Ammonia losses from surface-placed mixtures of urea-calcium-potassium salts in the presence of phosphorus

Abstract

Phosphorus compounds frequently are mixed with urea containing materials for economy in fertilizer operations. There is little published information on NH₃ losses from surface application of these mixtures. However, there is evidence that P can react and precipitate with adsorbed and added Ca and increase the potential for NH₃ loss. This paper compares NH₃ losses from surface applied urea plus KCl or CaCl₂ in the presence of 5 common P sources. The N, with Ca, K, and P salts, was surface-applied to a calcareous (Harkey) and an acid soil (Cuthbert) in a laboratory and the NH₃ losses determined by passage of the exhaust air through a 2% boric acid solution. Ammonia losses were increased with (in the presence of KCl or CaCl₂) KH₂PO₄ (KP) (calcareous soil only) and K₂HPO₄ (K₂P), unaffected by Na₅P₃O₁₀ (PP) but decreased with Ca(H₂PO₄)₂ (CaP) and H₃PO₄ (HP) (No HP or PP applied to the acid soil). Urea which hydrolyses in environments with lower soluble and desorbable Ca levels is susceptible to higher NH₃ losses. The effectiveness of KCl for control of NH₃ loss depended on the existence of desorbable Ca to react with the decomposing urea. Therefore the deleterious impact of P on NH₃ loss was greater with KCl than with CaCl₂. Adding Ca directly with the urea made additional Ca available for reaction with P and urea. Monocalcium phosphate (CaP) alone with urea, in a calcareous soil, did not reduce NH₃ loss; however, NH₃ loss was reduced in the acid soil. The addition of CaCl₂ with urea + CaP reduced NH₃ loss more than CaCl₂ with urea. The HP reaction with CaCO₃ was more rapid and complete than occurred with the acidic CaP. Sodium tripolyphosphate (PP) with urea had little impact on NH₃ loss over that produced by the KCl or CaCl₂ salts alone. The HP and CaP chemicals did not appear to function strictly as acid sources (calcareous soil). The Harkey soil has 8% CaCO₃ which would appear adequate to neutralize any acidity introduced by the P fertilizers. The explanation may lie in double salt formation between the Ca-urea-P materials.