Page 38 - Nutrient Management Planning Guide

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Soil Nutrient Cycling

Crop removal

Plant uptake

Dissolved metals

Sorption

Desorption

Precipitation

Dissolution

Immobilization Mineralization

Fe/Al oxides, clay, O.M.

Soil organic matter

Metal minerals

Erosion

Plant residue

Ca and Mg entering the exchangeable pool. Generally, there is less Mg in the soil solution than Ca. Even though plants require Mg in smaller amounts it is more likely to be deficient than Ca. This is partly because Mg binding to cation exchange sites is weaker than competing cations such as K + , Ca 2+ , and NH

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+ . Consequently, plants growing on soils with excesses of any of these cations are more likely to show symptoms of Mg deficiency. Erosion is the main route of Mg and Ca loss from the soil.

Micronutrients

Although micronutrients (Cl, Fe, Mn, Zn, B, Cu, and Mo) are required in “micro” quantities, they are significant in terms of their contribution to plant growth.

Adapted from Jones and Jacobsen 2003b

Figure 2.1.13 General Pattern of Micronutrient Cycles in Soils

Soil characteristics can influence micronutrient availability. Clay soils are less likely to be deficient in micronutrients than sandy soils. Soils with low (i.e., less than 1 to 2%) or very high organic matter content (i.e., greater than 30%) often have low levels of micronutrient availability. As soil pH increases, availability of micronutrients tends to decrease. The exception is molybdenum whose availability increases with soil pH. Most soil micronutrient cycles follow much the same general pattern as in Figure 2.1.13.

Inorganic micronutrients occur naturally in mineral soils. As parent minerals break down during soil formation, micronutrients slowly become available to plants. Organic matter is also an important source of micronutrients. Microbial decomposition helps to release micronutrients into plant-available forms.