Agriculture Reference
In-Depth Information
Chapter 9
Calcareous and High pH Soils
Balancing soil minerals to the Ideal Soil ratios requires three sets of information:
1. An accurate assessment of the potentially available soil reserves of eleven
elements: S, P, Ca, Mg, K, Na, B, Fe, Mn, Cu, and Zn.as well as soil pH
2. An accurate measurement or estimate of the soil's CEC, its functional Cation
Exchange Capacity.
3. An accurate measurement of the ratio and amount of the base cations Ca, Mg,
K, and Na presently held on the negative exchange sites in the soil. This is the
Base Cation Saturation Ratio,Albrecht's BCSR.
Number1 is straightforward enough. There are several soil test methods or
combinations of tests that can measure available or potentially available soil
minerals. The Ideal Soil ratios were developed using the Mehlich 3 test, which
gives a reasonably accurate assessment of all eleven elements in most soils
below pH 7. Problems arise when the Mehlich 3 or any of the common
multi-element soil tests are used to estimate CEC and exchangeable cations in
soils above pH 7 and soils with undissolved limestone particles.
Asoil's Cation Exchange Capacity is the sum total of the negative charges
available to make an electrostatic bond, an ionic bond, with a positively charged
ion. None of the common soil tests actually measure the soil's total negative
charge. What they do instead is extract the base cations Ca, Mg, K, and Na (along
with other elements) from the soil. The sum of the extracted base cations is used
to estimate the cation exchange capacity of the soil. Obviously this estimate will
only be valid if the amounts of Ca, Mg etc being used for the calculation have
actuallybeen extracted from negative exchange sites. In soils of pH 7.0 andbelow
this will usually be the case. To understand why we need to review what the
symbol pH stands for.
pH (p
otenz
H
ydrogen
) is the ratio and concentration of H+ Hydrogen ions to
OH- hydroxyl ions in an H
2
O (water) solution
.Another way of saying that is: pH
is the proportion and amount of negative charges compared to positive charges in
a water solution.At pH 7.0, + and - are equal and balanced. If there are more H+
ions than OH- ions, the pH is below 7 and is called acidic. If there are more OH-
than H+ ions the pH is above 7 and is called alkaline or basic. Importantly, the
solution will always strive for equilibrium, for all of the charges to be equalized.
Excess OH- ions will be attracted to and react with a source of positive charge +,
excess H+ ions will be attracted to and react with a source of negative charge -.
This will continue until all unbalanced charges have been neutralized, or until
there are no more readily available sources of + or - charges to react with.
