Phosphorus Adsorption Isotherm in Soil Under Organic Farming Using Patch and Flow Methods

Abstract

A substantial amount of effort has been put into understanding soil P availability for soils under long-term organic farming. Soils of two organic farms were collected to evaluate phosphorus (P) adsorption isotherm. S1 and S2 were Sicum farm soils (loamy sand soils) with 23 years and 12 years organic farming, respectively. F1 and F2 were Faium farm soils (sandy clay loam soils) with 10 years and 2 years organic farming, respectively. Organic matter percentage for S1, S2, F1, and F2 were 10.4, 6.1, 22.9, and 16.8g/kg, respectively. P-adsorption isotherm was studied using two different methods, namely the standard patch method and a new flow method. In the new method, soil column were placed in glass column and continuous flow was induced by controlled suction. Solution, 10 times the weight of the soil, was applied and recycled until apparent equilibrium concentrations were observed. P adsorption was evaluated on the basis P partition coefficients (Kd) calculated from Freundlich isotherm equations. Amount of phosphorus adsorbed, at equilibrium, and Kd was higher for soils with lower organic matter, for both farms and both methods. Flow method show more phosphorus adsorption at equilibrium and higher Kd than batch method, for the two farms. Flow method provides a way to obtain adsorption-isotherm data under more realistic conditions.