Phytoavailability Soil Test: Development and Verification of Theory

Abstract

AbstractA new approach to soil testing, designed to improve accuracy and usefulness, is being developed. A spherical, mixed‐bed ion‐exchange resin capsule is embedded in saturated‐paste soil samples and allowed to accumulate all nutrients the soil can deliver, as a function of diffusive ion movement. This simulates the action of nutrient movement to plant roots. The approach is called the Phytoavailability Soil Test (PST). Objectives were to determine if nutrient accumulation by the resin capsule was described by diffusion models, and to investigate rate‐limiting diffusion processes. Nutrient‐uptake curves were obtained on Ap horizon samples from 20 Montana agricultural soils by measuring time dependence of K, P, and S accumulation in resin capsules during 15 d. Also, a planar system was used in which both sides of a cylinder of resins accumulated P, K, and S from saturated pastes during 96 h. Fluxes to the resin were compared with nutrient gradients, measured by two other extraction methods, on soil slices from successive distances beyond the resin‐soil interface. The mass of nutrients adsorbed by the resin as a function of time was consistent with a diffusion process, as was the concentration of nutrients in the soil vs. distance from the resin‐soil interface. The process appeared to be that termed film diffusion. Results demonstrated independence of specific nutrient diffusion and resin adsorption. The PST may provide an improved approach to soil testing because it is sensitive to nutrient diffusion, a governing factor in plant availability of most nutrients.