General Composition of Soil
mineral solids
air-filled pores
organic solids
water-filled pores
Figure indicates ½ total volume is pore space,
or 0.50 porosity. This varies from soil to soil.
Figure shows ½ of pore space filled with water.
This depends on recent rain, internal drainage
and evapotranspiration.
This is a mineral soil. There are organic soils.
For a mineral soil, the indicated amount of
organic matter is higher than typical.
Problem
Assume the average density of solid particles = 2.60 g / cm3 and porosity = 0.50.
What is the density of this soil when it is dry (zero water content)?
bulk density = 1.30 g / cm3
High bulk density is bad for plants because it impedes root development.
A dense zone therefore restricts the volume of soil occupied by roots, increasing
susceptibility to drought.
You can determine bulk density by taking a core sample (known volume), drying in
oven to evaporate water, then weighing.
Often people assume a value of 2.65 g / cm3 for the density of solid particles,
determine bulk density, then calculate the porosity from,
porosity = 1 – (BD / PD)
The water content of the core sample can determined by weighing fresh from the
field besides when oven-dried.
gravimetric water content = mass of water / mass of dry soil
volumetric water content = volume of water / volume of soil (core)
The mineral particles range in size. Excluding particles gravel-size and larger, the
fine-earth particles include sand, silt and clay sizes.
Sand
Silt
Clay
2.000 mm – 0.050 mm
0.050 mm – 0.002 mm
< 0.002 mm
Texture is the proportion of sand, silt and clay. Crudely speaking, there are sandy
soils, loamy soils and clay soils. However, since texture strongly affects many
other properties, people split textural classification further into 12 textural classes.
The main effect of texture on other soil properties is through particle surface area.
Decreasing size of particles (sand → clay), greatly increases surface area. Greater
surface area means greater chemical and physical reactivity, like retention of
nutrients and capacity to hold water.
What’s the
textural
class of a
soil that’s
50 % sand
10 % silt
Adsorption and Exchange of Ions
The clay-size particles typically have formed in the soil from prior minerals by
chemical weathering. Besides just large surface area, the clay minerals have
the specific capacity to adsorb positively (mostly) and negatively charged nutrient
and other ions because they carry electrostatic charge.
The adsorbed ions are in chemical equilibrium with ions in the soil water (solution),
meaning that there is more or less free exchange.
The humus fraction of soil organic matter behaves similarly.
The cations shown are typically the
most common ones in soil.
Of these, Ca2+, Mg2+ and K+ are
important nutrients and are called
base cations to distinguish them from
acid cations (e.g., H+). Al3+ is also
an acid cation because when it reacts
with water, H+ is produced.
Within a volume of soil, the amount of cations, bases or acids, adsorbed onto clay
minerals and humus, is many times greater than the amount these cations in the
soil solution.
Adsorbed nutrients are a reservoir –when nutrients in the soil solution are depleted
by plant uptake, they are replenished by release from clay minerals and humus.
An important concept pertaining to adsorbed cations is cation exchange capacity,
CEC. This is the amount of adsorbed cation charge per mass of soil (cmol(+) / kg).
It includes charge due to both acid and base cations.
Soil Reaction, Acidity, Alkalinity and pH Adjustment
Intuitively, if a soil is dominated by acid cations, H+ and Al3+ (and others), it is acid,
has relatively little nutrient base cations, Ca2+, Mg2+, K+ (and others) and is
chemically infertile.
Percentage base saturation, %BS = (charge due to bases / CEC) x 100
The level of acidity, as measured by soil solution pH, is related to the %BS in
what is called a buffer curve.
Shape varies from soil
to soil but pH always
increases as %BS
increases.
To refresh memory:
pH = -log [H+]
So, low pH means
large [H+] and high
acidity.
The below data are for an acid sandy loam in North Louisiana:
Base Cations
Acid Cations
----------mmol (+) / 100 g or cmol (+) / kg ---------Ca Mg K Na
Al
H
2.0 0.6 0.3 0.1
6.4
0.6
CEC = ? and % BS = ?
Soil A Soil B
cmol (+) / kg
Basic cations
Acidic cations
90
10
5
5
Which soil has the lower pH?
Which soil is more fertile?
USDA NRCS categories
pH
Most plant do best in pH range
5.5 – 7.0, though some do better
under more acidic or basic conditions.
< 4.5
4.5 – 5.0
5.1 – 5.5
5.6 – 6.0
6.1 – 6.5
6.6 – 7.3
7.4 – 7.8
7.9 – 8.4
8.5 – 9.0
> 9.0
extremely acid
very strongly acid
strongly acid
medium acid
slightly acid
neutral
mildly alkaline
moderately alkaline
strongly alkaline
very strongly alkaline
Especially high or low pH bad for plant directly and indirectly through effects on the
solubility and plant-availability of other nutrients besides Ca2+, Mg2+ and K+, like P,
B, Mo, Fe etc. Also, high or low pH adversely affects beneficial soil microorganisms
involved in nutrient cycling (release of nutrients bound in organic matter) and others
that carry out N-fixation.
Therefore, an important soil management practice is pH adjustment.
Just how much base or acid that needs to be applied depends on initial and target
pHs, shape of the buffer curve and the CEC.
In practice, different amounts of base or acid are added to a set of soil samples
and the pH measured to develop a response curve.
Different lime materials may be used to neutralize soil acidity and raise pH.
CaCO3
CaMg(CO3)2
CaO
Ca(OH)2
calcitic limestone
dolomitic limestone
burned lime (quicklime)
hydrated lime
These latter two are more soluble
so react faster.
To lower pH, sulfur (S) is typically added. Although it is not itself an acid, it is
oxidized in the soil (primarily by certain microorganisms) to produce
sulfuric acid, H2SO4.
Fertilizer Recommendations
The nutrients most commonly applied are N, P and K because these are the
most commonly deficient with respect to crop needs.
Some states have recommendations for N based on soil tests for N. Louisiana
does not. The problem with N is that the overwhelming amount of soil N is bound
in organic matter. Release of this N depends on microbial activity, which is
highly variable.
In Louisiana and some other states, N recommendations are based on long-term
data for crop response to different rates of N fertilization.
Some states have found that the concentration of soil nitrate-N (NO3-) can be
used to make recommendations.
When soil test concentrations of a nutrient are used, crop growth/yield are plotted
with respect to soil test value and modeled.
Relative Yield (%)
Based on the relationship
between yield and soil test,
you need to add sufficient
fertilizer P to raise the soil
test from 3 to 18 if you want
to increase expected yield
from 50% to 90% of max.
Soil Test P (ppm)
The recommendation is
in terms of pounds P / acre.
Assuming you wanted to apply 120 lbs of N per acre and used a 12-12-12 fertilizer
that cost $ 400 per ton (2000 lbs), how much would it cost to fertilize a 40 acre tract?
You need
40 ac x (120 # N ac-1) / (0.12 x 2000 # N ton fertilizer-1) = 20 ton fertilizer
costing
20 ton x $ 400 ton-1 = $8000
Things you need to know about fertilizer grade, e.g., 12-12-12. In order, the
numbers refer to % N, % P2O5 and % K2O in it. However, there is no P2O5
nor % K2O in it! It does, of course, contain P and K but not in these forms.
This representation is highly misleading because these oxides weigh much
more than the elements they represent. You have to use conversion factors,
0.44 and 0.83 for P and K, respectively.
Continuing with the above scenario, how much P and K are applied along with
the 120 lbs of N per acre? Conversion factors for P2O5 and K2O are 0.44 and
0.83, respectively.
You apply the 12-12-12 fertilizer at a rate of 0.50 ton per acre, so you apply,
# P ac-1 = 1/2 ton fertilizer x (0.12 x 2000 # P2O5 ton fertilizer-1) x 0.44 = 53 # P ac-1
# K ac-1 = 1/2 ton fertilizer x (0.12 x 2000 # K2O ton fertilizer-1) x 0.83 = 100 # K ac-1
Some Points on N
Taken up as nitrate (NO3-) or ammonium (NH4+). Little of either in soil at
any one time.
Concentrations naturally depend on complex,
interrelated microbial processes.
Deficiency seen as chlorosis.
Growth is stunted.
N is mobile within the plant,
therefore, when deficiency exists
N is translocated from older tissue
to younger tissue so chlorosis seen
on older tissue.
Ammonia or ammonium is oxidized to nitrate fairly quickly by a microbial
process called nitrification. This negatively charged ion is not retained by
adsorption on soil particle surfaces (since they are negatively charged), so
it will be leached if not taken up.
Can use a chemical inhibitor to stop nitrification.
Nitrification produces H+, so ammonia / ammonium fertilizers are acid-forming.
Besides likely to leach, if the soil is anaerobic, the nitrate is converted to
nitrous oxide or nitrogen gas and lost from the soil in these forms. This is
also a microbial process (called denitrication).
Nitrate fertilizers are used with rice (True / False).
A very important process involving N is biological N fixation, in which
atmospheric N is converted into organic N by certain soil microbes, particularly
types living symbiotically with certain plants (including legumes).
The plants are called N-fixing plants but it is the microbes that do it.
A
B
If the soil does not have the right bacteria, you need
to use seed innoculated with them.
Some Points on P
Soils are naturally very low in P, the solubility of P in the soil is low and there is
little atmospheric deposition of P. Therefore, P is limiting to plant growth.
Taken up as the phosphate ions, H2PO4- and HPO42-.
Deficiency symptom is purplish color
Growth is stunted.
Reasons why phosphate is not soluble:
Reacts with Al and Fe in acid soils to form insoluble minerals
Bonds to the surface of soil mineral particles
Reacts with Ca in alkaline soils to form insoluble minerals
Solubility of P is greatest in the pH range of 5.5 – 7.0.
Without fertilizer input, natural systems are very efficient
in cycling P –mineralize organic P to phosphate.
Some Points on K
Take up as K+.
Very abundant in soils but plants take up
a lot of it and although abundant, most K
is in the structure of soil minerals (unavailable).
K+ adsorbed on negatively charged particles
is the main reservoir but it is depleted.
Deficiency symptom is chlorosis and
necrosis about leaf margins.
Plants have a tendency to take up more
K than they need for normal growth and
development. Called luxury consumption.
Accelerates depletion.
K is known for luxury consumption but it happens with most nutrients.
Some Points on S
Taken up as sulfate, SO42-.
Deficiency seen as chlorosis but,
different from N, sulfur is relatively
immobile so chlorosis is seen on
younger tissue. Growth is stunted.
Reasonably abundant in soils but
can be depleted. There is atmospheric
deposition of S as an input. Some added
as an impurity in P fertilizers.
Like N and P, S exists in a cycle involving organic S. Depending on the form
of organically-bound S and whether the soil is aerobic or anaerobic, the S
that is mineralized in organic matter decomposition will be either SO42- or S2-.
The latter is toxic.
Some Points on Micronutrients
Macronutrients
C H O N P K S Ca Mg
Micronutrients
B Cl Co Cu Fe Mo Mn Ni Zn
primary nutrients, the others
secondary in terms of how
much is typically added
Micros are essential, only called micro because they are needed in much
smaller amounts. Weathering of soil minerals, nutrient cycling and deposition
usually adequate to supply micronutrients.
However, deficiencies can occur where biomass removal is high, high analysis
(i.e., low impurities) fertilizers are used, and on soils with few weatherable
minerals (sands and organic soils).
High pH can also produce deficiencies because it greatly reduces the solubility
of micronutrients except Cl and Mo.
High pH favors Mo solubility and availability.
Mn deficiency cotton
The opposite situation can also occur –toxicity.
This may occur from unmindful over-application
as with biosolids or arise from soil conditions.
Low pH increases the solubility of most of the
micronutrients.
Under anaerobic + acid conditions, Fe and Mn
solubilities are especially high.
Crinkle-leaf in cotton due to
Mn toxicity.
Soil likely too acidic and wet?
What to do?
Soil testing and plant analysis
would provide key information.
Soil Testing and Plant Analysis
Unbiased soil sampling is essential. Must take steps to account for variability.
Divide field into homogeneous areas
Take many random samples, combine, mix and take subsample
Avoid atypical areas.
Lab analyses
Based on soil test levels that
are calibrated to field tests.
Best to use lab in state because
types of tests and field data
are appropriate for local soil and
environmental conditions.
Plant analysis is based on the relationship between concentration of elements
in tissue and growth.
90 %
max
Also known as hidden hunger. Plants not so deficient as to show symptoms,
only stunted. Can’t see this because all plants around are equally stunted.
Salinity and Sodium Problems
Lab analyses can indicate or confirm problems to too high salt and Na.
Salinity may be directly toxic but indirectly a serious problem because high salts
reduce the capacity of plants to uptake water.
Plants take up water because the water potential (energy status of water) in roots
is less than the water potential in the soil. Salts in the soil solution reduce the
water potential in the soil, causing drought stress when the water content of the
soil is relatively high.
This problem is more
common in arid regions,
however can occur
anywhere irrigation is
used and drainage is
inadequate.
Drainage is important because it allows salts to leach below the root zone.
Avoid irrigation induced salinity by applying a bit more water than the crop
needs to force salt leaching. Called leaching requirement.
To reclaim saline soil, improve drainage and leach salts.
High Na is a more difficult problem. High Na leads to very high soil pH and
very poor hydraulic conductivity. The latter means you can’t fix the problem
by leaching excess Na.
To remediate a sodic soil, add gypsum, CaSO4. It works in a two-fold way:
The Ca2+ will form calcite, CaCO3, which will lower the pH to the equilibrium
Value set by calcite solubility, about 8.4, high but much better.
Second, whereas Na+ is a dispersing cation (tends to break up soil aggregates
and clog pores, reducing conductivity), Ca2+ does the opposite. So, gypsum
restores freer water flow so that the excess Na can be leached.
How do lab analysis show salinity and sodium problems?
Salts are indicated by the electrical conductivity (EC) of a saturated soil paste.
A soil is saline if the EC > 4 dS / m
Sodium problem is indicated if the concentration of Na, relative to Ca + Mg,
in a water extract of soil is above a certain threshold. This parameter is
called the sodium adsorption ratio, SAR.
A soil is sodic if the SAR > 13.
There can be situations when the soil is saline and sodic, saline-sodic.
Remediation of saline-sodic soils is the same as for sodic soils, gypsum.
Comments on Soil Water
There is a relationship between soil water potential and soil water content.
This is called the soil moisture characteristic curve. It is soil-specific.
This is the general shape.
As the water potential
decreases (left to right
in figure) water content
decreases. This occurs
because as a soil drains
or dries, the water it
contains is held ever more
tightly by soil solids –the
tension in the water
greatly increases.
Gravitational
Plant-Available
Unavailable
-0.2 Bar
This water is
held at too great
tension for
plants to extract
it from the soil.
-15 Bar, Wilting Point
Field Capacity
This water drains so quickly from the root zone that little is available for
plant use.
Factors affecting plant-available water
Texture
Organic Matter
Which soil is more droughty, a silt loam with high organic matter or a sand
with low organic matter?
Field hydrologic cycle
Want to conserve water by:
Increasing infiltration /
reducing runoff
Reduce evaporation
Reduce transpiration of
competing weeds
But have adequate
surface and internal
drainage so aeration is
good in root zone.
Surface drainage is a matter of smoothing, grading and making channels.
Subsurface drainage is through a network of parallel buried pipe.
Non-point Source Water Pollution from Agriculture
Growing issue with potential regulation of operations.
Main effect is on downstream water quality, particularly suspended solids and
nutrients (N, P and C), that affect levels of dissolved oxygen and / or change
the ecology of the water bodies.
Suspended solids is largely a matter of soil erosion and its control.
N and P lost in surface runoff or internal drainage to some outlet enrich
downstream water (eutrophy), leading the ecological changes, including
reduced oxygen. Gulf of Mexico hypoxia is thought to be caused by N and P.
A problem with using animal waste as a fertilizer is that the nutrient content is
not optimal. In particular, if you base the rate of application on N content, then
much more P is added than can be used by the crop so soil P builds-up with
repeated applications, increasing the likelihood of loss to surface waters.
Numerous best management practices (BMPs) can be used to reduce
soil erosion and nutrient losses.
Buffer or filter strips trap suspended
solids and reduce nutrient losses.
Cover crops reduce runoff and conserve
nutrients or reduce fertilizer needed.
Vetch
Conservation tillage leaves the soil
protected with residue. Less erosion,
greater infiltration and build-up of
organic matter in surface soil.
Water and Wind Erosion and Their Control
The two are conceptually similar. They involve detachment and movement of
surface soil by fluids. Consequently, the factors affecting them are similar.
Factor
Water Erosion
Wind Erosion
Climate
Heavy rainstorms
Dry and windy
Soil
Fine-sandy to silty
most erodible
Fine-sandy to silty
most erodible
Topography
More erosion on
long, steep grades
More erosion on smooth,
wide-open areas
Plant cover
Good cover protects
soil and limits erosion
Good cover protects
soil and limits erosion
Control practice
Contour rows, stripcropping, terracing
Rough-up surface, windbreaks
Soil Survey
Spatial map of soil types in county with chemical and physical data for each
and interpretation of their relative suitability for different uses.
Available in hardcopy, CD and web forms (Web Soil Survey).
Soil types referred to as mapping units.
Locate soil type at a site by using index map to find correct areal photo map sheet,
then go from there based on landmarks or legal description.
Physical and chemical data are given for each horizon.
Particularly relevant table is land capability classification (classes I to VIII).
Class
Severity of Limitation
I
II
V
VIII
None.
Minor.
Major.
Severe.
Can be intensively farmed.
Major limitation given as lower case letter, i.e., e = erodible, w = wet
Only perennial vegetation, i.e., pasture OK
Only recreation.
Soil Morphology and Classification
There are 5 master horizons (vertical zones) in soils. Some soils have all, some
have only 2. Except for the lowermost horizon, these developed as the soil
aged in place.
O
A
E
O = organic material above mineral soil
A = upper mineral, enriched in organic matter
B
B = clay (or salts) accumulated from above
C
C = close to presumed original geologic material
E = little organic material or clay (or salts)
Besides master horizons, there are numerous
secondary horizon designations and transitional
horizon designations, e.g., Ap = plowed, Bt = clay
Classification
US system is called Soil Taxonomy. It is analogous to biological classification
System, including soil order (parallel to phylum) as the most general level, and
series (parallel to species) as the most specific.
The simplest of the 12
soil orders is the Entisol.
It only has A and C
horizons. Thought to be
young.
The order Alfisol is common
in soils formed under forest.
It shows A, E, Bt and C
horizons and is considered
to have been developing
in place for a long time.
Mollisols are prairie
soils with typically
high organic matter.
Prized agricultural
soils.
This is an Oxisol from
the humid tropics. The
red color is from oxidized
iron. These are highly
weathered, infertile soils.
Similar to the Oxisol is this
Ultisol from Louisiana. These
forest soils are less highly
weathered and more fertile
than Oxisols.