Soil diffusible phosphate and dependence of phosphate diffusion coefficient exogenous factors

Samples of a loamy loess soil have been pretreated for 7 weeks with 32P-orthophosphate and afterwards formed in small blocks (2 cm x 3 cm x 1 cm). Subsequently, they were pressed with the front side to the surface of similarly formed anion exchanger blocks. The diffused phosphate content and the diffusion coefficient of phosphorus were calculated on the basis of the 32P and total P amounts found in the anion resin after a diffusion period of 80 minutes. The following relationships were obtained: The diffusible fraction of orthophosphate in the soil under study corresponded with the labile phosphate. The latter could be estimated nearly by water extraction. When the soil water content continued to rise and water-soluble orthophosphate was added prior to the diffusion experiment, the diffusion coefficient increased from 0.75 · 1010 cm2/sec. to 67.37 · 10-10 cm2/sec. This behaviour was attributed to the improved mobility of phosphate ions in thicker water films and the saturation of P sorption positions due to the added phosphate. (author)