| Abstract: | Abstract The influence of reaction time and ionic environments, on phosphate adsorption were studied using one calcareous soil from India, and one calcareous and two latosols from Hawaii. Phosphorus adsorption by soils has a initial rapid phase followed by a slow process. For plant nutrition studies, where emphasis is on P concentration of solutions from which plants derive P, isotherms should be constructed using data obtained after near‐equilibration has been attained. This condition does not obtain in a few hours and may require 6 days or more. Calcium chloride as suspending electrolyte always gave lower phosphate solubility than when KC1 was used as electrolyte. Phosphate retention increased with increasing ionic strength. The necessity for obtaining clear supernatant solutions and the desirability for maintaining reasonable constant equilibrium conditions make 0.01 M CaCl2 a reasonable choice for constructing P sorption isotherms, even though 0.01 M CaCl2 is not representative of Ca concentrations in many soil solutions. Saturation extracts of soils investigated here were in the range 0.0002 to 0.005 M Ca. Adsorption of calcium by highly weathered soils was high suggesting specific adsorption. Calcium adsorption was increased by phosphate additions to a Hydrandept. |
