Fertilizer Phosphorus Recovery from Gypsum-Amended, Reclaimed Calcareous Marsh Soils

Gypsum amendments in marsh soils may affect the recovery of applied P fertilizer as the addition of Ca and sulphate alters the P sorption and release capacities of soils through their effects on P adsorption and precipitation processes. The effect of gypsum on P recovery was studied in a laboratory experiment where gypsum was added at 0, 6, 20, and 100g kg -1 to six representative marsh soils from SW Spain fertilized with 200 or 2000mg Pkg -1 applied as NH 4 H 2 PO 4 . Recovery was measured as the increase in Olsen P after 30, 60, and 150 days. P forms were also studied at these times, using sequential chemical extraction. Recovery of applied P decreased with increase in initial Olsen P (P status), Ca saturation, and sulphate concentration in the 1:1 soil extract. Less than 10% of applied P was recovered as Olsen P from the soils with highest P status and Ca saturation (LB2, LB5) after 30 days at the lower fertilizer P rate. P recovered from soils with an initial low Olsen P and high Na saturation (LB3 and LB4) was much higher (more than 50% in LB4 at 30 days at the two P rates). The addition of gypsum increased recovery of applied P (measured as Olsen P) in soils with a high degree of reclamation (high Olsen P and high Ca saturation): in LB2 and LB5, the recovery was double in samples amended with gypsum at 100g kg -1 than in nonamended samples at 150 days. By contrast, recovery of applied P was decreased by gypsum in soils with high Na saturation and an initial low P status. The effect of gypsum on recovery in high P - high Ca soils is related to an interference of sulphate with sorption processes that reduces short-term P sorption and increases the proportion of applied P that precipitates as soluble (bicarbonate extractable) phosphates in the long run. The effect of gypsum on recovery in low P (high Na saturation) soils is due to the increased P sorption capacity resulting from increased Ca saturation. Gypsum amendments increase the efficiency of fertilizer P, contributing to enhanced productivity of freshly reclaimed saline and sodic marsh soils.