NUTRIENT FLUX TO MIXED-BED ION-EXCHANGE RESIN - TEMPERATURE EFFECTS

The phytoavailability soil test (PST) is designed to assay plant-available nutrient quantity as a function of diffusive ion movement to a spherical, mixed-bed ion-exchange resin sink. Temperature is known to alter many parameters relating to nutrient availability and plant uptake. This study was conducted to determine PST sensitivity to temperature-induced changes in availability of NO3--N, NH4+-N, P, K, and S. Nutrient flux to resin accumulators was measured during 96 h at 25, 35, and 45-degrees-C. Fluxes of P, K, S, and NH4+-N all increased with increasing temperature, while the flux of NO3--N decreased. The size of the temperature effect was nutrient and soil specific. Nutrient-accumulation characteristics were significantly represented by quadratic equations, corresponding to diffusion-controlled processes. Initial flux (0.5 h) of each nutrient to the resin was rapid and highly related to nutrient ion activity in saturated-paste soil solutions equilibrated for 24 h. The slope of the accumulation curve decreased after the initial rapid flux. This is interpreted to suggest that diffusion characteristics of the soil control, to a large degree, the flux of nutrients to the resin accumulator after an initial rapid phase related to high nutrient activity in the soil solution. These results provide strong evidence that the PST system is diffusion controlled and that it is sensitive to conditions known to influence nutrient availability in soils, such as temperature and nutrient ion activities.