STUDIES ON EXTRACTION AND ON AVAILABILITY TO PLANTS OF ADSORBED PLUS SOLUBLE SULFATE

Abstract

An 0.01 [image] solution of calcium monophosphate was chosen as an extractant for available S. For most soils this solution contained sufficient phosphate to displace maximum quantities of adsorbed sulfate at a solution soil ratio of 5:1. For some soils a wider solutioiusoil ratio was necessary. Displacement of adsorbed sulfate approached a maximum after about 24 hours. The decrease in extractable S was precisely related to the measured uptake of S by plants grown for 183 days in a glasshouse. This suggested that mineralization of S was unimportant. Yet mineralization of S could be demonstrated in incubation experiments. In a further experiment, mineralization measured after incubation in tubes was compared with apparent mineralization under plants {i.e., uptake + sulfate remaining in soil - sulfate initially present). The 2 values were correlated but the apparent mineralization was lower. The difference was almost as large as the average mineralization. Part of the difference is due to inability to recover, all the roots but this may not explain all of it. The observation that mineralization of S appeared to be lower under plants probably explains the good relationships between extraetable S and uptake.