Measuring interlayer potassium release rates from soil materials. II. A percolation procedure to study the influence of the variable ‘solute K’ in the < 1…10 μM range

Release rates of nonexchangeable K from Ap material of a Luvisol (‘Eckerde’ loess, 15% clay) are determined using a percolation procedure which avoids the common artifacts due to shaking or stirring soil suspensions and thus provides less biased kinetic data. CaCl 2 solution (10 mM c . 20 °C. pH 5.8) is percolated through packages of soil aggregates (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 μM and is shown to have a dominant influence on the rates of interlayer K release. These increase exponentially below 3.5 μM (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 μM K. The average rates, of a 10‐day‐interval, starting after 1.3 times the exchangeable K had been removed, are 40 μmol K kg −1 soil d −1 at 4 μM C K and 240 μmol kg −1 d −1 at 3 μM C K , respectively. It is concluded that larger quantities of interlayer K become plant available in the studied soil if the of soil solutions gets below 3.5 μM (for 10 mM c Ca, 20 °C, pH 5.8), probably because the dominant dioctahedral illites start to join the release process below this critical 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 μM K. It is further argued that existing diffusion or reaction kinetics approaches towards K release are incomplete because the influence of solute K is not considered.