Solubility and transport of phosphate and the accompanying ions as influenced by sulphate salts in a model calcareous soil system

• Published in: Canadian journal of soil science 2009-11, Vol.89 (5), p.589-601
• Main Authors: Olatuyi, S.O, Akinremi, O.O, Flaten, D.N, Crow, G.H
• Format: Article
• Language: English
• Subjects: ammonium dihydrogen phosphate; ammonium sulfate; calcareous soils; diffusive transport; ions; nutrient availability; phosphorus; potassium dihydrogen phosphate; potassium phosphates; soil solution; soil transport processes; solubility
• ISSN: 0008-4271; 1918-1841
• DOI: 10.4141/CJSS07117

Plant availability of phosphorus (P) in calcareous soil can be improved by modifying the chemical environment of the soil-P fertilizer reaction zone through the banding of non-phosphate fertilizer with P. We investigated the solubility and diffusive transport of P as influenced by addition of two sulphate salts [(NH 4 ) 2 SO 4 and K 2 SO 4 ] to NH 4 H 2 PO 4 and KH 2 PO 4 . The salts were applied to a series of wax columns packed with approximately 223 g of Ca 2+ -saturated cation exchange resin-sand mixture buffered with CaCO 3 . The background pH of the mixture was 8.8. Each treatment contained approximately 204.5 mg P kg -1 soil, while 632.3 mg SO 4 2- kg -1 soil was added to each P source for treatments containing the dual bands to provide a molar concentration of P and SO 4 2- of 6.6 mmol kg -1 soil. After 2 wk of incubation, column segmentation and extraction showed that H + moved deeper into the columns on addition of the sulphate salts compared with adding NH 4 H 2 PO 4 or KH 2 PO 4 alone. The maximum depth of P penetration in the columns containing NH 4 H 2 PO 4 was 4.2 cm, while P transport in the columns treated with KH 2 PO 4 was restricted to the top 4.0 cm depth. Addition of (NH 4 ) 2 SO 4 or K 2 SO 4 to NH 4 H 2 PO 4 increased the concentration of water-extractable P by 43 and 21%, respectively, above that in NH 4 H 2 PO 4 alone. Similarly, addition of (NH 4 ) 2 SO 4 or K 2 SO 4 increased the concentration of water-soluble P by 48 and 41%, respectively, above the amount in KH 2 PO 4 alone. The increased water solubility of P on addition of the sulphate salts was attributed to anion competition between HPO 4 2- and SO 4 2- for precipitation with Ca 2+ . We also observed ionic competition between NH 4 + and K + when both cations were added together, causing K + to travel farther into the column and with increased solubility than when applied alone. Our results showed that anion and cation competition can be used to modify the transport and solution concentration of ions through dual banding. These results also suggested that the combination of anion competition by SO 4 2- and pH reduction due to salt effect could have a positive influence on the availability of P in calcareous soils. Key words: Phosphate, sulphate, columns, solubility, diffusive transport, resin