Download or read book Factors Affecting the Availability of Potassium Through Fixation and Release in Soils of Diverse Mineralogical Composition written by Gordon Lawrence Rees and published by . This book was released on 2015 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The management of soil potassium is complicated by sorption and release of K by soil minerals, with fixation of K by vermiculite removing it from the pool of readily-available exchangeable K. Several laboratory methods have been developed to characterize different pools of soil potassium, including ammonium acetate extraction of exchangeable K (XK), sodium tetraphenylboron extraction of exchangeable and some nonexchangeable K (TPB-K), and a method to estimate the potential of soils to fix K (Kfix). In order to understand the relationship of these methods to each other and to K uptake by plants, we undertook a series of experiments to evaluate the effect of K concentration, incubation time, soil moisture content, and mineralogy on soil K pools. Additions of K equal to initially measured Kfix values only partially reduced K fixation, and increased XK and TPB-K by less than the amount of K added, indicating that some K was fixed strongly enough to be removed from the nominally plant-available pool. Duration of incubation with K from 1 to 16 days did not significantly impact K fixation or availability as measured by these methods, indicating that fixation reactions took place within the first 24 hours. Incremental additions of potassium up to an amount equal to the cation exchange capacity resulted in increasing amounts of K fixed. For some soils, a plateau was reached indicating a maximum level of K fixation, but for other soils it is unclear if a maximum was achieved. Recovery of K by the TPB-K method was roughly twice as efficient as by the XK method. Approximately 50% of fixed K was plant-available nonexchangeable K (PANK), meaning it was recovered by the TPB-K method but not by the XK method. A single air-drying event, relative to soils maintained at field-moist water content, resulted in an increase in Kfix for all K-fixing soils by an average of 55 mg kg−1, but the change in Kfix for non-K-fixing soils was not consistent. The change in Kfix was not correlated with XK values. Changes in XK with drying were less than 20 mg kg−1 for most samples, with XK increasing for most K-fixing and low-XK soils, with less consistency for high-XK soils. Multiple cycles of wetting and drying did not result in further changes in soil K measurements. In a greenhouse pot study, K uptake by annual ryegrass (Lolium multiflorum) was best correlated with TPB-K (r=0.8966), and yield was best correlated with total K by aqua regia digestion (r=0.7465) followed by TPB-K (r=0.6692). The critical value for yield response determined by the TPB-K method was most successful in predicting significant responses to increased K rates. The XK method also performed well in this respect. K fixation by one soil resulted in reduced uptake of K, and in yield responses to higher K rates than predicted by critical values for all methods. For other soils, the effect of K fixation potential was unclear. Atomic force microscopy of clay grains from selected K-fixing soils and standard clays revealed complex surface morphology for most soils which made it impossible to delineate changes in layer spacing with K sorption or release. Nearly-flat surfaces and stacks of relatively few layers in smectite clays were more conducive to this analysis. For both montmorillonite and beidellite standards, layers collapsed with displacement of Na+ by K+, and re-expanded after extraction with a TPB solution. No zones of preferential K fixation or release were observed.