A paste method for estimating concentrations of ammonium, nitrate, and phosphate in soil solution

Abstract

A paste method was evaluated for estimating the concentrations of NH₄, NO₃, and PO₄ in soil solution. The method involves the addition of enough water, e.g. 60 mL, to a fresh soil sample, e.g. 240 g, to prepare a quasi-saturated paste. The paste is then equilibrated for 1 h at room temperature before centrifugation and filtration. Solutes in the filtrate are analysed by routine methods. Solute concentrations in the original soil solution are estimated using a formula to account for the change in water content and an appropriate solid{liquid partition coefficient (K_d) of the soil for each particular solute. The method was tested on 3 soils (clay loams derived from basalt, sandstone, or siltstone), each with and without fertilisers applied in field experiments. Dilution effects were evident with all soils for NH₄ and NO₃, which were poorly buffered, but not for PO₄, which was well buffered. Dilution effects for NO₃ were adequately accounted for by assuming no buffering, i.e. K_d = 0, but for NH₄ a K_d value derived from a desorption isotherm by using the paste method needed to be assumed. Based on theoretical relationships, a guide is provided to the error associated with particular combinations of dilution ratio and K_d, when the latter is assumed. Discrepencies that occurred between estimated concentrations using the paste method and those collected using in situ soil solution samplers soon after fertilisation were consistent with the expected differences between the 2 methods in terms of the temporal and spatial sampling of soil solution plus the possibility of microbial immobilisation of extracted nutrients after in situ collection. We conclude that the proposed paste method will be useful for estimating concentrations of nutrients in soil solution.