It is known that PO4 is retained by soils through ligand exchange, i.e., inner sphere complexation, but the mechanism for SO4 adsorption at the mineral‐water interface has been in debate. By studying the effects of ionic strength on ion adsorption, it is possible to distinguish between inner and outer sphere ion surface complexes. This study was conducted to evaluate ionic strength effects on SO4 and PO4 adsorption on γ‐Al2O3 and kaolinite at varying solution pH (3–11), and to infer SO4 and PO4 adsorption mechanisms at the mineral‐water interface. The adsorption of SO4 on γ‐Al2O3 and kaolinite decreased monotonically with increasing solution pH and was markedly reduced by increasing the concentration of background electrolyte. On the other hand, PO4 adsorption on γ‐Al2O3 and kaolinite increased from pH 3 to 4 and decreased from pH 6 to 11, with an adsorption plateau between pH 4 and 6. Effects of change in ionic strength on PO4 adsorption varied with pH. At low pH, PO4 adsorption demonstrated a slight decrease with increasing ionic strength, whereas at high pH, PO4 adsorption increased slightly with increasing ionic strength, resulting in a crossover point where there was no ionic strength effect. The triple‐layer model (TLM) was applied to model the adsorption of SO4 and PO4 with both inner and outer sphere complexes using the FITEQL 3.1 computer program. Sulfate adsorption was better modeled by assuming outer sphere complex formation, while PO4 adsorption was better modeled by assuming inner sphere complex formation.

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Efficiency of fertilizer uptake may be improved by selecting genotypes that absorb a greater proportion of the nutrients in the soil. The objective of this investigation was to examine genotypic differences in ion influx (uptake rate per cm of root) properties of corn (Zea mays L.) roots that may influence efficiency of fertilizer uptake. Six genotypes developed in Florida and six genotypes developed in Indiana were investigated. Each represented two single‐cross hybrids and their parents. They were grown for 15 days in nutrient solution in a controlled climate chamber and the influx parameters of K, Ca, Mg, and P were determined by measuring the depletion of these ions from solution. All hybrids exhibited heterosis in the growth of shoots and roots. Florida genotypes had finer roots than Indiana genotypes. Florida genotypes had higher average Imax (maximum influx) values for Ca and P. Parent lines of Florida hybrids had higher average Imax values for Ca and lower for K and Mg than hybrids. Indiana genotypes had higher average Km, Michaelis‐Menten, values for K uptake and lower Km values for P uptake than Florida genotypes. Florida genotypes had significantly higher average Cmin (concentration in solution where net influx is zero) values for Mg and P and lower for K than Indiana genotypes. Even though Florida and Indiana genotypes originated in different geographic regions they exhibited little difference in their average net influx rates for Ca and Mg, however, there were small but significant differences between these genotypes in average influx values for P and K. Florida inbreds with higher P influx rates tended to have hybrids with higher P influx rates. While the differences in this study were not large, investigation of a larger number of genotypes is needed to determine the possibilities of developing hybrids with roots that are more effective nutrient absorbers.

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