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

In drip-irrigated soil treated with high rates of NH4 fertilizer, acid ification 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 physio chemical properties, the cation dynamics involving Al in acidic bulk a nd rhizosphere soil may also differ. We examined cation-exchange equil ibria between Al-K and Al-Ca pairs in bulk and rhizosphere soils gener ated by growing peach seedlings. Overall, compared with a calculated n onpreference line, K and Ca were preferred over Al in Arbuckle soil (f ine-loamy, mixed, thermic Typic Haploxeralf), but soil selectivity for each cation was highly dependent on the soil conditions. The selectiv ity 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 individua l soil properties on the exchange reactions could not be fully determi ned, there were significant differences in the exchange equilibria bet ween the bulk and rhizosphere soil. When we compared the cation exchan ge of bulk and rhizosphere soil, the preference for Al over K was appa rent in the bulk soil and the preference for Al over Ca was apparent i n the rhizosphere soil. Because a strong Al adsorption fan reduce Al l evel in solution and the Al toxicity in acidic conditions, the less pr eferred Ca ion will be less effective than K in displacing Al. This ef fect would be greater in the rhizosphere soil where Al selectivity was greater than that of Ca.