AMMONIUM-POTASSIUM AND AMMONIUM-CALCIUM EXCHANGE EQUILIBRIA IN BULK AND RHIZOSPHERE SOIL

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 pA decreased, the selectivity for NH4 over Ca increased but pA 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 not 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.