AIR ,TANAH
&
TANAMAN
1
Proses fotosintesis memerlukan air
2
CO2 dari Udara
Fotosintesis:
CO2 + H2O ---- Karbohidrat
(Glukosa)
Glukosa
Pati
dan senyawa organik lain dalam buah
dan biji
Air dari tanah
3
CO2 dari Udara
Fotosintesis:
CO2 + H2O
Karbohidrat
(Glukosa)
Glukosa
Pati
dan senyawa organik lain
dalam biji
Stomata:
Pintu lalulintas CO2,
O2, dan H2O
Air dari tanah
4
Budidaya
tanaman padi sawah
memerlukan banyak air
5
KEBUTUHAN AIR
TANAMAN
A plant has different
water needs at different
stages of growth. While
a plant is young it
requires less water than
when it is in the
reproductive stage.
Kurva Penggunaan Air Musiman
oleh Tanaman
When the plant
approaches maturity, its
water need drops.
Curves have been
developed that show the
daily water needs for
most types of crops.
6
Komposisi tanah menurut volume
Tanah subur yg ideal:
• Mineral 45%
• Organic matter 5%
• Water 25%
• Air
25%
7
Tiga komponen tanah
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.
8
Plants develop the tension, or
potential, to move soil water
from the soil into
the roots and distribute the
water through the plant by
adjusting the water potential,
or tension, within their plant
cells.
The essence of the process is
that water always moves
from higher to lower water
potential.
For water to move from the
soil, to roots, to stems, to
leaves, to air the water
potential must always be
decreasing.
Ilustrasi tentang penurunan potensial air
untuk suatu tanaman
9
HUB. TanahAir-Tanaman
Hub.TAT 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
10
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
11
Air Tersedia untuk pertumbuhan tanaman
12
).
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.
13
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
14
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
15
Representasi bola air yang menyelubungi partikel padatan tanah
16
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.
17
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
18
persen air tanah
Hubungan antara kadar air tanah dan tegangan air
tanah untuk tekstur lempung
19
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
20
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
21
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 + …………………………
22
( 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
Potensial osmotik
(hisapan)
Potensial matrik
(hisapan)
Energi bebas dari air tanah
23
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.
24
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
25
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
KANDUNGAN
AIR DAN
TEGANGAN
Tegangan air tanah, Bar
10.000
Liat
Lempung
Pasir
0.01
10
Kadar air tanah, %
70
26
Tekstur tanah dan air tersedia
27
Hubungan antara kadar air tanah dengan tegangan air tanah
28
Jelaskan bagaimana tektur tanah mempengaruhi jumlah air tersedia bagi
29
tanaman? Sebanyak 250 kata
Jelaskan tanah-tanah yang tekturnya halus mampu menahan lebih banyak air
dibandingkan dgn tanah-tanah yang teksturnya kasar? Sebanyak 250 kata
30
Kapasitas air tersedia dalam tanah yang teksturnya berbeda-beda
31
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
32
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
33
Pola Penetrasi dan Pergerakan Air pada tanah Berpasir dan
tanah Lempung-liat
34
Pola pergerakan air gravitasi dalam tanah
35
Pengaruh struktur tanah terhadap pergerakan air tanah ke arah
bawah
36
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
37
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
38
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.
39
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
40
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.
41
Klasifikasi Air
Tanah
Klasifikasi Fisik:
1. Air Bebas / air gravitasi (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
42
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
43
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
Klasifikasi
Biologi
Air tanah
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
44
Faktor yg berpengaruh:
1. Hubungan tegangan dengan kelengasan
2. Kedalaman tanah
3. Pelapisan Tanah
Faktor yg
mempengaruhi
Air Tersedia
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
45
SUPLAI AIR
ke TANAMAN
Dua proses yg memungkinkan akar tanaman mampu menyerap air dlm
jumlah banyak, yaitu:
1. Gerakan kapiler air tanah mendekati permukaan akar penyerap
2. Pertumbuhan akar ke arah zone tanah yang mengandung air
LAJU GERAKAN KAPILER
Bulu akar
menyerap
air
Gerakan
kapiler
2.5 cm
sagt penting
Jumlah
air tanah
berkurang
Laju gerakan
tgt perbedaan
tegangan dan daya
hantar pori tanah
Tegangan
air tanah
meningkat
Terjadi
gerakan kapiler
air menuju
bulu akar
Terjadi
perbedaan
Tegangan
dg air tanah di
sekitarnya
LAJU PERPANJANGAN AKAR
Selama masa pertumbuhan tanaman, akar tanaman tumbuh memanjang dengan
cepat, sehingga luas permukaan akar juga tumbuh terus.
Jumlah luas permukaan akar penyerap yang bersentuhan langsung dengan
sebagian kecil air tanah (yaitu sekitar 1-2%)
46
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.
KEHILANGAN
UAP AIR
DARI TANAH
Awan hujan
presipitasi
Pembentukan Awan
transpirasi
evaporasi
Run off
infiltrasi
Tanah permukaan
perkolasi
Batuan
Groundwater
Sungai - laut
47
Hadangan hujan
oleh tanaman
semusim
Sekitar 5 - 25% dari curah hujan dihadang tanaman dan dikembalikan
ke atmosfer.
Besarnya tergantung pada kesuburan tanaman dan stadia pertumbuhan
tanaman .
Dari curah hujan 375 mm, hanya sekitar 300-350 mm yang mencapai
tanah.
Hadangan curah hujan oleh jagung dan kedelai
Keadaan hujan
Persen dari curah hujan total untuk:
Jagung
Kedelai
Langsung ke tanah
Melalui batang
70.3
22.8
65.0
20.4
Jumlah di tanah
Yang tinggal di atmosfer
93.1
6.9
85.4
14.6
Sumber: J.L.Haynes, 1940.
48
HUBUNGAN ENERGI LTTA:
Perubahan tegangan air pd saat bergerak dari tanah melalui akar, batang, daun , ke atmosfer
Atmosfer
Daun
Batang
Akar
Tanah berkadar air rendah
500
300
100
25
20
Tanah berkadar air tinggi
15
10
5
Tanah
0
Potensial negatif air (Tegangan air)
49
EVAPOTRANSPIRASI
Kehilangan uap air dari tanah:
1. EVAPORASI: penguapan air dari permukaan tanah
2. TRANSPIRASI: Penguapan air dari permukaan tanaman
3. EVAPOTRANSPIRASI = Evaporasi + Transpirasi
Laju penguapan air tgt pd perbedaan potensial air = selisih tekanan uap
air = perbedaan antara tekanan uap air pd permukaan daun (atau
permukaan tanah) dengan atmosfer
Faktor Iklim dan Tanah:
1. Energi Penyinaran
2. Tekanan uap air di atmosfer
3. Suhu
4. Angin
5. Persediaan air tanah
Air tanah
Jagung
Tinggi
Sedang
17.7
12.7
Evapotranspirasi (cm:
Medicago sativa
24.4
20.5
Sumber: Kelly, 1957.
50
Ketersediaan Air
Tanah vs
Evapotranspirasi
Ketersediaan air di daerah perakaran sangat menentukan besarnya
evapotranspirasi.
Kedalaman daerah perakaran tanaman 50 - 60 cm.
Air tanah pada lapisan olah mengalami pengurangan karena evaporasi
permukaan
Air tanah pd lapisan bawah mengalami pengurangan karena diserap
akar tanaman
Kedalaman tanah (cm)
0 - 17.5
17.5 - 180.0
Evapotranspirasi (cm):
Jagung Padang Rumput
Hutan
24.25
20.75
23.27
22.25
23.45
21.17
Sumber: Dreibelbis dan Amerman, 1965.
51
PEMAKAIAN
KONSUMTIF
(PK)
Pemakaian Konsumtif merupakan jumlah kehilangan air melalui
evaporasi dan transpirasi.
Lazim digunakan sebagai ukuran dari seluruh air yg hilang dari tanaman
melalui evapotranspirasi
Ini merupakan angka-praktis untuk keperluan pengairan
Dua faktor penting yg menentukan PK adalah:
1. KEDALAMAN PERAKARAN TANAMAN
2. FASE PERTUMBUHAN TANAMAN
PK dapat berkisar 30 - 215 cm atau lebih:
1. Daerah basah - semi arid dg irigasi: 37.5 - 75 cm.
2. Daerah panas dan kering dg irigasi: 50 - 125 cm.
EVAPORASI vs TRANSPIRASI
Faktor yg berpengaruh adalah:
1. Perbandingan luas tutupan tanaman thd luas tanah
2. Efisiensi pemakaian air berbagai tanaman
3. Perbandingan waktu tanaman berada di lapangan
4. Keadaan iklim
Di daerah basah : EVAPORASI  TRANSPIRASI
Di daerah kering:
1. EVAPORASI  70 - 75 % dari seluruh hujan yg jatuh
2. TRANSPIRASI  20 - 25%
3. RUN OFF  5%
52
WUE : Water Use
Efficiency
WUE  Produksi tanaman yg dapat dicapai dari pemakaian sejumlah air
tersedia
WUE dapat dinyatakan sbg:
1. Pemakaian konsumtif (dalam kg) setiap kg jaringan tanaman yg
dihasilkan
2. Transpirasi (dalam kg) setiap kg jaringan tanaman yg dihasilkan
……… NISBAH TRANSPIRASI
Jumlah air yg diperlukan untuk menghasilkan 1 kg
bahan kering tanaman
NISBAH TRANSPIRASI
Untuk tanaman di daerah humid: 200 - 500, di daerah arid duakalinya
Tanaman
Nisbah Transpirasi
Beans
Jagung
Peas
Kentang
209 - 282 - 736
233 - 271 - 368
259 - 416 - 788
385 - 636
Sumber: Lyon, Buckman dan Brady, 1952.
53
Faktor yang mempengaruhi WUE: Iklim, Tanah, dan Hara
WUE tertinggi lazimnya terjadi pd tanaman yg berproduksi
optimum;
Adanya faktor pembatas pertumbuhan akan menurunkan WUE
FAKTOR
WUE
Nisbah evapo-transpirasi tanaman di lokasi yg mempunyai defisit kejenuhan dari
atmosfer
800
Kentang
Kacang polong
400
Jagung
0
0
Defisit kejenuhan dari atmosfer (mm Hg)
12
14
Jumlah air unt menghasilkan 1 ton bahan kering
30
Kadar air tanah rendah
15
Kadar air tanah tinggi
0
54
0
Pupuk P, kg/ha
600
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
55
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
56
Beberapa proses penting dalam siklus air:
Precipitation is condensed water vapor that falls to the
Earth's surface.
Most precipitation occurs as rain, but also includes snow,
hail, fog drip, graupel, and sleet.
Approximately 505,000 km³ of water fall as precipitation
each year, 398,000 km³ of it over the oceans.
57
Canopy interception
is the precipitation that is
intercepted by plant
foliage and eventually
evaporates back to the
atmosphere rather than
falling to the ground.
58
LIMPASAN = Runoff
includes the variety of ways by
which water moves across the land. This includes both surface
runoff and channel runoff.
As it flows, the water may infiltrate into the ground, evaporate
into the air, become stored in lakes or reservoirs, or be extracted
for agricultural or other human uses.
Infiltration is the flow of water from the ground surface into
the ground.
Once infiltrated, the water becomes soil moisture or groundwater.
59
Subsurface Flow is the flow of water underground, in
the vadose zone and aquifers. Subsurface water may return
to the surface (eg. as a spring or by being pumped) or
eventually seep into the oceans.
Water returns to the land surface at lower elevation than
where it infiltrated, under the force of gravity or gravity
induced pressures.
Groundwater tends to move slowly, and is replenished
slowly, so it can remain in aquifers for thousands of years.
60
Evaporation
is the transformation of water from liquid to gas
phases as it moves from the ground or bodies of water into the
overlying atmosphere.
The source of energy for evaporation is primarily solar radiation.
Evaporation often implicitly includes transpiration from plants,
though together they are specifically referred to as
evapotranspiration.
Approximately 90% of atmospheric water comes from evaporation,
while the remaining 10% is from transpiration. Total annual
evapotranspiration amounts to approximately 505,000 km³ of water,
434,000 km³ of which evaporates from the oceans.
61
SUBLIMASI is the state change directly from solid
water (snow or ice) to water vapor.
ADVEKSI is the movement of water — in solid, liquid,
or vapour states — through the atmosphere. Without
advection, water that evaporated over the oceans could not
precipitate over land.
KONDENSASI is the transformation of water vapour
to liquid water droplets in the air, producing clouds and
fog.
62
Aktivitas manusia yang dapat mempengaruhi siklus air :
Pertanian
Alteration of the chemical composition of the atmosphere
Construction of dams
Deforestation and afforestation
Removal of groundwater from wells
Water abstraction from rivers
Urbanization .
63
KAPASITAS PENYIMPANAN AIR:
WATER HOLDING CAPACITY
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 pre-irrigate. And there would be no such
thing as dryland farming.
64
Soil Moisture Level (Depletion, %) vs. Soil Moisture Tension (Bars).
65
Hubungan antara Potensial Air
Tanah dnegan Air Tersedia pada
tiga macam tekstur tanah
66
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).
67
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.
68
Graphically, the relationship can be described by the Figure SWP-1.
Looking at the lowest line for a coarse soil.
You can see that at A, the soil moisture level is very high and the
water-holding forces are low.
This means that the plant can extract water easily from the soil.
At B, the soil moisture level is lower but the water-holding forces
haven't gone up that much.
The plant can still extract water easily.
However at C, the soil moisture level is very low and the waterholding forces have increased greatly.
The plant cannot extract water easily and will be stressed.
69
Looking at the top line for a finer soil.
At A, as with the coarse soil, the water-holding forces are low
when the soil moisture level is high.
However, at B, the soil moisture level has dropped somewhat but
the water-holding forces have gone up greatly.
And at C, where the soil moisture level is low, the water-holding
forces have gone up very high.
We will be coming back to this idea of increasing soil waterholding forces with decreasing soil moisture many times
70
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.
71
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.
72
The amount of soil water is
usually measured in terms of
water content as percentage
by volume or mass, or as soil
water potential.
Jumlah air tersedia dipengaruhi
tekstur 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.
The only information
provided by water content is
the relative amount of water
in the soil.
73
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.
74
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
75
Kapasitas Lapangan
Field Capacity
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.
76
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.
77
You can use the sponge again to demonstrate this important fact.
With the sponge at "field capacity", use a cup to pour water on it.
The water will soak in, there will be open pores in the sponge that will
take in water. But you will see that the sponge starts dripping again
as the excess water starts to drain off the bottom.
Because of this ability to hold water against the pull of gravity, soil
does not act like a bathtub during irrigations.
That is, irrigation water does not have to go to some "bottom" and
then fill back up to the top. Rather soil fills to field capacity from the
top down.
78
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 EFEKTIF
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".
80
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 a pre-irrigation though, you may want to consider the maximum
potential root zone as the effective root zone for that irrigation.
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.
81
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.
82
The pore volume is actually a reservoir for holding water. Not all of
the water in the reservoir is available for plant use.
Figure 3 represents a "wet" (saturated) soil immediately after a large
rainfall.
Note that all of the pores are filled with water. Gravity will pull some
of this water down through the soil below the crop's root zone.
The water that is redistributed below the root zone due to the force of
gravity is gravitational water. In general, gravitational water is not
available to plants, especially in sandy soils, because the
redistribution process occurs quickly (in two days or less).
83
Kapan tanah perlu ditambah air agar tanaman tidak terganggu pertumbuhannya?
Jelaskan pendapat Saudara dnegan 250 kata?
84
Sumber dan perilaku air yang ditambahkan ke tanah
85
Saturated (wet) soil. All pores (light areas) are filled with water. The dark areas
represent soil solids.
86
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
87
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.
88
Plant-available water, PAW, adalah volume air
yang disimpan dalam tanah yang dapat
digunakan oleh tanaman .
It is the difference between the volume of water
stored when the soil is at field capacity and the
volume still remaining when the soil reaches
the permanent wilting point (the lower limit), as
shown in Figure 6.
89
Figure 6. HUBUNGAN ANTARA AIR-TERSEDIA DAN DISTRIBUSI AIR
DALAM TANAH .
90
Kapasitas tanah menyimpan air
91
Jumlah air tanah pada tiga macam tekstur tanah
92
Tabel 1. Jumlah air tersedia dalam tanah yang teksturnya
berbeda-beda
93
AIR-TANAH dan CEKAMAN (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.
94
AIR-TANAH dan CEKAMAN (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.
95
Gambar.
Kalau tanaman
menyerap air, tanah di
sekitar perakaran
menjadi mengering .
If the rate of water
movement from moist
zones is less than the
PET, the plant
temporarily wilts.
96
Pada malam hari, pada saat PET menurun hingga
mendekati nol , air tanah bergerak dari tanah yang lebih
basah memasuki zone tanah yang lebih kering di sekitar
akar tanaman.
The plant recovers turgor and wilting ceases (Figure 8).
This process of wilting during the day and recovering at
night is referred to as temporary wilting.
Proper irrigation scheduling reduces the length of time a
crop is temporarily wilted.
97
Gambar .
At night when the
PET is low, the plant
recovers from
wilting as water
moves from moist
zones (dark areas)
to eliminate the dry
zones around the
roots.
98
Hubungan antara distribusi air dalam tanah dan konsep jadwal irigasi
ketika 50 percent air tersedia telah habis
99
FAKTOR TANAMAN
Three plant factors must be considered in developing a
sound irrigation schedule: the crop's effective root depth, its
moisture use rate, and its sensitivity to drought stress (that
is, the amount that crop yield or quality is reduced by drought
stress).
KEDALAMAN EFEKTIF AKAR
Rooting depth is the depth of the soil reservoir that the plant
can reach to get PAW. Crop roots do not extract water
uniformly from the entire root zone. Thus,the effective root
depth is that portion of the root zone where the crop extracts
the majority of its water. Effective root depth is determined by
both crop and soil properties.
100
PENGARUH TANAMAN thd KEDALAMAN EFEKTIF AKAR
Different species of plants have different potential rooting depths.
The potential rooting depth is the maximum rooting depth of a crop when grown
in a moist soil with no barriers or restrictions that inhibit root elongation.
Potential rooting depths of most agricultural crops important in North Carolina
range from about 2 to 5 feet. For example, the potential rooting depth of corn is
about 4 feet.
Water uptake by a specific crop is closely related to its root distribution in the soil.
About 70 percent of a plant's roots are found in the upper half of the crop's
maximum rooting depth. Deeper roots can extract moisture to keep the plant alive,
but they do not extract suffficient water to maintain optimum growth.
When adequate moisture is present, water uptake by the crop is about the same as
its root distribution. Thus, about 70 percent of the water used by the crop comes
from the upper half of the root zone (Figure 10). This zone is the effective root
depth.
101
JUMLAH AIR YANG DAPAT DISERAP TANAMAN DIPENGARUHI OLEH
DISTRIBUSI AKAR DLAMA TANAH
102
PENGARUH TANAH thd KEDALAMAN EFEKTIF AKAR.
The maximum rooting depth of crops in North Carolina is usually less than
their potential rooting depth and is restricted by soil chemical or physical
barriers.
North Carolina subsoils have a pH of about 4.5 to 5.0, which presents a
chemical barrier to root growth, as shown in Figure 11.
Liming practices rarely improve soil pH below the 2-foot depth. Shallow
soils (Carolina slate belt soils) or soils with compacted tillage pans (coastal
plain soils) are examples of soils with physical barriers that restrict root
penetration below the plow depth (usually less than 12 inches unless
subsoiling is practiced).
Thus, for example, while corn has a potential rooting depth of 4 feet, when
grown under North Carolina conditions, its maximum rooting depth is about
2 feet. Maximum rooting depths for several crops under North Carolina
conditions are given in Table 2.
103
CIRI-CIRI TANAH YANG MEMPENGARUHI KEDALAMAN PERAKARAN
TANAMAN
104
The effective root depth is the depth that should be used to compute
the volume of PAW in the soil reservoir.
The effective root depth for a mature root zone is estimated to be
one-half the maximum rooting depth listed in Table 2.
For example, under North Carolina conditions corn has a maximum
rooting depth of 2 feet; thus, the maximum effective root depth is
estimated to be 1 foot.
Effective root depth is further influenced by the stage of crop
development. Effective root depths for most aops inaease as top
growth inaeases until the reproductive stage is reached. After this
time, effective root depth remains fairly constant.
105
Kedalaman perakaran tanaman jagung pada berbagai umur
pertumbuhannya. Jadwal irigasi harus didasarkan pada kedalaman efektif
akar dan bukan pada kedamalan maksimum perakaran .
106
LAJU PENGGUNAAN AIR TANAMAN
Often, irrigation scheduling requires an estimate of the rate at which PAW
is being extracted. A "checkbook" approach is often used to keep a daily
accounting of water additions and removal.
Traveling irrigation systems usually require several days to complete one
irrigation cycle. Soil-water measurements should be used to schedule
irrigation for these systems, but continued PAW extraction during the
irrigation cycle must also be estimated so that the last part of the field
does not get too dry.
In the above situations, the crop's water use rate must be estimated.
Estimates of the water use rate for most crops are available from county
Extension Service or Soil Conservation Service offices. As with rooting
depth, water use rate is a function of the crop's stage of development, as
shown in Figure 13.
For example, corn uses water three times as fast during the pollination
period (65 to 75 days after planting, 0.25 inch per day) as during the kneehigh stage (35 to 40 days after planting, 0.08 inch per day).
107
Penggunaan air harian tanaman jagung dipengaruhi oleh fase
pertumbuhan tanaman . Jadwal irigasi harus disesuaikan dengan
perubahan konsumsi air tanaman selama musim pertumbuhannya
108
KEPEKAAN TANAMAN TERHADAP
KEKERINGAN
The reduction in crop yield or quality resulting from drought stress
depends on the stage of crop development. For example, corn is most
susceptible to stresses caused by dry conditions at the siLicing stage
(Figure 14).
For a given level of stress, the yield reduction for corn would be four times
greater at the silking stage than at the knee-high stage. From the yield
standpoint, applying irrigation water at silking would be worth four times
more than if the same amount of water was applied during the knee-high
stage.
Knowledge of this relationship is most useful when the irrigation capacity
or water supply is limited. When water is in short supply, irrigation should
be delayed or cancelled during the least susceptible crop growth stages.
This water can then be reserved for use during more sensitive growth
stages.
109
Kepekaan tanaman jagung terhadap kekeringan dipengaruhi oleh
fase pertumbuhannya. Semakin besar tingkat kepekaannya, maka
pengaruh kekeringan terhadap hasil semakin besar.
110
Kepakaan tanaman jagung terhadap kekeringan
dipengaruhi oleh umur tanaman.
This relationship is typical for most agricultural crops irfigated.
The most critical irrigation period typically begins just before the
reproductive stage and lasts about 30 to 40 days to the end of the fruit
enlargement or grain development stage. Because the root system is
fully developed by the beginning of the reproductive period,
irrigation amounts should be computed to replace the depleted PAW
within the effective root zone (12 inches).
Exceptions include tobacco and other transplanted crops where
irrigation is often scheduled immediately after transplanting to
ensure stand establishment.
111
When if rigation is scheduled before the crop root system is fully
developed, the amount of irrigation to apply should be based on the
depleted PAW within the actual effective root depth at the time of
irrigation.
For example, irrigation scheduled when corn is at the knee-high
stage (35 to 40 days after planting) should apply only about twothirds as much water as an irrigation scheduled during the tasseling
stage (65 days after planting) because the effective rooting depth at
the knee-high stage is only two-thirds as deep (8 inches compared to
12 inches).
For soils that have an abrupt textural change within the effective
root depth, such as a loamy sand surface texture overlying a sandy
clay loam, a correction may be necessary to account for the different
amounts of PAW within each soil texture.
112
113
Jumlah air tanah tersedia dalam berbagai tipe tanah
114
115
116
Bagaimana mycorrhiza dapat membantu penyerapan air dari
dalam tanah? Uraian 250 kata
117
Jelaskan
mengapa
air
bergerak
dari akar
menuju
daun
tanaman ?
250 kata
118
Jelaskan klasifikasi biologis air tanah, dengan 250 kata
119
Pengaruh Potensial Air tanah thd konduktivitas hidraulik tanah
120
Pengaruh ketersediaan air terhadap pertumbuhan tanaman
121
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
122
Sistem Perakaran Serabut dan Perakaran
Tunggang pada Tanaman umur dua bulan
123
Penyerapan air BAWANG PUTIH (Allium cepa)
Tanaman mempunyai sistem perakaran yang dangkal dan
akar-akar terkonsentrasi pada tanah klapisan atas sedalam 0.3
m.
Pada umumnya 100% penyerapan air terjadi dari lapisan
tanah atas sedalam 0.3-0.5 m (D=0.3-0.5 m ).
Untuk memenuhi sekuruh kebutuhan air tanaman (ETm) tanah
harus dijaga tetap lembab; pada laju evapotranspirasi 5-6
mm/hari ternyata laju penyerapan air mulai menurun kalau
sekitar 25% dari total air tanah tersedia telah habis (p = 0.25).
124
Penyerapan air tanaman LOMBOK (Capsicum annum dan Capsicum
frutescens)
Tanaman lombok mempunyai akar utama yang patah pada saat transplanting dan kemudian menumbuhkan banyak akar-akar lateral.
Kedalaman akar dapat meluas hingga 1 m tetapi pada kondisi irigasi
ternyata akar terkonsentrasi pada lapisan tanah atas sedalam 0.3 m.
Pada kondisi evapoytranspirasi maksimum 5-6 mm/hari, 25-30% total air
tersedia dapat dihabiskan sebelum terjadi reduksi penyerapan air (p=0.250.30).
Biasanya 100% penyerapan air terjadi dalam keda;laman lapisan tanah
0.5 - 1.0 m (D = 0.5-1.0 m).
125
Penyerapan air tanaman jeruk
Tanaman jeruk menumbuhkan satu akar tunggang utama.
Akar-akar cabang membentuk semacam jaring horisontal yang dilengkapi
dengan bulku-bulu akar. Perkembangan akar snagat tergantung pada tipe
batang bawah yang digunakan dan karakteristik profil tanah.
Kedalaman perakaran beragam antara 1.20 dan 2.0 m. Pada umumnya
60% akar berada pada lapisan tanah atas 0.5 m, 30% dalam lapisan tanah
0.5 m ke dua, dan 10% pada lapisan tanah di bawah 1 meter.
Kalau persediaan air irigasi mencukupi, biasanya 100% air diekstraks dari
lapisan tanah atas 1.2 - 1.6 m (D = 1.2-1.6 m) tetapi pada kondisi kering
ternyata kedalaman ekstraksi air lebih dalam lagi.
Selama periode defisit air yang panjang, air dalam tanah yang kedalaman
efektifnya tebal dan drainasenya bagus dapat digunakan oleh tanaman
hingga kedalaman 2 atau 3 meter.
126
Pergerakan air dari lapisan tanah basah ke lapisan tanah kering
dengan bantuan sistem perakaran tanaman
127
BAGAIMANA TANAMAN MENGAMBIL AIR?
Apa kebutuhan tanaman?
Plants need water. We all know that. Why do they need water? For the following
reasons:
Firstly, they need water in order to stand up. Some will eventually make woody
tissue to help this process, but basically plants are full of pressurised water which
makes them turgid. The leaves offer themselves to the sun....their stomata (pores)
open....and moisture evaporates. Water is drawn upward from the roots and
through the stems to replace this lost water. This process is called
"evapotranspiration". The more sun, the greater the pressure to take up water.
This process takes energy from the plant, and obviously requires a healthy root
system and the presence of AVAILABLE water in the root zone (I'll explain the
"availability" shortly). If it's not there, the plant will wilt. In cases of root disease
and diseases like Fusarium, you will see whole crops crash down.
128
Secondly, they need water to carry nutrients into themselves which
are dissolved in the soil water. They can't munch on dry fertiliser.
No water.....or I should say, "no passage of water into the plant"......
and no nutrient uptake.
If the plant can't take up water, it will become starved of nutrients.
It's not so uncommon to see high nutrient soils and pale, nutrientstarved crops because of an inability of the plant to take up water.
Thirdly, plants need water to photosynthesize.
To summarise a fairly complex process, photosynthesis is the
synthesis of sugar (energy) from light, carbon dioxide and water, with
oxygen as a by-product.
Take away any of those factors, and the plant won't grow. It has no
energy.
129
Apa lagi kebutuhan tanaman ?
They need oxygen, and they need it in the root zone.
Like all aerobic organisms (including us), they need to respire as part
of the process of utilising the sugars they created in photosynthesis,
and this requires oxygen.
No oxygen, and no respiration. No respiration, and no functionality.
The roots can't grow....and can't take up water....and can't supply the
plant with the nutrients and water that it needs.
This is why we talk about a plant needing DRAINAGE.
The problem in a waterlogged situation is not too much water......it's
too little oxygen!
130
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).
131
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.
132
AIR HIGROSKOPIS
This is the water which is held so tightly (by surface
tension) to the soil particles that the plant roots can't
take it up.
It's there.......but it's unavailable.
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
merupakan sifat
tanah yang
penting bagi
pertumbuhan
tanaman.
Mengapa
demikian?
Jelaskan
dengan 250 kata
133
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. The higher the salinity level, the harder it is for a plant to take a drink,
despite apparently sufficient moisture present.
134
Jelaskan pendapat
Saudara
mengenai
pentingnya
sirkulasi air
dalam sistem
Tanah-Tanaman
250 kata
135
Bibit tanaman
tomat yang baru
ditanam ini
memerlukan cukup
banyak air dari
dalam tanah.
Mengapa
demikian?
Jelaskan
dengan 250 kata
136
Struktur
Sistem TanahTanaman.
Jelaskan
bagaimana air
dari tanah
memasuki
sistem tanahtanaman.
250 kata
137
Bagaimana
peranan
tumbuhan
dalam siklus
air di alam?
Jelaskan
pendapat
Saudara
250 kata
138
Representasi ketersediaan air dalam tanah bagi
pertumbuhan tanaman
139
AIR TERSEDIA BAGI TANAMAN
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.
140
RAW - Readily Available Water
(Air 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.
141
PAW - Plant Available Water
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.
142
Jelaskan
bagaimana
hubungan antara
Evapotranspirasi
dan Irrigasi
Dengan 250 kata
143
Evapotranspirasi dan Irrigasi
Evapotranspiration (ET) is the combined process of plant
transpiration and soil evaporation .
Plant transpiration is the movement of moisture from the plant
to the air through tiny pores in the leaves known as stomates.
The water enters the plants through the roots in a
liquid form and leaves the plants through the
stomates in a gaseous form.
Soil evaporation is the direct evaporation of water from the
surface of the soil into the atmosphere.
144
Hubungan
antara profil
tanah dengan
air tanah.
Jelaskan
pendapat
Saudara
tentang hal ini
250 kata
145
Hubungan
antara kadar
air tanah
dnegan nilai
pF, pada tiga
macam tekstur
tanah.
Jelaskan
pendapat
Saudara
tentang hal ini
250 kata
146
Transport air dalam tanaman
Plants need raw materials like CO2, water and minerals for
photosynthesis and for various other purposes such as making of
proteins. For plants soil is the richest source of water and minerals.
Roots absorb these substances and transport to the various parts of the
plant.
The water and minerals dissolved in it move through special tissue
present in plants called xylem.
Xylem consists of two kinds of elements called tracheids and vessels.
Vessels and tracheids of the roots, stems and leaves are interconnected
to form a continuous system of water conducting channels reaching all
parts of the plant.
147
148
Struktur jaringan pembuluh tanaman
149
Struktur jaringan pembuluh tanaman
150
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.
151
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 .
152
A few points to consider:
Only after heavy rainfall does the water “flow” through the soil. This is especially
true in our area where evapotranspiration exceeds precipitation. During most of
the growing season the water can be said to be “pulled” through the soil by
capillarity.
Field Capacity can be thought of as “all the water a soil can hold against the pull of
gravity”.
When the field capacity of a particular soil is exceeded, water begins to flow
downward. One last point to consider is that available water to the plant is only the
water held in the soil at tensions between field capacity and wilt point, or
realistically, the water held at tensions less than wilt point.
The characteristic annual cycle of changes in soil moisture content deserves study
because it leads to a better understanding of the principles of ground-water
movement, surface runoff, and various aspects of the sculpturing of the land by
running water.
153
Hubungan Air –
Tanah – dan
Tanaman
Suatu sistem yang
kontinum.
Jelaskan pendapat
Saudara mengenai
hal ini
(sebanyak 250
kata)
154
Air tanah pada berbagai kondisi kelengasan (kadar air)
155
Tanaman menyerap
air dari dalam tanah
melalui akarakarnya, kemudian
diangkut ke daun
untuk fotosintesis
Jelaskan bagaimana
akar tanaman
menyerap air dari
dalam tanah?
dengan 250 kata
Struktur Tanaman
156
AKAR TANAMAN
Often roots are overlooked, probably because they are less visible
than the rest of the plant. However, it's important to understand
plant root systems because they have a pronounced effect on a
plant's size and vigor, method of propagation, adaptation to soil
types, and response to cultural practices and irrigation.
Roots typically originate from the lower portion of a plant or cutting.
They have a root cap, but lack nodes and never bear leaves or
flowers directly.
Their principal functions are to absorb nutrients and moisture, anchor
the plant in the soil, support the stem, and store food. In some plants,
they can be used for propagation.
157
Struktur akar tanaman
158
Penampang melintang akar tanaman
159
Pengolahan tanah sawah memerlukan
banyak air
Pengolahan tanah
sawah untuk
menanam padi
memerlukan
banyak air.
Mengapa
demikian?
Jelaskan
dengan 250 kata
160
Penanaman bibit padi juga memerlukan banyak air
161
How Rice Is Grown
The two major types of rice, indica (long-grain) and japonica
(medium- and short-grain) do well in different environments.
Long-grain indica rices (basmati and jasmine, for example) do
well in hot, equatorial climates. Medium- and short-grain
japonica rices grow well in temperate and mountainous
regions.
Rice cultivation has traditionally been well-suited to countries
and regions with low labor costs and high rainfall. Without
modern technology, rice is very labor-intensive to cultivate;
either way it requires plenty of water for irrigation.
162
Kebutuhan air tanaman :
"kedalaman (jumlah) air yang diperlukan untuk
memenuhi kehilangan air melalui evapotranspirasi
(ETtanaman) tanaman yang sehat, tumbuh pada
sebidang lahan yang luas dengan kondisi tanah yang
tidak mempunyai kendala (kendala lengas tanah dan
kesuburan tanah) dan mencapai potensi produksi
penuh pada kondisi lingkungan tumbuh tertentu".
163
AIR TANAMAN
Water is essential in the plant environment for a number of
reasons. Water transports minerals through the soil to the roots
where they are absorbed by the plant. Water is also the principal
medium for the chemical and biochemical processes that support
plant metabolism. Under pressure within plant cells, water
provides physical support for plants.
It also acts as a solvent for dissolved sugars and minerals
transported throughout the plant. In addition, evaporation within
intercellular spaces provides the cooling mechanism that allows
plants to maintain the favorable temperatures necessary for
metabolic processes.
164
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.
165
Komponen Neraca Air pada Suatu
Lahan
Air Irigasi
166
Hubungan antara Kadar Air Tanah dan
Pertumbuhan Tanaman
Growth of most
agricultural crops is
favored by a soil water
content that is high enough
to encourage crop growth
and development, but not
so high that aeration
becomes restrictive.
If soil water is plantextracted to levels
approaching the PWP,
water is held so tenaciously
by the soil that plants can
no longer obtain sufficient
water to meet the potential
for transpiration.
Transpiration is restricted
and yield losses take place.
167
IRRIGATION
A. Definition: Supplying water to plants in an
artificial manner. (39% of all freshwater in
US is used to irrigate crops)
the
1. Ancient practice – first irrigation used
ditches to divert rivers and streams.
2. California agriculture relies on irrigation.
a. Mediterranean climate
b. Crop diversification
c. Economics
168
Pola pergiliran
tanaman
berdasarkan
curah hujan
Jelaskan mengapa
demikian?
Dengan 250 kata
169
Soil Water and Groundwater (1)
170