Influence of lime and phosphate on potassium and magnesium leaching in soil with predominantly variably charged surfaces

Abstract

The effect of lime (Ca(OH)₂) on K⁺ and Mg²⁺ leaching was investigated under laboratory conditions in the A and B horizons of three New Zealand soils with predominantly variable surface charge. The A horizon of the three soils was used to study the influence of monocalcium phosphate (Ca(H₂PO₄)₂.H₂O; MCP) on K⁺ leaching.

Factors influencing the soils' surface charge properties in relation to cation leaching were studied initially. The effect of drying of soil on the cation (CEC) and anion (AEC) exchange capacity, and on K⁺ and Mg²⁺ retention was determined. Air drying resulted in small but non-significant changes in these properties. Oven drying significantly decreased the CEC of all soils. This was associated with a decrease in K⁺ and Mg²⁺ retention by the Taupo sand and Naike clay soils. Potassium retention by the Ahaura silt loam increased following oven drying which was attributed to K⁺ fixation. It was concluded that air drying would not significantly influence the ability of these soils to retain K⁺ or Mg²⁺ on the exchange sites during leaching.

The effect of leaching on the permanence of CEC resulting from lime and/or MCP addition was studied. Liming to pH 6 and 7 increased the CEC of the A horizon soils by an average of 3.5 and 12.0 me per 100 g, and the B horizon soils by 1.0 and 5.1 me per 100 g, respectively. The average increase in CEC following MCP addition at rates of 6 and 147 mg P per 5 g of soil was 1.6 and 4.8 me per 100 g for the A horizon soils, and 1.5 and 8.4 me per 100 g for the B horizon soils, respectively. Simultaneous addition of lime and MCP caused a smaller increase in CEC compared with the summation of the CEC for individual lime and MCP treatments. This was attributed to the possible formation of calcium phosphates of low solubility. The CEC of treatments receiving MCP at the high rate initially increased by up to 5 me per 100 g with leaching. This was attributed to the redistribution of MCP through the soil. With prolonged leaching, i.e. approximately 500 pore volumes, the CEC for all lime and/or MCP treatments gradually decreased. The decrease in CEC commonly fell within the range of 20 to 40 per cent and was associated with an increase in soil acidity. Similar results were obtained when either deionised water or CaCl₂ was used as the leaching solution. The use of less than 20 pore volumes of deionised water in the column leaching experiment would not measurably decrease the lime or MCP induced CEC.

Packed columns of < 2 mm air dry soil were used to investigate the effect of lime on K⁺ and Mg²⁺ leaching. Various rates of KCl or MgCl₂ were applied to the column surface and leached. Liming caused increased leaching losses of K⁺ and Mg²⁺ from the untreated soils through their displacement from the exchange sites by Ca²⁺. Liming to pH 6 reduced the leaching of applied K⁺ and Mg²⁺ in all soils. For most treatments, this was related to the increase in CEC. Potassium was leached to a greater extent at pH 7 than at pH 6 in all A horizon soils, despite over a doubling in CEC. The opposite was found for K⁺ leaching in the B horizon and Mg²⁺ leaching in the A horizon, for all soils. Liming to pH 6 caused an increase in the Gapon exchange coefficient for K⁺ (kKCa) and Mg²⁺ (kMgCa) for soils where A1³⁺ dominated the exchange sites. For soils with a naturally high concentration of exchangeable ca²⁺, liming to pH 6 had the opposite effect. For all kKCA and kMgCA was lower at pH 7 than at pH 6. The direction of change in kKCa and kMgCa following liming was attributed to a decrease in exchangeable A1³⁺ or an increase in exchangeable ca²⁺. Changes in the Gapon exchange coefficient resulting from liming were consistent with changes in the distribution of applied K⁺ and Mg²⁺ observed within the column following leaching.

Generally, the addition of MCP did not reduce K⁺ leaching compared with the control, despite a significant increase in CEC. This result was similar to that found for the high lime treatment. The absence of any effect of MCP on K⁺ leaching was attributed to the strong preference of the exchange sites for ca²⁺ relative to K⁺.