Page 34 - Nutrient Management Planning Guide

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Soil Nutrient Cycling

Plant uptake

Soil solution K

Sorption

Release

Plant release and residue

Fixation

Desorption Weathering

Fixed (non-exchangeable K)

Leaching

Exchangeable K

K

K

Clay

K K K

K

Parent minerals

Clay K

Erosion

Crop removal

Adapted from Jones and Jacobsen 2002b

Figure 2.1.9 The Agricultural Potassium Cycle

K Cycling in Soils

The main pathways for K in the soil are shown in Figure 2.1.9. Potassium, much like P, exists in pools with differing abilities to replenish crop available K. In soil, K occurs in four pools: soil solution, exchangeable, fixed and parent minerals.

The soil solution and exchangeable pools of K are in equilibrium with each other. Plants absorb K exclusively as the K + ion, which is the only form that exists in soil solution. Exchangeable K refers to ions adsorbed to exchange sites on soil particles. It accounts for 1 to 2% of soil K. When K is removed from soil solution by

plant uptake it is replenished by K released from the exchangeable pool. Likewise, if the concentration of K in soil solution exceeds that in the exchangeable pool, K will adsorb to the exchange sites. This equilibrium ensures a steady pool of available K.

Potassium fixation is the entrapment of the K + ion in the structure of clay minerals. Fixation accounts for 1 to 2% of soil K (Figure 2.1.10). The fixed pool is not able to release K at rates sufficient to meet the demands of growing crops. However, a portion of this pool will become available as the exchangeable and soil solution K supplies are depleted.