Phosphorus diffusion from monocalcium phosphate co-applied with salts in a calcareous soil

Abstract

. 2004. Phosphorus diffusion from monocalcium phosphate coapplied with salts in a calcareous soil. Can. J. Soil Sci. 84: 447-458. Mixing non-phosphatic salts with fertilizer P influences the solubility and mobility of P in soils. Little evidence, however, is available regarding the mechanisms causing such effects. The objectives of this study were to investigate the effects of mixing fertilizer P with (NH 4 ) 2 SO 4 , MgSO 4 or (NH 2 ) 2 CO on the diffusion of P in a calcareous soil (Gleyed Rego Black Chernozem), and to identify the causes for such effects. To the surface of 50-mm-long soil columns, maintained at field capacity water content, 32 P-labelled monocalcium phosphate (MCP) was applied alone or in combination with (NH 4 ) 2 SO 4 , MgSO 4 or (NH 2 ) 2 CO. Ratios of applied P:N, P:Mg and P:S were 1:5, 1:4.5 and 1:6, respectively. Extraction and analysis of each 2-mm layer of the columns after incubation for 1, 2, 3, and 4 wk revealed that the addition of (NH 4 ) 2 SO 4 and MgSO 4 with MCP significantly increased P diffusion whereas (NH 2 ) 2 CO had little or no effect. The mechanisms of such effects were identified using a multi-ionic, mechanistic, diffusion model. According to model predictions, the dissolution of MCP was increased by more than twofold when mixed with (NH 4 ) 2 SO 4 and MgSO 4 , and by 1.2-fold when mixed with urea. The main difference between SO 4 salts and urea in affecting P diffusion was the competition between the anion of the salt and P for precipitation with Ca. Sulphate competed strongly with P, reducing the precipitation of Ca phosphates. Application of urea increased soil pH initially, but eventually soil pH decreased with nitrification of NH 4 . Initial increase in pH to above 8.0 favoured precipitation of Ca phosphate, but the pH was not high enough to favour CaCO 3 precipitation. The application of P fertilizers with fertilizers containing SO 4 could be beneficial in calcareous soils due to enhancement of P solubility and mobility. Mélanger des sels non phosphatés à un engrais phosphaté (P) modifie la solubilité et la mobilité du P dans le sol. Toutefois, on sait peu de choses sur les mécanismes à l'origine de ce phénomène. L'étude devait préciser comment le fait de mélanger un engrais P avec du (NH 4 ) 2 SO 4 , du MgSO 4 ou du (NH 2 ) 2 CO agit sur la diffusion du P dans un sol calcaire (tchernoziom noir gléyifié Rego) et établir les causes des modifications observées. À cette fin, les auteurs ont appliqué du phosphate monocalcique (PMC) marqué au 32 P avec ou sans (NH 4 ) 2 SO 4 , MgSO 4 ou (NH 2 ) 2 CO au sommet de colonnes de sol de 50 mm de hauteur dont la teneur en eau était maintenue à la capacité maximale du terrain. Les ratios appliqués étaient respectivement de 1:5, 1:4,5 et 1:6 pour P:N, P:Mg et P:S. Après incubation pendant 1, 2, 3 ou 4 semaines, l'extraction et l'analyse de chaque tranche de 2 mm des colonnes révèle que l'addition de (NH 4 ) 2 SO 4 et de MgSO 4 au PMC accroît sensiblement la diffusion du P alors que le (NH 2 ) 2 CO n'a pas d'effet sur ce phénom...

Accompanying cations and anions affect the diffusive transport of phosphate in a model calcareous soil system

“…Hammond (1997) reported that, of all the salts evaluated, ammonium sulphate was the most effective at increasing the crop utilization of P when banded with monoammonium phosphate fertilizer. Kumaragamage et al (2004) also found that the solubility of P was increased by more than twofold when monocalcium phosphate fertilizer was mixed with ammonium sulphate or magnesium sulphate, and by 1.2-fold when mixed with urea. The difference between the effect of sulphate salts and urea on P solubility was attributed to the competition between SO 4 2( and HPO 4 2( for precipitation reaction with Ca 2' .…”

“…The enhanced solubility and diffusion of P in the K 2 SO 4 column was probably due to both pH reduction and competition of SO 4 2( against HPO 4 2( for precipitation with Ca 2' (Kumaragamage et al 2004;Olatuyi 2006). There was no marked differences in P solubility between KH 2 PO 4 alone and KH 2 PO 4 'KNO 3 treatment, but KNO 3 had greater P solubility than K 2 CO 3 or KCl.…”

“…Reagent grade salts were used for all treatments. The salts were thoroughly mixed together prior to application (particularly for the treatments containing the phosphate and non-phosphate salt) and uniformly applied to the surface of the resin-sand columns using a spatula, to simulate a one-dimensional diffusive movement of ions Cho 1991a, b, 1993;Kumaragamage et al 2004).…”

“…The accompanying anions in the phosphate fertilizer band also affect the fate of P in the soil by competing against P for adsorption sites (Satnam et al 1996) and for precipitation with Ca 2' in calcareous soils (Kumaragamage et al 2004;Olatuyi 2006). Hammond (1997) reported that, of all the salts evaluated, ammonium sulphate was the most effective at increasing the crop utilization of P when banded with monoammonium phosphate fertilizer.…”

“…While researchers have reported the roles of cation interactions such as that between K ' and Ca 2' Cho 1991a, b, 1993), and the effects of anions such as SO 4 2( (Kumaragamage et al 2004;Olatuyi 2006) on the fate of P in a calcareous soil system, there is no comprehensive information on the influence of various cations and anions on P movement and solubility. Therefore, the objective of this study was to determine the influence of three cations in combination with four anions on the solubility, diffusive transport and chemical precipitation of P in a model calcareous soil system.…”