Determination of Available Interlayer Potassium and Its Uptake by Ryegrass

Nonexchangeable soil K + trapped between interlayers of clay minerals is an important source of K + for plants grown on many soils. The availability of nonexchangeable K + depends primarily on the release rate of K + from the interlayers of clay minerals. The objective of this investigation was to test whether the K + release rate of nonexchangeable K + can be measured with a modified electroultrafiltration (EUF) technique, which essentially involved removal of soil K + from an electrodialysis cell at 20°C for 30 min at 200 V, followed by a second 30‐min extraction at 80°C and 400 V. The experiments were carried out with two Alfisols (Argalfs), one Vertisol (Orthert), and one Entisol (Aquent). The cumulative K + extraction curves obtained with an extraction at 400 V and 80°C were described ( P < 0.01) by the first‐order equation, the parabolic diffusion equation, the power function, and the Elovich equation, with the latter showing the best fit. An important term of these equations is the constant b , which is indicative of the release rate of nonexchangeable K + . The b values obtained by the Elovich function and by the parabolic diffusion equation decreased with the depletion of nonexchangeable K + brought about by the uptake of K + by ryegrass ( Lolium perenne L. cv. Taptoe). Potassium uptake by ryegrass grown in Mitscherlich pots was correlated with the b values of the parabolic diffusion equation, the power function, and particularly the Elovich function. The latter finding shows that the b value represents a reliable indicator for the availability of nonexchangeable K + . The same is true for the NH 4 + ‐exchangeable K + , provided the soil is low in exchangeable K + .