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-degrees-C for 30 min at 200 V, followed by a second 30-min extraction at 80-degrees-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-degrees-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 NH4+-exchangeable K+, provided the soil is low in exchangeable K+.