Page 43 - Nutrient Management Planning Guide

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Chapter 2.2

(K + ) or calcium (Ca 2+ ) and positively charged sites attract negatively charged ions (anions) such as nitrate (NO

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- ) and chloride (Cl - ).

Adsorption retains nutrients in the root zone making them easily accessible to growing crops. Adsorbed cations are loosely held to the negatively charged surfaces of soil particles. This association is strong enough that adsorbed ions resist being leached by the downward movement of water through the soil profile. However, it is also weak enough for adsorbed ions to be replaced by other cations in soil solution. This substitution, known as cation exchange, occurs largely through competition between ions for the negatively charged exchange sites on the particle surface. The common cations in soil are listed below, in order of increasing adsorption strength:

Na + < K + = NH

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+ < Mg 2+ = Ca 2+ < Al 3+ < H +

The amount of exchangeable cations per unit weight of soil (on a dry basis) is referred to as cation exchange capacity (CEC). It estimates the number of exchange sites in a given soil sample that would be capable of holding positively charged crop nutrients. The larger the CEC, the more cations the soil can hold. Increasing the organic matter content of soils with low clay content will help to increase the CEC. Managing soil pH will also help optimize the CEC.

Soil Fertility Implications of CEC

Two soil characteristics related to CEC are base saturation and buffering capacity. Base saturation refers to the percentage of the CEC occupied by K + , Ca 2+ , Mg 2+ , and Na + . Soils with higher percent base saturation have higher levels of available K + , Ca 2+ , and Mg 2+ for growing crops.

Buffering capacity refers to the ability of a soil to replenish ions in soil solution. Soils with a high buffering capacity usually have large amounts of clay and organic matter. Soils with lower buffering capacity have a limited ability to replenish nutrients; therefore, they require more frequent nutrient additions to maintain fertility.

Organic Matter

Soil organic matter consists of materials, such as animal and plant residues, at various stages of decay. The organic matter content of a soil depends on the balance of two activities—the addition of organic residues to the soil and the decomposition of residues by soil macro- and microorganisms (Figure 2.2.1). The result of decomposition is a dark, stable end product (i.e., it does not change much with time) called humus. Over the long-term, however, all nutrients found in soil organic matter are converted into simple end products such as carbon dioxide, water and nutrients.