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Ammonium-Potassium and Ammonium-Calcium Exchange Equilibria in Bulk and Rhizosphere Soil

Res. Inst. of Industrial Science and Technology, Dongkwangyang, Chunnam, Korea

Robert J. Zasoski^*

Soils and Biogeochemistry Program, Dep. of Land, Air, and Water Resources, Univ. of California, Davis, CA 95616

*Corresponding author.

ABSTRACT

This study was conducted to determine whether differential soil properties in bulk and rhizosphere soil could influence cation-exchange equilibria. The pH of rhizosphere samples collected from peach [Prunus persica (L). Batsch var. persica] seedling roots grown in Arbuckle gravelly loam (fine-loamy, mixed, thermic Typic Haploxeralf) were 0.4 units lower than bulk soil. Organic C content, cation-exchange capacity, clay, and amorphous oxide content were higher in the rhizosphere. Mineralogical differences apparent to x-ray analysis were not evident. As soil pH decreased, the selectivity for NH₄ over Ca increased but pH effects on NH₄-K exchange were insignificant. Organic matter addition increased the selectivity for NH₄ over K, and Ca over NH₄. As exchange solutions were diluted, the selectivity of the higher valent cation increased. Although the effect of individual soil properties on the exchange reactions could be fully determined, there were significant differences in the exchange equilibria between the bulk and rhizosphere soil. In the rhizosphere samples, selectivity for NH₄ over K was relatively higher at an ionic strength of 0.02, but selectivity reversed at an ionic strength of 0.05. At ionic strengths of 0.02 and 0.05, the selectivity for NH₄ over Ca was higher in the bulk soil. The differential exchange equilibria between bulk and rhizosphere soils are expected to affect cation distributions in the soil environment around roots, nutrient uptake, and toxicity relations of plant roots.

Received for publication October 15, 1993.

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