storage Capacity
The total soil water storage capacity refers to when all the soil pores or voids
are filled with water. This occurs when the soil is saturated or flooded.
A peat soil usually has the highest total soil water storage capacity of around 70
to 85% by volume. Sands and gravels will have the lowest total porosity of
around 30 to 40% by volume.
Total porosity for silt soils ranges from 35 to 50%, and clay soils typically range
from 40 to 60%. Restricted drainage conditions can cause the soil to attain its
total porosity water content, at which time free water is observed and perched
water tables develop (in layered soils) or the apparent water table is found near
the surface.
When the total soil water storage capacity is reached, air is pushed out of the
pores or void spaces and oxygen and other gaseous diffusion in the soil is
severely restricted.
Diunduh dari: http://nrcca.cals.cornell.edu/soil/CA2/CA0211.5.php …… 13/11/2012
SOIL WATER STORAGE.
Total Soil Water Storage Capacity
Most agricultural plants cannot tolerate this condition very long (usually no
more than a day or two) as plant root respiration requires some oxygen
diffusion to the roots.
Without air-filled pores, the concentration of carbon dioxide and other gases
like ethylene increase, producing toxic conditions and limiting plant growth.
Root cells switch to anaerobic respiration, which is much less efficient than
aerobic respiration in converting glucose molecules to ATP (adenosine
triphosphate, the chemical energy within cells for metabolism and cell division).
Diunduh dari: http://nrcca.cals.cornell.edu/soil/CA2/CA0211.5.php …… 13/11/2012
SOIL WATER STORAGE.
Total Soil Water Storage Capacity
As anaerobic (reduced) conditions develop in the soil, nitrification ceases and
denitrification is enhanced.
Corn plants will quickly yellow in response to this saturated soil state as
nitrogen becomes limiting, and the plant tries to adjust by producing more
adventitious roots.
Prolonged anaerobic conditions in the soil starts to reduce manganese, iron
(causing phosphorus to be more soluble), sulfur (producing hydrogen sulfide),
and eventually methane gases.
Hydrophytic (wetland type) plants are adapted to saturated soils because they
are able to obtain oxygen through other forms of plant structure adaptations
(i.e. pneumataphores, lenticels, aerenchyma).
Diunduh dari: http://nrcca.cals.cornell.edu/soil/CA2/CA0211.5.php …… 13/11/2012
.
SOIL WATER STORAGE.
Drainable porosity is the amount of water that drains
from macropores by gravity between saturation to
field capacity typically representing three days of
drainage in the field.
Drainable Porosity
The drainable porosity is the pore
volume of water that is removed (or
added) when the water table is
lowered (or raised) in response to
gravity and in the absence of
evaporation.
Consider a soil that is saturated with
the water table at the surface.
If this soil has a subsurface drainage
pipe (tile) buried several feet down
and it is discharging to the
atmosphere at some lower elevation,
the drainable porosity water content
will be released to the tile drain until
the water table is lowered to the depth
of the drain.
Diunduh dari:
http://www.nature.com/scitable/knowledge/library/soil-waterdynamics-59718900…… 13/11/2012
Diunduh dari:
http://nrcca.cals.cornell.edu/soil/CA2/CA0211.5.php
…… 13/11/2012
SOIL WATER STORAGE.
Drainable Porosity
Any nutrients or pesticides dissolved or suspended in this readily drainable
pore space will also be carried along with this water, either flowing to the tile
drain or continuing downward to the water table via deep percolation if no
drainage restriction exists.
In large pores, nutrients that might otherwise adsorb to the soil particles
(ammonium or phosphate) will bypass the soil because of limited time for
contact and chemical reactions to occur with the soil surface area.
Soils with a wide range of different pore sizes (sandy loams) or soils with mostly
small sized pores are better at filtering nutrients and pesticides as they leach
through the soil profile.
Diunduh dari: http://nrcca.cals.cornell.edu/soil/CA2/CA0211.5.php …… 13/11/2012
SOIL WATER STORAGE.
Drainable Porosity
The variability of drainable porosity with soil texture
and structure
Soil Texture
Field
Capacity (%
by vol.)
Wilting Point
(% by vol.)
Drainable
Porosity (%
by vol.)
clays, clay
loams, silty
clays
30-50%
15-24%
3-11%
well
structured
loams
20-30%
8-17%
10-15%
sandy
10-30%
3-10%
18-35%
Diunduh dari:
http://www.extension.umn.edu/distribution/cropsystems/dc7644.ht
ml ………..13/11/2012
The combined aspect of low
available water holding
capacity and high drainable
porosity for sandy soils causes
these soils to have a high
leaching potential.
It will not take much rain or
irrigation (or application of
liquid manure) to replenish the
available soil water and to raise
the soil water content to a
drainable state.
Applying the proper amount
(depth) of irrigation to these
soils will both conserve water
and enhance irrigation and
nutrient use efficiency.
Diunduh dari:
http://nrcca.cals.cornell.edu/soil/CA2/CA021
1.5.php …… 13/11/2012
SOIL WATER STORAGE.
The soil texture and
structure fundamentally
determines the number
and sizes of soil pores,
which will influence the
fate and transport of air
(gas) and water
exchange.
The figure provides an
illustration of how
various parameters of
soil water storage may
be influenced by different
texture and structure
aspects
(From H.M. van Es).
Diunduh dari: http://nrcca.cals.cornell.edu/soil/CA2/CA0211.5.php …… 13/11/2012
SOIL POROSITY
Saturation:
all soil pores are filled
Gravitational water:
drainable water 0 to -33 kPa.
Field capacity
(after 1-2 days of drainage)
-33 kPa, usually 1/2 of saturation water.
Permanent wilting point:
-1500 kPa
Plant available water:
difference in water% at FC and PWP.
Diunduh dari:
http://passnano.com/8.html…… 13/11/2012
SOIL PORE
A well aggregated soil has a range of
pore sizes. This medium size soil
crumb is made up of many smaller
ones. Very large pores occur
between the medium size
aggregates.
The ideal soil can be found in a well
aggregated medium-textured loam
soil.
Such a soil has enough large pore
spaces between the aggregates to
provide adequate drainage and
aeration during wet periods, but also
has adequate amounts of small
pores and water-holding capacity to
provide sufficient water to plants and
soil organisms between rainfall or
irrigation events.
Diunduh dari:
http://saret.ifas.ufl.edu/publications/bsbc/chap6.htm…… 11/11/2012
Types of soil pores
1. Macropores (d>0.08mm) occur
between aggregates (interped
pores) or individual grains in
coarse textured soil (packing
pores) and may be formed by soil
organisms (biopores). They allow
ready movement of air and the
drainage of water and provide
space for roots and organisms to
inhabit the soil.
Pore space can be filled with either water or air.
The volume of soil water and soil air, however,
cannot exceed the total soil porosity. As soil
water increases, soil air must decrease and vice
versa.
2. Micropores (d<0.08mm) occur
within aggregates. They are
usually filled with water and are too
small to allow much movement of
air. Water movement in micropores
is extremely slow and much of the
water held by them is unavailable
to plants.
Diunduh dari: http://www.landfood.ubc.ca/soil200/interaction/water_air.htm …… 13/11/2012
SOIL STRUCTURE
Only about 50% of soil is solid material. The remainder is
pore space.
Small pores within the aggregates provide storage and
refuge. The larger pores (and fissures) between the
aggregates are the pathways for liquids, gases, roots and
organisms.
Diunduh dari:
Soil structure is the
arrangement of pores and
fissures (porosity) within a
matrix of solid materials (soil
particles and organic matter).
The solid materials bond and
aggregate to give the pores
and fissures.
The quantity, distribution and
arrangement of pores
determines water holding
capacity, infiltration,
permeability, root
penetration, and, respiration.
http://vro.dpi.vic.gov.au/dpi/vro/vrosite.nsf/pages/soilhealth_soil_structure…… 13/11/2012
SOIL POROSITY.
(A) Porosity of cultivated structured soil (schematic):
1--thin, predominantly capillary pores in
aggregates, which fill with water on wetting; 2-medium-sized pores (cells, channels), upon wetting
they will fill with water for a short period and
subsequently, after the resorption of water, with air;
3--capillary pores; 4--large pores, between
aggregates almost always filled with air (according
to Kschinakii, 1956);
(B) Visible porosity of soil aggregate (reproduction
from a microsection). Thin chernozem (southern):
1--micro-aggregates; 2--visible pores (according to
Kachinskii et al., 1950).
The total porosity is determined according to its volume and
specific gravity. It is determined by the following formula:
P = (1 - vol/sp) x 100
where P--porosity, sp--the specific gravity of the solid phase,
vol--weight by volume.
Diunduh dari:
http://www.soilandhealth.org/01aglibrary/010112krasil/010112krasil.ptII.html…… 11/11/2012
SOIL AGGREGATION
Soil management practices that promote good soil structure include:
Minimizing tillage; Timing tillage for optimum moisture conditions (approximately field capacity
or a little drier); Maintaining plant litter/residues on the soil surface; Incorporating a constant
supply of decomposable organic material; Using sod crops whenever possible; Using green
manure and cover crops whenever possible; Applying gypsum and soil conditioners
Diunduh dari: http://faculty.yc.edu/ycfaculty/ags105/week05/soil_physical_properties/soil_physical_properties_print.html …… 13/11/2012
AGGREGATE STABILITY
Relationship between
aggregate stability
and soil organic
matter in some
selected soils from
the Cornell
University research
sites in NY.
Diunduh dari: http://ipmguidelines.org/FieldCrops/Chapters/CH02/CH02-5.aspx …… 13/11/2012
SOIL POROSITY.
Porosity refers to the amount of space between the solid soil particles.
Pore space can be filled with either water or air.
Smaller pores tend to be filled with water.
The amount of water-filled pores is referred to as capillary porosity. Large
pores are typically filled with air.
These air-filled pores are referred to as non-capillary porosity.
When a growing media has approximately equal amounts of water-filled
and air-filled pore space, the soil is said to have balanced porosity.
Soils text books indicate that growing media with balanced porosity
provide an ideal environment where beneficial microbes, nutrients, water,
and air can interact and thrive.
This provides advantageous conditions for desirable soil gas exchange,
good mineral/water holding, vigorous root growth, and healthy plants.
Diunduh dari:
http://www.turfdiag.com/porosity.htm…… 13/11/2012
SOIL POROSITY.
A vertical cross sectional view
of a highly structured soil. The
largest soil units shown are
macroaggregates (~ 2 mm
diameter).
They are composed of
microaggregates (~ 0.1 mm)
and sand grains, as shown in
the center left of the
macropore.
Four hierarchical classes of
soil pore space are illustrated:
(1) macropore, (2)
intermacroaggregate, (3)
intermicroaggregate (includes
intramacroaggregate space,
see arrow) and (4)
tramicroaggregate space.
(Illustration by S. L. Rose).
Diunduh dari:
http://www.certifiedorganic.bc.ca/rcbtoa/training/soil-article.html…… 13/11/2012
SOIL POROSITY.
Water is retained in many of
these pores when the soil is at
field capacity and pore space
is large enough to be inhabited
by nematodes.
The pores between
microaggregates but within
macroaggregates are large
enough to accommodate small
nematodes and protozoa and
may be the chief habitat of
fungi.
The smallest class of pores,
those within microaggregates,
may be only about 1 mm,
maximally, and may be
inhabited mostly by bacteria
(Kilbertus 1980).
Diunduh dari:
http://www.certifiedorganic.bc.ca/rcbtoa/training/soil-article.html…… 13/11/2012
Soil Pore spaces
Soil particles do not fit together
snugly. There are spaces between
particles. These spaces are called
pore spaces and contain water and
air.
The pore spaces provide the route for
the downward movement of water and
allow roots to grow into them. They
also provide air space, which is
essential for plant growth.
The larger the pore spaces the better
the drainage of water and the less
water retained in the soil. Conversely,
the smaller the pore spaces the less
water drains away and the more water
is retained in the soil.
Diunduh dari:
http://www.swtafe.vic.edu.au/toolbox/turf/html/pages/office/grass_roots/soil_structure.html…… 13/11/2012
Water holding capacity.
After a soil has been completely soaked by a downpour of rain all its pore spaces
are filled with water and it is regarded as being saturated. Any air in the pore spaces
is forced out and the soil is said to be waterlogged.
Once the rain stops and the water has a chance to drain away, the total amount of
water that remains in the soil, against the force of gravity, is called the field capacity
of the soil. Due to capillary forces, water will drain away from the large pore spaces
first and remain in some of the smaller pore spaces. So, at field capacity, the larger
pores will contain air whilst the smaller pores hold water.
Plant roots in the soil are able to suck the water out of many of the small pores.
However, eventually water in the small pore spaces is held too tightly by capillary
forces for the roots to take it up. This stage, when the turf plants are unable to
extract any more water from the soil and begin to wilt, is called the permanent
wilting point.
Due to differences in particle and pore space size, soils differ in their field capacity
and permanent wilting point.
Have a look at the different water holding capacities of sand, loam and clay.
Diunduh dari:
http://www.swtafe.vic.edu.au/toolbox/turf/html/pages/office/grass_roots/soil_structure.html…… 13/11/2012
What is water holding capacity of soil?.
Water holding capacity of soil is just that, the specific ability of a particular
type of soil to hold water against the force of gravity.
Different types of soils have difference capacities, for example a sand soil
had a lower capacity to hold water when compared to a clay soil. The
nature of the soil, composition of the soil, amount of organic component
and size of the soil particles determine its ability to retain water. Water
molecules are held closely to the individual soil particles by forces of
cohesion.
The maximum amount of water a soil can hold before it is saturated and
starts to loose water by gravity is known as "field capacity“.
Diunduh dari: http://wiki.answers.com/Q/What_is_water_holding_capacity_of_soil …… 13/11/2012
A plant's available water holding capacity for soils with different textures.
The texture of a soil is
important for soil water
availability because it
controls not only how
well a soil can hold water
but also how well water
is absorbed into the soil.
Any water that infiltrates
into a soil does so
primarily through large
pores in the soil called
“macropores” that are
created by plant roots,
microorganisms, and
physical processes such
as freezing and thawing
and drying and wetting.
Diunduh dari: http://www.extension.org/pages/33617/soils-and-water-availability …… 13/11/2012
WHC = WATER HOLDING CAPACITY
General relationship between
soil texture and available
water-holding capacity. As
clays increase in a soil, so
does water-holding capacity.
Typically, clay loam soils hold
more than twice as much water
as sandy textured soils.
The presence of humus in
topsoil increases waterholding capacity of loams and
sandy loams at a rate of 2.25%
water to each percent rise in
soil humus (Jenny 1980) which
equates to approximately
0.75% increase in water for
every 1% increase in organic
matter.
Diunduh dari: http://www.nativerevegetation.org/learn/manual/ch_5.aspx …… 13/11/2012
Mitigating for Textures with Low Water-Holding Capacities
Organic Amendments — Incorporation of organic amendments (e.g.,
compost) can increase the water-holding capacity of a soil.
Because the water-holding capacity of each type of organic matter varies
by composition and degree of weathering, the effect on soil waterholding capacity by any organic matter being considered must be
assessed prior to application.
Sandy textured soils benefit most from organic matter additions,
especially those with plant available water of 9% or less (Claassen 2006),
which are typically sands, loamy sands, and sandy loam soils.
Testing several different rates of incorporated organic matter on soil
moisture-holding capacity should be done to prior to selecting the
source and the amount of material to apply.
Diunduh dari: http://www.nativerevegetation.org/learn/manual/ch_5.aspx …… 13/11/2012
Mitigating for Textures with Low Water-Holding Capacities
Clay — The water-holding capacity of sandy textured
soils can be increased by incorporating clay loam, sandy
clay loam, and silty clay loam textures in the soil.
The addition of clays should be at rates that result in new
soil textures similar to loams, silt loams, or sandy clay
loams.
Higher rates of clay addition are not recommended. It is
always important to test the additions of any soil to
another to understand what the effects on water-holding
capacity and structure might be.
Ideally this should be in the field in small plots.
Diunduh dari:
…… 13/11/2012
Soil moisture retention curve
The soil moisture retention curve
(pF curve) gives the relation
between soil moisture suction and
soil moisture content.
A soil is at F.C. (field capacity) or
has a pF-value of 2, some 2 to 3
days the soil has been saturated by
rainfall or irrigation.
When the soil becomes dry and
plants cannot take up water
anymore the soil is at W.P (wilting
point) or has a pF=4.2.
The amount of water held by a soil
in the root zone between F.C. and
W.P. and which can be used by
plants is described as available
water. (F.C.- W.P.= available water)
For sand, loam and clay the values
are 6, 20 and 17 volume percent
respectively.
Diunduh dari: http://www.tankonyvtar.hu/hu/tartalom/tamop425/0032_talajtan/ch07s05.html …… 13/11/2012
Plant available water.
Plant available water is
considered the amount of
water held between field
capacity and the
permanent wilting point.
Limitations to the concept
of plant available water:
1. Roots are not distributed
throughout the profile.
2. Water content is not
uniform throughout the
profile.
3. Not all plants have the
same wilting point.
4. Water may not be as
‘easily’ obtained (i.e.
available) as the soil dries
andpotential decreases
toward the permanent
wilting point.
Diunduh dari: http://www.tankonyvtar.hu/hu/tartalom/tamop425/0032_talajtan/ch07s05.html …… 13/11/2012
SOIL
AGGREGATION
Struktur Tanah & Agregasi
Soil may be a loose assemblage of individual and random
particles, or consist of distinctly structured aggregates of
distinctive size and shape; the particular arrangement of
which is called soil structure.
Most methods of measurement are indirect, and measure
various properties that are dependent or at the least
influenced by specific structural properties; e.g., total
porosity, pore size distribution, liquid retention/
transmission, and infiltration.
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Struktur Tanah & Agregasi
Soils may be non-structured (e.g., single grain or massive) or
consist of naturally formed units known as peds or aggregates.
The initial stage in the formation of soil structure is the
process of flocculation.
Individual colloids typically exhibit a net negative charge which
results in an electrostatic repulsion.
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Struktur Tanah & Agregasi
Reduction of the forces of electrostatic repulsion allows the
particles to come closer together.
Flocculation
This process allows other forces of attraction to become more
dominant. The formation of these “flocs” in suspension represents the
early stages of aggregation.
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Struktur Tanah & Agregasi
As this process continues, the flocs become larger and larger forming the
more refined structural units.
On their own, these units are pretty fragile and the process
is easily reversed. But in the presence of natural or
artificial binding become more strongly cemented together
forming stable soil aggregates.
Bahan perekat (pengikat) dapat berupa :
Inorganic – Fe & Al oxides, carbonates,
amorphous gels and sols; or
Organic – polysaccharides, hemicellulose, and
other natural or manufactured organic polymers.
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Struktur Tanah & Agregasi
The arrangement or organization of individual soil particles (soil
separates) into a specific configuration is called “soil structure”. Soil
structure is developed over a geologic time frame, is (or can be)
naturally fragile, and is affected by changes in climate, vegetation,
biological activity, and anthropogenic manipulation.
Soil structure influences the mechanical properties of soil such as
stability, porosity and compaction, as well as plant growth,
hydrologic function, and erosion.
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Struktur Tanah & Agregasi
There are three broad categories of soil structure; single grained,
massive, and aggregated.
When particles are entirely unattached the structure is
completely loose and such soils are labeled single grained.
When packed into large cohesive blocks the structure is called
massive.
Neither have any visible structural characteristics.
Between these two extremes particles are present as
aggregates or peds.
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The observable shapes of soil structure in the field are classified
as:
Platy: Horizontally layered, thin, flat aggregates similar to
wafers.
Spherical: Rounded aggregates generally < 2.0 cm in diameter
that are often found in loose condition called “granules or
crumbs”.
Blocky: Cube-like blocks, sometimes angular with well-defined
sharp faces or sub-angular with rounded faces up to 10cm in
size.
Columnar or Prismatic: Vertically oriented pillars up to 15cm in
diameter.
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Platy and spherical soil structure is common to the surface soil horizons, blocky
and columnar/prismatic are associated with the deeper subsurface soil horizons
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Struktur Tanah & Agregasi
• Non-Structured
– Single Grain
• Structured
– Platy: horizontal & flat
– Spherical (Grannular):
rounded and <2.0 cm
– Blocky: cubes up to 10 cm
that are angular (sharp
edges) or subangular
(rounded)
– Prismatic (Columnar):
longer than wide, often 6
sided, sharp or rounded, <
15 cm
– Massive
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Struktur Tanah & Agregasi
Aggregate size distribution also influences the pore
size distribution.
Macropores: Inter-aggregate cavities that
influence infiltration, drainage, and aeration.
Micropores: Intra-aggregate capillaries
important to water and solute retention.
Mesopore: Inbetween.
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DISTRIBUSI UKURAN AGREGAT
Similar to particle size distribution, the aggregate size
distribution also is determined by sieving.
An index known as the Mean Weight Diameter (X) based on the
size and weight distribution of aggregates is derived by
weighing the mass of aggregates within the respective size
classes, and characterizing the overall size distribution.
(MWD) X = ∑ xiwi
xi = mean diameter
wi = dry mass fraction
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Stabilitas Agregat
Since aggregation and stability is time
dependent, another useful characterization is that
of “aggregate stability”.
Aggregate stability expresses the resistance of individual soil aggregates to
disruptive forces such as mechanical, wind, and water erosion;
freezing/thawing; wetting/drying; and air entrapment.
The level of stability is assessed by determining the fraction of the original
aggregate mass which has withstood disruptive forces. The laboratory
approach uses wetting (misting and/or from bottom up with de-aired water)
followed by sieving.
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TIPE-TIPE STRUKTUR TANAH
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STRUKTUR TANAH
10 structure = soil particles + organic matter (humus) + roots +
microorganisms
20 structure = aggregate or ped = stability
Humic-like substances secretion
hydrophobic region
Bacterial colonies
Polysaccharide secretion - hydrophobic region
binding of clay particles
Polysaccharide secretion
binding of clay particles
Fungi
Soil aggregate
Physical entanglement
Cross-section
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AGREGAT TANAH
Clay Particles
C
Polysaccharide secretion
Cell wall
Fissure
Fungal hyphae
MicroNon perturbed clay
environment =
oriented, packed
and glued clay
1 micron
Soil aggregates are formed and stabilized by clay-organic
complexes, microbial polysaccharides, fungal hyphae and plant
roots.
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AGREGAT TANAH
Soil aggregates are associated with relatively large inter-aggregate pore spaces that
range from um to mm in diameter. Each aggregate also has intra-aggregate pore
spaces that are very small, ranging from nm to um in diameter.
Interaggregate pore space (m to mm in size)
Intraaggregate pore space
(nm to  m in size)
Enlargement
Aggregate
particle
Intra-aggregate pores can exclude bacteria (called micropore exclusion). However, after a
spill, contaminants can slowly diffuse into these pores. This creates a long-term sink of
pollution as the contaminants will slowly diffuse out again.
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Berapa banyak “Pori” dalam tanah?
Assume a soil aggregate that is 2 x 2 x 2
mm. Further assume that the volume of the
aggregate is 50% pore space. How many
pores of diameter 15 um does the aggregate
have? How many pores of 50 um?
2 mm
(the volume of a sphere is: 4/3π r3)
2 mm
2 mm
Calculation for 15 um pores:
The volume of the aggregate is 2 mm x 2 mm x 2 mm = 8 mm3
Pore space is 50% of 8 mm3 = 4 mm3
A pore of 15 um diameter has volume = 4/3 π (7.5 um)3 = 1.77 x 103 um3
4 mm3 (1000 um)3 / 1.77 x 103 um3 = 2.3 x 10 6 pores of 15 um per aggregate!
mm3
pore
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Where are the bacteria?
In soil 80 to 90% of the bacteria are attached to surfaces and only 10-20% are
planktonic. Cells have a patchy distribution over the solid surfaces, growing in
microcolonies. Colony growth allows sharing of nutrients and helps protect against
dessication and predation or grazing by protozoa.
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Pergerakan dan Potensial Lengas Tanah
Increasing distance from particle surface
m
A
Soil water potential depends on
how tightly water is held to a soil
surface. This in turn depends on
how much water is present.
Capillary forces have water
potentials ranging from –31 to –
0.1 atm. Optimal microbial
activity occurs at approximately 0.1 atm.
At greater distances there is little
force holding water to the surface.
This is considered free water and
moves downward due to the force
of gravity.
Capillary forces
Soil particles
Surface forces have water
potentials ranging from –10,000
to –31 atm.
Surface forces
Soil air
FREE WATER
Gravitational
forces
Soil air
0%
% Saturation of
the soil pore
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100%
Estimating effects of compaction on pore size distribution of soil aggregates by mercury
porosimeter
J. Lipiec, M. Hajnos, R. Świeboda. Geoderma. Volumes 179–180, June 2012, Pages 20–27
The aim of this study was to describe quantitatively the effect of vehicular traffic on pore
size distribution (PSD) of topsoil (0.05–0.15 m) and subsoil (0.25–0.35 m) aggregates
(3 mm and 8 mm) of silty loam.
The total aggregate porosity, average pore radius and volume of larger pores, > 1–3 μm
at 0.05–0.15 m depth, and > 0.3–0.4 μm at 0.25–0.35 m decreased with increasing soil
compaction, mostly from NC to MC. At 0.25–0.35 m depth this decrease was
accompanied by an increase in the volume of smaller pores (< 0.3 μm) mostly from MC to
SC. As a consequence, the volume of pores retaining plant available water (0.1–15 μm
radius) decreased in compacted soil.
The differential pore curves exhibited peaks at the pore throat radius of 1–6 μm. At 0.05–
0.15 m depth the peaks under SC were lower than under NC and MC, whereas at 0.25–
0.35 m depth they were lower under MC and SC than NC.
At all compaction treatments and aggregate fractions the volume of larger pores > 1–
3 μm was greater at 0.05–0.15 m depth than at 0.25–0.35 m depth and the inverse was
true for smaller pores (< 0.3 μm). The observed changes in pore size distribution in the
subsoil are considered as almost irreversible and thus long-lasting or even permanent.
Diunduh dari:
http://www.sciencedirect.com/science/article/pii/S0016706112000900…… 13/11/2012
Estimating effects of compaction on pore size distribution of soil aggregates by mercury
porosimeter
J. Lipiec, M. Hajnos, R. Świeboda. Geoderma. Volumes 179–180, June 2012, Pages 20–27
Cumulative
curve of pore
volume vs.
equivalent pore
radius of 3–
5 mm
aggregates.
Diunduh dari:
http://www.sciencedirect.com/science/article/pii/S0016706112000900…… 13/11/2012
Estimating effects of compaction on pore size distribution of soil aggregates by mercury
porosimeter
J. Lipiec, M. Hajnos, R. Świeboda. Geoderma. Volumes 179–180, June 2012, Pages 20–27
Cumulative
curve of
pore
volume vs.
equivalent
pore radius
of 8–10 mm
aggregates.
Diunduh dari:
http://www.sciencedirect.com/science/article/pii/S0016706112000900…… 13/11/2012
Estimating effects of compaction on pore size distribution of soil aggregates by mercury
porosimeter
J. Lipiec, M. Hajnos, R. Świeboda. Geoderma. Volumes 179–180, June 2012, Pages 20–27
Differential
curve of
pore
volume vs.
equivalent
pore radius
of 3–5 mm
aggregates.
Diunduh dari:
http://www.sciencedirect.com/science/article/pii/S0016706112000900…… 13/11/2012
Crop-Pasture Rotation for Sustaining the Quality and Productivity of a Typic
Argiudoll
Guillermo A. Studdert, Hernan E. Echeverría, Elda M. Casanovas. 2007.
Aggregate Stability Index
The relationship between SOC and
ASI. It can be seen that ASI values
under cropping were relatively
constant across all the explored
range of SOC (29.6-37.7 g kg-1).
On the other hand, ASI increased
with increases in SOC (r = 0.64, P <
0.01) when periods under pasture
were analyzed.
These results agree with the fact
that aggregate stability and SOC
are closely related (Greenland,
1981; Oades, 1984).
It appears that SOC was not
enough by itself to explain ASI
variations because ASI values were
different at the same SOC under
cropping or under pasture,
respectively.
Diunduh dari: http://en.engormix.com/MA-agriculture/articles/croppasture-rotation-sustaining-quality_475.htm ……
13/11/2012
GEOLISTRIK
PENDUGAAN
LENGAS TANAH
GEOLISTRIK.
Geolistrik merupakan metoda geofisik yang mempelajari sifat aliran listrik di
dalam bumi dan bagaiman cara mendeteksinya di permukaan bumi.
Dalam hal ini meliputi pengukuran potensial, arus dan medan elektromagnetik
yang terjadi baik secara alamiah ataupun akibat injeksi arus ke dalam bumi.
Ada beberapa macam metoda geolistrik, antara lain : metoda potensial diri, arus
telluric, magnetotelluric, IP (Induced Polarization), resistivitas (tahanan jenis)
dan lainnya.
Penyelidikan air tanah dilakukan untuk memperkirakan tempat terjadinya air
tanah, kedalaman muka pembentukan (kerikil, pasir, dan lain-lain), serta
ciri-ciri fisik air tanah (suhu, kerapatan, dll).
Penyelidikan air tanah dapat dilakukan dari permukaan tanah maupun dari
bawah permukaan tanah.
Penyelidikan air tanah yang biasa dilakukan dari permukaan tanah adalah
dengan menggunakan metode Geolistrik.
Sumber:
Penggunaan Metode Geolistrik Untuk Mendeteksi Keberadaan Air Tanah . Eva Rolia . TAPAK Vol. 1 No. 1
PENDUGAAN GEOLISTRIK.
Geolistrik merupakan salah satu metode geofisika yang mempelajari sifat aliran
listrik di dalam bumi dan untuk mengetahui perubahan tahanan jenis lapisan
batuan di bawah permukaan tanah dengan cara mengalirkan arus listrik DC (direct
current) yang mempunyai tegangan tinggi ke dalam tanah. Metode ini lebih
efektif jika digunakan untuk eksplorasi yang sifatnya dangkal, contohnya
penentuan kedalaman batuan dasar, pencarian reservoir air, dan juga digunakan
dalam eksplorasi geothermal.
Tujuan survey geolistrik tahanan jenis adalah untuk mengetahui resistivitas bawah
permukaan bumi dengan melakukan pengukuran di permukaan bumi. Resistivitas bumi
berhubungan dengan mineral, kandungan fluida dan derajat saturasi air dalam batuan.
Metode yang bisa digunakan pada pengukuran resistivitas secara umum yaitu dengan
menggunakan dua elektroda arus (C1 dan C2), dan pengukuran beda potensial dengan
menggunakan dua elektroda tegangan (P1 dan P2), dari besarnya arus dan beda
potensial yang terukur maka nilai resistivitas dapat dihitung menggunakan persamaan:
Dengan k adalah faktor geometri yang tergantung penempatan elektroda
permukaan.
Sumber:
Penggunaan Metode Geolistrik Untuk Mendeteksi Keberadaan Air Tanah . Eva Rolia . TAPAK Vol. 1 No. 1
Metode Geolistrik Resistivitas
Pendugaan potensi air tanah menggunakan Metode Geolistrik untuk mengetahui nilai resistivitas
batuan dan menentukan potensi atau kandungan air tanah. Dari nilai resistivitas batuan dan potensi
atau kandungan air tanah, maka akan diketahui adanya air tanah di Suatu lokasi.
Metode geolistrik resistivitas atau tahanan
jenis adalah salah satu dari kelompok metode
geolistrik yang digunakan untuk mempelajari
keadaan bawah permukaan dengan cara
mempelajari sifat aliran listrik di dalam
batuan di bawah permukaan bumi. Metode
resistivitas umumnya digunakan untuk
eksplorasi dangkal, sekitar 300 – 500 m.
Prinsip dalam metode ini yaitu arus listrik
diinjeksikan ke alam bumi melalui dua
elektrode arus, sedangkan beda potensial
yang terjadi diukur melalui dua elektrode
potensial. Dari hasil pengukuran arus dan
beda potensial listrik dapat diperoleh variasi
harga resistivitas listrik pada lapisan di
bawah titik ukur.
Diunduh dari: http://trisusantosetiawan.wordpress.com/2011/01/04/metode-geolistrik-resistivitas/ …… 13/11/2012
GEOLISTRIK.
1. Geolistrik merupakan metode geofisika yang cukup efektif
untuk digunakan dalam mendeteksi keberadaan air tanah
dengan memanfaatkan sufat batuan yang mampu mengalirkan
arus listrik.
2. Geolistrik merupakan alat alternatif yang dapat digunakan
dalam kegiatan teknik sipil untuk mengetahui lapisan tanah di
dalam bumi, selain dengan menggunakan metode hand bor,
sondir, dan metode lain dalam ilmu teknik sipil.
3. Geolistrik memiliki cara kerja yang efisien karena mudah
dioperasikan, mudah dibawa, murah, dan akurasi data yang
dapat diandalkan.
Sumber:
Penggunaan Metode Geolistrik Untuk Mendeteksi Keberadaan Air Tanah . Eva Rolia . TAPAK Vol. 1 No. 1
METODE GEOLISTRIK
Metoda geolistrik adalah salah satu metoda
geofisika yg didasarkan pada penerapan
konsep kelistrikan pada masalah kebumian.
Tujuannya adalah untuk memperkirakan sifat
kelistrikan medium atau formasi batuan
bawah-permukaan terutama kemampuannya
untuk menghantarkan atau menghambat
listrik (konduktivitas atau resistivitas).
Aliran listrik pada suatu formasi batuan terjadi
terutama karena adanya fluida elektrolit pada
pori-pori atau rekahan batuan. Oleh karena itu
resistivitas suatu formasi batuan bergantung
pada porositas batuan serta jenis fluida
pengisi pori-pori batuan tsb. Batuan porous
yg berisi air atau air asin tentu lebih konduktif
(resistivitas-nya rendah) dibanding batuan yg
sama yg pori-porinya hanya berisi udara
(kosong).
Diunduh dari:
http://geofisikaonline.blogspot.com/2009/05/metoda-geolistrik.html…… 13/11/2012
METODE GEOLISTRIK
Temperatur tinggi akan lebih menurunkan
resitivitas batuan secara keseluruhan karena
meningkatnya mobilitas ion-ion penghantar
muatan listrik pada fluida yg bersifat elektrolit.
Cara kerja metoda geolistrik secara sederhana
dapat dianalogikan dengan rangkaian listrik.
Jika arus dari suatu sumber dialirkan ke suatu
beban listrik (misalkan kawat seperti terlihat
pada gambar) maka besarnya resistansi R
dapat diperkirakan berdasarkan besarnya
potensial sumber dan besarnya arus yg
mengalir.
Dalam hal ini besaran resistansi tidak dapat
digunakan untuk memperkirakan jenis material
karena masih bergantung ukuran atau
geometri-nya. Untuk itu digunakan besaran
resistivitas yg merupakan resistansi yg telah
dinormalisasi terhadap geometri.
Diunduh dari:
Dalam prakteknya pengukuran
geolistrik dilakukan dengan
mengalirkan arus ke dalam tanah
melalui 2 elektroda (C1 dan C2)
dan respons-nya (beda
potensial) diukur melalui 2
elektroda yg lain (P1 dan P2).
Berdasarkan konfigurasi
elektroda dan respons yg terukur
maka sifat kelistrikan medium
bawah-permukaan tersebut
dapat diperkirakan.
http://geofisikaonline.blogspot.com/2009/05/metoda-geolistrik.html…… 13/11/2012