Exchangeable Calcium/Magnesium Ratio Affects Phosphorus Behavior in Calcareous Soils

Calcium and magnesium are two dominant cations in calcareous soils that undergo precipitation reactions with applied P fertilizers. Unlike Ca, the reactions of Mg with P have not been thoroughly studied. The objectives of this study were to determine the effect of the exchangeable Ca/Mg ratio on the solubility and mobility of P in model calcareous soils and to identify the P species using 31 P magic angle spinning (MAS) nuclear magnetic resonance (NMR). A cation exchange resin was saturated to achieve five different saturation ratios: approximately 100% Ca, 60:40 Ca/Mg, 50:50 Ca/Mg, 30:70 Ca/Mg, and 100% Mg. The resin was mixed with quartz sand and CaCO 3 to form model calcareous soils. Fertilizer monoammonium phosphate (MAP) was surface applied on soil columns. According to the analysis of 2‐mm segments, water‐soluble P increased with decreasing Ca saturation while the acid‐soluble P decreased with decreasing Ca saturation. The decreasing Ca saturation (or increasing Mg saturation) enhanced deeper penetration of added P. Dicalcium phosphate dihydrate (DCPD) was the dominant P species formed in soils with 100 to 50% Ca saturation. Magnesium phosphate trihydrate (MPTH) was identified as a prominent P phase with a poorly ordered structure in soils with 70 to 100% Mg saturation. The DCPD and MPTH were identified as minor P phases in 30:70 and 60:40 Ca/Mg soils, respectively. Both chemical and NMR results confirmed that Mg, unlike Ca, was not a strong cation for P precipitation. The role of exchangeable Mg in reducing the formation of Ca‐P precipitates could have a positive effect on the P availability in soils with low Ca/Mg saturation.