AVAILABILITY OF POTASSIUM TO RYEGRASS FROM SCOTTISH SOILS II. UPTAKE OF INITIALLY NON‐EXCHANGEABLE POTASSIUM

The stable desorption parts of soil Quantity/Intensity isotherms were used to determine the contributions of initially exchangeable and non‐exchangeable potassium to plant uptake from ten soils. The activity ratio, AR^K= a_K/√^aC_a, Mg at which K was first taken up from non‐exchangeable sources varied from 3 × 10⁻³ to 8 × 10⁻³ M^1/2 depending on the soil. Uptake rates of two categories of initially non‐exchangeable K were linearly related to √times;. The first category appeared to be close to equilibrium with the initially exchangeable K, and gave effective diffusion coefficients of 10⁻⁷ cm² s⁻¹ for four soils. The second category gave diffusion coefficients from 10⁻²⁰ to 10⁻²² cm² s⁻¹, probably came from internal surfaces of micaceous clays, and began to be released at activity ratios below 3 × 10⁻⁴ to 6 × 10⁻⁴ M^1/2 depending on soil type. The soils fell into three groups, broadly consistent with soil series, on their ability to release the second category of potassium.