AVAILABILITY OF POTASSIUM TO RYEGRASS FROM SCOTTISH SOILS I. EFFECTS OF INTENSIVE CROPPING ON POTASSIUM PARAMETERS

Scottish soils were cropped with two sowings of perennial ryegrass (Lolium perenne, cv, Dutch Barenza) in the glasshouse, without addition of potassium, until growth virtually ceased. Potassium uptake and the corresponding changes in soil K properties were examined. Uptake of K correlated well over the entire cropping period with the initially labile K predicted from the Q/I isotherms of the soils, although K uptake was at least a factor of 1.9 greater than the initially labile K. The Q/I isotherms were similar for soils from the same series, and the form of the lower part remained virtually unchanged during intensive cropping. The equilibrium activity ratio, AR₀, of soil K measured in 0.01 M calcium chloride was reduced in all soils to a narrow range close to a mean value of 3 × 10 ⁻⁴ M^1/2. Drying‐and‐wetting fresh soil samples severely depleted of ryegrass‐available K raised their K status. The increases were negatively correlated with the percentage K saturation of the fresh depleted soils, and a K saturation of 1.4 per cent must apparently be left in the fresh cropped soils for no K to be released. Freezing‐and‐thawing did not affect the K status of these soils.