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.