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figure 4.9 Potassium solubility of various soils related to their type of clay minerals (Adapted from A.N. Sharpley, Soil Sci. 149:44-51, 1990.)

with 400 V at 80oC. The first fraction contains the nonhydrated adsorbed K+ plus the K+ in the soil solution, whereas the second fraction contains the interlayer K+. The extraction curves are shown for four different soils in Figure 4.10, from which it is clear that the K+ release of the second fraction is a first-order reaction (101). The curves fit the first-order equation, the Elovich function, the parabolic diffusion function, and the power function, with the Elovich function having the best fit with R2 > 0.99.

4.5.3 Plant-Available Potassium

Several decades ago it was assumed that the 'activity ratio' between the K+ activity and the Ca2+ plus Mg2+ activities in the soil solution would describe the K+ availability in soils according to the equation (102)

In diluted solutions such as the soil solution, the K+ activity is approximately the K+ concentration. It was found that this activity ratio does not reflect the K+ availability for plants (103). Of utmost importance for the K+ availability is the K+ concentration in the soil solution. The formula of the AR gives only the ratio and not the K+ activity or the K+ concentration. The K+ flux in soils depends on the diffusibility in the medium, which means it is strongly dependent on soil moisture and on the K+ concentration in the soil solution, as shown in the following formula (104):

where J is the K+ flux toward root surface, D1 the diffusion coefficient in the soil solution, q the K+ concentration in the soil solution, D2 the diffusion coefficient at interlayer surfaces, c2 the K+ concentration at the interlayer surface, x the distance, dc/dx the concentration gradient, c3 the K+ concentration in the mass flow water, and v the volume of the mass flow water.

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