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

• Published in: Canadian journal of soil science 2009-02, Vol.89 (2), p.179-188
• Main Authors: Olatuyi, S.O, Akinremi, O.O, Flaten, D.N, Crow, G.H
• Format: Article
• Language: English
• Subjects: ammonium carbonate; ammonium sulfate; anions; calcareous soils; cation exchange resins; cations; chemical precipitation; diffusion; ion interactions; magnesium phosphates; nutrient availability; phosphorus; phosphorus fertilizers; potassium phosphates; salts; solubility
• ISSN: 0008-4271; 1918-1841
• DOI: 10.4141/CJSS07118

Mixing non-phosphate salts with phosphate fertilizer modifies the chemical environment of the soil-phosphorus (P) fertilizer reaction zone due to induced changes in soil pH and the interactions of P with soil components and other ions. The objective of this study was to examine the effects of cation and anion interactions on the solubility and diffusive transport of P in columns packed with a mixture of inert quartz sand and Ca 2+ -saturated cation exchange resin, buffered with CaCO 3 . The background pH of the resin-sand mixture was 9.4. Three types of cations (K + , NH 4 + , and Mg 2+ ) were combined with four anions (NO 3 - , Cl - , SO 4 2- , and CO 3 2- ) to produce 12 non-phosphate salts. Each of these salts was mixed with 0.4 g of KH 2 PO 4 to provide 408.9 mg P kg -1 soil. The experiment was carried out in three replicates. Following 2 wk of incubation, columns were sectioned at 3-mm intervals and the pH of each section was measured. The samples were extracted with deionized water and subsequently with 1 mol L -1 HCl. Addition of KH 2 PO 4 alone reduced the pH at the surface (first 3-mm section) of the resin-sand column to 7.1, while pH remained unchanged at 9.1 on addition of K 2 CO 3 and KH 2 PO 4 . Addition of MgCl 2 and KH 2 PO 4 resulted in the lowest pH in the column and the greatest depth of H + penetration compared with other treatments. None of the treatments containing NO 3 - or Cl - salts enhanced the solubility and movement of P. Addition of (NH 4 ) 2 SO 4 or (NH 4 ) 2 CO 3 to KH 2 PO 4 produced the greatest amount of water-extractable P, followed by K 2 SO 4 . The lowest solubility of P occurred on addition of K 2 CO 3 (P < 0.05). We attributed the enhanced solubility of P by SO 4 2- and CO 3 2- associated with NH 4 + to competition between these anions and HPO 4 2- for precipitation with solution Ca 2+ . Phosphate ion moved to depths of 5.0, 6.0 and 7.5 cm on addition of K 2 SO 4 , (NH 4 ) 2 SO 4 , and MgSO 4 to the column, respectively. These results suggested that salts such as K 2 SO 4 , (NH 4 ) 2 CO 3 , (NH 4 ) 2 SO 4 and MgSO 4 would enhance the lability of fertilizer P in a calcareous soil system. Key words: Solubility, precipitation, diffusive transport, resin, cation exchange, anion competition, dual banding