Phosphorus Adsorption Parameters in Relation to Soil Characteristics

Abstract

Understanding of soil phosphorus (P) adsorption chemistry is essential for evolving best fertilizer management practices aimed at enhancing fertilizer P use efficiency and environment quality. Soil chemical parameters which govern P adsorption maxima and binding strength in calcareous and non-calcareous soils were determined. Five soils representing three different parent materials i.e. mixed mineralogy alluvium, loess and noncalcareous sandstone were sampled at genetic horizons in each profile. Phosphorus adsorption isotherm was developed for each of the 32 soil samples by equilibration of two g air dry soil with 0 to 4000 mg l⁻¹ (0 to 125 mM) P in 20 ml of 10 mM KCl and fitted to Langmuir and Freundlich equations. The chemical parameters i.e. pH, CaCO₃, total and dissolved organic carbon, exchangeable calcium, dithionite extractable iron (Fed) and aluminum (Ald), oxalate extractable iron (Feo) and aluminum (Alo) and as a test for plant available P extractable with NaHCO₃ were determined. The Fruendlich equation, an empirical relation between adsorbed and solution P, fitted the adsorption isotherms better than the Langmuir equation which relates adsorption to solid surface coverage by adsorbate. Adsorption maxima at low affinity sites positively correlated with soil CaCO3 due to precipitation as Ca-P and to extractable iron oxide; and the binding strength of high affinity (k₁) sites correlated positively with soil Fed, Alo, Ald, and crystalline iron oxide (Fed-Feo) contents. The Freundlich parameter bf, adsorption exponent which signifies sorption intensity also positively correlated with soil Fed, Fecr and Ald contents. Phosphorus adsorption at its low concentration in equilibrium solution seems to be governed by soil iron oxides and precipitation as Ca-P is important only at high P concentration, less frequently achieved in farm fields.