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Diffusion Kinetics of Multinutrient Accumulation by Mixed-Bed Ion-Exchange Resin

Dep. of Agricultural Chemistry, Kangweon National Univ., Chuncheon 200-701, Korea

Earl O. Skogley^*

Dep. of Plant and Soil Science, Montana State Univ., Bozeman, MT 59717

* Corresponding author.

ABSTRACT

Conventional batch or miscible-displacement methods for studying nutrient release in the soil-solution-root continuum have been confined to single-ion-release reactions that result in the disintegration of concentration gradients due to excessive water and chemicals used in the procedure. The Phytoavailability Soil Test (PST) accumulates all nutrients simultaneously, as a function of diffusion to a mixed-bed resin sphere. Objectives of this research were to compare kinetic models to describe empirically multinutrient accumulation by the PST method, and to test its sensitivity to conditions that influence diffusion kinetics. Accumulation of nutrients by resin was measured by inserting resin spheres into saturated soil pastes for 96 h at 25, 35, or 45°C, and stripping resin-accumulated nutrients with HCl. Existing kinetic models all described multinutrient accumulation, except for NO₃-N. The parabolic diffusion law model consistently provided the best fit. Values for reaction rates and net reaction energy (E_an) were of similar magnitude to those reported in the literature. Positive values of G^o and H^o and negative values of S^o (pseudo thermodynamic parameters) for K and P accumulation were similar to those reported for release of these nutrients. Significant relationships were found between rate constants of K, P, S, and NH₄-N and respective ion activities in soil solution. Accumulation of all nutrients was directly related to temperature, except NO₃-N, which was temperature and soil specific. These results suggest that the PST method has advantages over conventional methods for measuring nutrient dynamics, as it more closely simulates field conditions, while providing for multinutrient studies.

Contribution no. J-2537 from Montana Agric. Exp. Stn.

Received for publication July 12, 1990.

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