An experimental approach to quantifying chemical and positional availability of soil potassium

Abstract

We propose a new experimental approach that enables to evaluate the chemical and positional availability of soil potassium (K) to pot-grown plants. The proposed approach makes use of the inherent differences between the removal of soil K by plants and by chemical methods, and comprises the following procedures: 1) preparation of both planted and unplanted soils by conducting a pot experiment; 2) stepwise extraction of K in these soils with neutral 1 M NH4OAc, followed by repeated treatment with 0.01 M HCl; and 3) calculation of the chemical and positional availability of K by determining the amount of K removed in each step of extraction. By applying this method to K-bearing minerals and a Fluvisol sampled from pots in which wheat (Triticum aestivum L.) and komatsuna (Brassica campestris L. var. rapa) had been grown for 30 d, we could estimate that more than half of the exchangeable K in a pot was located within the range of root removal (positionally available), and that the percentage of K in a positionally available form decreased with the decrease in its chemical availability. Furthermore, the application of this method to other published data convinced us that, in principle, the method is applicable to all the pot-grown plants and also to various soils and minerals. Since the proposed approach does not require a special pot for regulating root growth, it may further provide a clue for the development of a quantitative field method.