Magnesium-fortified phosphate fertilizers improve nutrient uptake and plant growth without reducing phosphorus availability

Magnesium (Mg) deficiency can significantly limit crop yield and quality. Separate application of straight Mg fertilizer is unattractive because of additional labor costs. Meanwhile, bulk blending Mg with other macronutrient fertilizers is also a suboptimal solution because bulk blended fertilizers often yield poor nutrient distributions. One rapid and economical alternative to alleviating Mg deficiency is to co-granulate macronutrient fertilizers with Mg. However, few commercial products have implemented this approach. One of the barriers hindering the production of Mg-fortified phosphorus (P) fertilizers is the assumption that precipitation of P with Mg will reduce P solubility. In this study, four Mg compounds, anhydrous magnesium sulfate (MgSO 4 ), magnesium oxide (MgO), anhydrous magnesium chloride (MgCl 2 ), and dolomite (CaMg(CO 3 ) 2 ), were co-granulated with mono-ammonium phosphate (MAP), and their granule strength, Mg and P availabilities, and agronomic effectiveness were evaluated. Results showed that there were no significant differences in P solubility between Mg-fortified MAP and MAP treatments. X-ray diffraction (XRD) indicated that the Mg species after co-granulation were boussingaultite (Mg(NH 4 ) 2 (SO 4 ) 2 ·6H 2 O), schertelite (Mg(NH 4 ) 2 H 2 (PO 4 ) 2 ·4H 2 O), magnesium hydrogen phosphate (Mg(H 2 PO 4 ) 2 ), and dolomite (CaMg(CO 3 ) 2 ). A pot experiment using an acidic soil demonstrated an average 9.6-fold increase in shoot Mg uptake, 3.0-fold increase in shoot P uptake, and 3.2-fold increase in soybean shoot dry matter in Mg-fortified MAP treatments, compared to those in MAP treatment. The current study provides a simple, effective, and low-cost approach for the addition of Mg to macronutrient fertilizers, to minimize Mg deficiency.