Release of nonexchangeable potassium from Finnish mineral soils

Abstract

Release of nonexchangeable potassium by treatment with 1 N HCI at 50°C was studied on basis of a material consisting of 330 samples of Finnish mineral soils. The results ranged from 1 to 830mg K/100g. The mean content of nonexchangeable acid-soluble potassium was in the surface samples of sand and fine sand soils 95±26 mg/100 g, in loam soils 165±31 mg/100 g, in silt soils 195±52 mg/100 g, in clay loam soils 258±32 mg/100 g, in silty clay soils 283±43 mg/100 g, and in heavy clay soils 345±126 mg/100 g. In the subsoil samples of loam, clay loam, silty clay and heavy clay soils the mean content was significantly higher than in the surface samples, or 283±51 mg/100 g, 404±56 mg/100 g, 535±53 mg/100 g, and 580±37 mg/100 g, respectively. The results seem to be high as compared with data reported from Sweden, Norway and Germany. The content of nonexchangeable potassium released by acid was to some extent connected with the clay content: the correlation coefficient in the whole material was r = 0.74***, but only about 0.5*** both in the separate groups of the 178 nonclay samples and the 152 clay samples. There was only a very low correlation between the contents of nonexchangeable acid-soluble potassium and readily exchangeable potassium. A somewhat higher correlation, r = 0.65***, was found for the relationship between the former and fixation of added potassium under »wet» conditions, but it was markedly decreased by the elimination of the effect of the clay content. Nonexchangeable acid-soluble potassium usually represented a lower part of the total potassium in the surface samples than in the subsoil samples, and also the proportion tended to be higher in the clay soils than in the coarser soils. It varied from 0.2 to 26.3 per cent in the small material studied. In most cultivated soils less nonexchangeable potassium was released from the samples of plough layer than from samples of deeper layers. In a podsol profile the minimum content of nonexchangeable and exchangeable potassium and the maximum of fixation of added potassium was found in the A2 horizon; in a brown podsolic soil all these test values decreased fairly regularly with depth. From some silt and silty clay soils incubated for three months at room temperature a large part, even more than 40 per cent of the added potassium was not recovered by the acid extraction. Ammonium acetate extracted from 9 to 85 per cent of the potassium applied before incubation, and the part of added potassium found as nonexchangeable acid-soluble form varied from 5 to 53 per cent. The equilibrium between the different potassium fractions in soil was discussed. It was supposed that differences in the ability of plants to utilize nonexchangeable potassium may partly depend on the level to which plant roots are able to decrease potassium concentration in the solution around the minerals.