A PASTE METHOD FOR ESTIMATING CONCENTRATIONS OF AMMONIUM, CITRATE, AND PHOSPHATE IN SOIL SOLUTION

A paste method was evaluated for estimating the concentrations of NH4, NO3, and PO4 in soil solution. The method involves the addition of en ough water, e.g. 60 mt, to a fresh soil sample, e.g. 240 g, to prepare a quasi-saturated paste. The paste is then equilibrated for Ih at roo m temperature before centrifugation and filtration. Solutes in the fil trate are analysed by routine methods. Solute concentrations in the or iginal soil solution are estimated using a formula to account for the change in water content and an appropriate solid-liquid partition coef ficient (K-d) of the soil for each particular solute. The method was t ested on 3 soils (clay loams derived from basalt, sandstone, or siltst one), each with and without fertilisers applied in field experiments. Dilution effects were evident with all soils for NH4 and NO3, which we re poorly buffered, but not for PO4, which was well buffered. Dilution effects for NO3 were adequately accounted for by assuming no bufferin g, i.e. K-d = 0, but for NH4 a K-d value derived from a desorption iso therm by using the paste method needed to be assumed. Based on theoret ical relationships, a guide is provided to the error associated with p articular combinations of dilution ratio and K-d, when the latter is a ssumed. Discrepencies that occurred between estimated concentrations u sing the paste method and those collected using in situ soil solution samplers soon after fertilisation were consistent with the expected di fferences between the 2 methods in terms of the temporal and spatial s ampling of soil solution plus the possibility of microbial immobilisat ion of extracted nutrients after in situ, collection. We conclude that the proposed paste method will be useful for estimating concentration s of nutrients in soil solution.