MEASURING INTERLAYER POTASSIUM RELEASE RATES FROM SOIL MATERIALS - II- A PERCOLATION PROCEDURE TO STUDY THE INFLUENCE OF THE VARIABLE SOLUTE K IN THE LESS-THAN-1 - 10 MU-M RANGE

Release rates of nonexchangeable K from Ap material of a Luvisol ('Eck erde' loess, 15% clay) are determined using a percolation procedure wh ich avoids the common artifacts due to shaking or stirring soil;suspen sions and thus provides less biased kinetic data. CaCl2 solution (10 m M(c), 20 degrees C, pH 5.8) is percolated through packages of soil agg regates (0.5-1 mm grain size, 0.5 g samples) with 0.02 to 25 ml h(-1) Solute K (C-K) was varied between <1 and > 10 mu M and is shown to hav e a dominant influence on the rates of interlayer K release. These inc rease exponentially below 3.5 CIM (no steady state but steady decrease of release rates in each sample). The difference between the related C-K between moderate and high release rates is as small as 1 mu M K. T he average rates of a 10-day-interval, starting after 1.3 times the ex changeable K had been removed, are 40 mu mol K kg(-1) soil d(-1) at 4 mu M C-K and 240 mu mol kg(-1) d(-1) at 3 mu M C-K,C- respectively It is concluded that larger quantities of interlayer K become plant avail able in the studied soil if the C-K Of soil solutions gets below 3.5 m u M (for 10 mM, Ca, 20 degrees C, pH 5.8), probably because the domina nt dioctahedral illites start to join the release process below this c ritical limit. The higher K concentration range was accounted for by K -Ca exchange isotherms which, by alteration of shape, indicate that K- nex release becomes measurable below 10 to 20 mu M K.It is further arg ued that existing diffusion or reaction kinetics approaches towards K release are incomplete because the influence of solute K is not consid ered.