Different
types of clays also vary in
cation-exchange capacity (CEC).
Smectites have the highest
cation-exchange capacity (CEC) (80-100
millequivalents 100 g-1), followed by illites (15-40 millequivalents 100
g-1) and kaolinites (3-15 millequivalents 100 g-1).
Examples of
CEC values for different soil textures are as follows:
Soil texture
CEC (meq/100g soil)
Sands (light
colored) 3-5 (meq/100g soil)
Sands (dark
colored) 10-20 (meq/100g soil)
Loams 10-15
(meq/100g soil)
Silt loams
15-25 (meq/100g soil)
Clay and
clay loams 20-50 (meq/100g soil)
Organic
soils 50-100 (meq/100g soil)
In general,
the CEC of most soils increases with an increase in soil pH. Two factors
determine the relative proportions of the different cations adsorbed by
clays.
First, cations
are not held equally tight by the soil colloids. When the cations are
present in equivalent amounts, the order of strength of adsorption is: Al3+
>Ca2+ > Mg2+ > K+ = NH4+ > Na+.
Second, the
relative concentrations of the cations in soil solution helps determine
the degree of adsorption. Very acid soils will have high concentrations of
H+ and Al3+. In neutral to moderately alkaline soils, Ca2+
and Mg2+ dominate.
Poorly drained
arid soils may adsorb Na in very high quantities.
Base
Saturation:
The proportion of
cation-exchange capacity (CEC) satisfied
by basic cations (Ca, Mg, K, and Na) is termed percentage base saturation
(BS%). This BS% is inversely related to soil acidity.
As the BS% increases, the
pH increases. High base saturation is preferred but not essential for tree
fruit production. The availability of nutrient cations such as Ca, Mg, and
K to plants increases with increasing BS%.
Base
saturation is usually close to 100% in arid region soils. Base saturation
below 100% indicates that part of the CEC is occupied by hydrogen and/or
aluminum ions. Base saturation above 100% indicates that soluble salts or
lime may be present, or that there is a procedural problem with the
analysis.
CEC and
nutrient availability:
Exchangeable cations, as
mentioned above, may become available to plants. Plant roots also possess
cation exchange capacity. Hydrogen ions from the root hairs and
microorganisms may replace nutrient cations from the exchange complex on
soil colloids. The nutrient cations are then released into the soil
solution where they can be taken up by the adsorptive surfaces of roots
and soil organisms. They may however, be lost from the system by drainage
water.
Additionally,
high levels of one nutrient may influence uptake of another nutrient. This
is called an antagonistic relationship. For example, K uptake by plants is
limited by high levels of Ca in some soils. High levels of K can in turn,
limit Mg uptake even if Mg levels in soil are high.
Anion Exchange:
In contrast to CEC, anion
exchange capacity (AEC) is the degree to which a soil can adsorb and
exchange anions. AEC increases as soil pH decreases. The pH of most
productive soils in the US and Canada is usually too high (exceptions are
for volcanic soils) for full development of AEC and thus it generally
plays a minor role in supplying plants with anions.
Because the
anion exchange
capacity (AEC)
of most agricultural soils is small compared to their
CEC, mineral anions such as nitrate (NO3 - and Cl-) are
repelled by the negative charge on soil colloids.
These ions
remain mobile in the soil solution and thus are susceptible to leaching.
When leaching occurs, there are often chelated subordinate materials which
will leave with the leaching elements in a peristaltic manner. This can
create serious trace mineral imbalances in the soil and if not corrected
can cause sufficient damage to the soil structure that it becomes bound
and cannot any longer release chelates.
This
sterilization is also known as chemical burning and is unfortunately a
serious soil condition in many parts of the world at this present time.
The addition
of earth worms (Eisenia fetida), specifically the hybrid red
wiggler, to a healthy soil environment increases the the extent to which a
nutrient can be utilized by the plant body by nearly 150%.
This is so because the
worms further break down the humus and the resultant castings are rich in
fulvic acid. Humic acid and Fulvic acid makes the ion salts available to
plant roots regardless of soil pH.
As you
understand the importance of the previous information, you can see why
Plant by Nature® formulations truly mitigate many otherwise
untenable soil conditions and really improve crop yield.
In addition
the plant sap is much richer in plant-specific content resulting in
sharply increasing growth parameters and sharply reduced susceptibility to
disease and parasitic elements.
As noted above
the pH is normally very relevant to plant nutrition. In addition to the
above, soils rich in humic acid also provide a perfect habitat for soil
micro flora and fauna where the substances tryptophan and sucrose become
optimally available for optimally efficient rhizome biom biomass activity.
Actinomycetes and algae are
also important fixers of atmospheric nitrogen that can used later by soil
organisms.
A single gram
of soil (about 1/5 teaspoon) can contain over 100 million bacteria, 1
million actinomycetes and 100,000 fungi with hyphae if strung together
would measure 5 meters in length. The weight of these organisms would only
account for 0.05 percent of the weight of the soil. The size of a single
bacteria is approximately 1 micron or 1 millionth of a meter - 100,000
bacteria placed end to end would measure 1 cm.
A special kind of
fungus called Mycorrhizae invades plant root cells and aid in
