Aluminum‐Potassium and Aluminum‐Calcium Exchange Equilibria in Bulk and Rhizosphere Soil

Abstract

AbstractIn drip‐irrigated soil treated with high rates of NH4 fertilizer, acidification can be significant and the level of soluble Al may be high. Whether these changes in bulk soil are reflected in the rhizosphere is unknown. Because rhizosphere and bulk soils may have different physiochemical properties, the cation dynamics involving Al in acidic bulk and rhizosphere soil may also differ. We examined cation‐exchange equilibria between Al‐K and Al‐Ca pairs in bulk and rhizosphere soils generated by growing peach seedlings. Overall, compared with a calculated nonpreference line, K and Ca were preferred over Al in Arbuckle soil (fine‐loamy, mixed, thermic Typic Haploxeralf), but soil selectivity for each cation was highly dependent on the soil conditions. The selectivity for Al over K or Ca was greater at higher pH and at higher organic matter level. As exchange solutions were diluted, selectivity for the higher valent cation (Al) 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. When we compared the cation exchange of bulk and rhizosphere soil, the preference for Al over K was apparent in the bulk soil and the preference for Al over Ca was apparent in the rhizosphere soil. Because a strong Al adsorption can reduce Al level in solution and the Al toxicity in acidic conditions, the less preferred Ca ion will be less effective than K in displacing Al. This effect would be greater in the rhizosphere soil where Al selectivity was greater than that of Ca.