Soils - Part 4: Soil pH
pH Capacity and Intensity
Two types of soil pH are of concern: capacity (reserve) and intensity (activity).
Some H+ ions will be adsorbed on the cation exchange sites (not in solution). These adsorbed H+ ions are sometimes referred to as reserve or potential acidity. The buffer pH is the test used to estimate lime requirement. The lime requirement is the amount of ag lime (calcium and/or magnesium carbonate) required to neutralize the H+ from the soil exchange sites and in the soil solution. The cations of Ca++ and Mg++ in the ag lime replace the H+ on the soil and/or organic matter negatively charged exchange sites.
Cation Exchange Capacity (CEC) refers to the exchange of one cation for another on the surface of a soil particle or colloid. Soils vary in CEC value because of makeup of the solid part of the soil. The solid part includes decaying plant materials, humus, clays, sands, silts, and other mineral components. The CEC value of a soil is measured in milliequivalents /100 grams. Clay components of soil may have a CEC value of 10 to 80 me/100 g. while organic humus particles will have CEC values of 100 to 200 me/100 g. A Valentine sand might have CEC values near 3-6 me/100 g., while Sharpsburg soils might be closer to 16-25 me/100 g. The CEC value of a soil is mainly a function of clay content and organic matter (see Figure 4.1).
Table 4.2 illustrates the relationship between soil pH, buffer pH and lime requirement for soils with different soil textural classifications. Each soil in Table 4.2 is 70 percent base saturated (30 percent hydrogen saturated); that is, 70 percent of the exchange capacity is filled by basic cations such as Ca++, Mg++, K+, and sodium (Na+) and very small amounts of micronutrient bases such as Zn++. The soil pH (measurement of active hydrogen) values are basically the same for all three soils, but note differences in “reserve H+”, 1.8 to 7.5 me/100g which is measured by the buffer pH and why the amount of lime required to raise the soil pH to 6.5 is 4 tons/acre for the silty clay loam soil compared to 1 ton/a for the loamy sand.
This is the soil pH that is most familiar. It is important to remember that soil pH is a measure of the H+ activity of the soil solution. It is not a measure of the total acidity of the soil. The amount of H+ in the soil solution is a function of the proportion of soil cation exchange capacity that is satisfied by H+ ions, not the total amount of exchangeable H+ in a given soil (Table 4.2). Table 4.3 shows the expected pH range for most agricultural soils.
1These are acid peat soils not found in Nebraska.
2Alkali mineral soils may be found only in extreme alkali spots in Nebraska.
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