Availability of phosphate fertilizers in unlimed and limed acid soils as affected by wetting and air-drying

The interactions between air-drying and rewetting, and liming or timing of phosphate (P) fertilizer application on plant availability of monocalcium phosphate (MCP) or reactive phosphate rock (PR) fertilizers were studied under glasshouse conditions. A subsidiary isotopic exchange study was carried out using ³²P to determine the mechanism causing changes in the extractability of P following air-drying and rewetting. An additional study was undertaken to test the hypothesis that an increase in the degree of structural order of precipitated amorphous hydroxy-Al polymers may occur following air-drying of limed acid soil. Changes in P extractability and P adsorption were determined. Yield and P uptake responses were assessed using wheat (Triticum aestivum L.) as the test plant. Two acid soils from the South Island of New Zealand, the Craigieburn silt loam and the Onepunga sandy loam, were used. The Onepunga sandy loam was selected to represent a soil with low P adsorption capacity and low initial exchangeable Al concentration. The Craigieburn silt loam soil was chosen to represent a soil with high P adsorption capacity, and high initial exchangeable A1 concentration. Liming decreased P extractability in the Craigieburn soil, but caused large increases in wheat yields and P uptake. Liming had no significant effect on P extractability or on wheat yields and P uptake in the Onepunga soil. The decrease in P extractability following liming in the Craigieburn soil was attributed to the formation of sparingly soluble Ca-P compounds. It was suggested that the major agronomic effect of liming this soil was to ameliorate factors due to soil acidity (e.g. A1 toxicity) and thus allow plants to utilize available P more effectively. Air-drying and rewetting significantly increased P extractability, and yields and P uptake of wheat in both soils. Yield and P uptake increases were greater for the added P treatments than for the control (zero added P) treatment in the Craigieburn soil. In the Onepunga soil, air-drying and rewetting caused greater increases in the control treatment compared to the added P treatment Yield and P uptake increases due to air-drying and rewetting were also greater in the limed soil treatment compared to the unlimed soil treatment in the Craigieburn soil. This interaction between air-drying and rewetting, and liming, on yield and P uptake was not observed in the Onepunga soil. It was suggested that liming ameliorated factors due to soil acidity in the Craigieburn soil and thus allowed the plants to utilize available P more effectively. The increase in P extractability following air-drying and rewetting was attributed to net mineralization of soil organic P. The hypothesis that air-drying a limed soil may increase the degree of structural order of precipitated amorphous hydroxy-A1 polymers was tested by raising the initial exchangeable A1 concentration of the low P-adsorbing Onepunga soil. The samples were subsequently limed, and subjected to air-drying and rewetting. Liming decreased the initial exchangeable A1 concentrations and this was attributed to the formation of amorphous hydroxy-A1 polymers, P adsorption was increased by liming. This result was attributed to the adsorption of P by the newly precipitated amorphous hydroxy-A1 polymers. Air-drying and rewetting decreased P adsorption in the soil samples where the initial exchangeable A1 concentrations were high. And had no effect on the sample with low initial exchangeable A1 concentration. A limed treatment of the high P-adsorbing Craigieburn soil (high initial exchangeable Al concentration) included in the study was unaffected by air-drying and rewetting. The decrease in P adsorption by the limed Onepunga soil samples with initially high exchangeable A1 concentrations indicated that an increase in the degree of structural order of precipitated amorphous hydroxy-A1 polymers might have occurred following air-drying and rewetting cycles. The isotopic exchange study supported the conclusion that the increased extractability of P following air-drying and rewetting resulted mainly from net mineralization of soil organic P. It was concluded that the release of P from net mineralization of soil organic P may mask an increase in P availability due to changes in the degree of structural order of precipitated amorphous hydroxy-A1 compounds. Air-drying and rewetting decreased ³²P specific activity of both the soil and plant P extracts. In addition, the results provided further evidence that the decrease in P extractability following liming the Craigieburn soil was caused by the formation of sparingly soluble Ca-P compounds. The interaction effect between air-drying and rewetting, and the timing of MCP application on the availability of P was investigated using limed treatments of both soils. The extractable P concentrations decreased with increasing time of P fertilizer reaction with the soils and this indicated the slow and continuing reactions of P fertilizer with soil. The decreases in extractable P concentrations were reflected by wheat yields and P uptake responses particularly in the Craigieburn soil. Air-drying and rewetting had no significant effect on the decreases of extractable P concentrations with time, a finding which indicated that air-drying and rewetting probably had no significant influence on the slow reactions. The interaction effects between air-drying and rewetting, and the timing of reactive PR fertilizer application on PR dissolution, P extractability, and wheat yields and P uptake were studied using unlimed samples of the Craigieburn soil. Two reactive PR fertilizers were used: North Carolina PR (NCPR) and Jordan PR (JPR). The extent of dissolution of NCPR increased with time, while that of JPR was unaffected by the timing of application. P extractability declined with increasing period of reaction of both fertilizers with soil, but this had no significant effects on yields and P uptake. The decline in extractable P concentrations with the period of PR reaction with soil indicated that application of PR to soils of high P retention several weeks before sowing may not increase P availability. Air-drying and rewetting had no significant effect on dissolution of both PR materials, but increased P extractability. The increases in the extractable P concentrations were attributed to the Birch effect. From an agronomic point of view, NCPR was a slightly better fertilizer than MCP, a finding which appeared to be caused by a slight liming effect resulting from NCPR application. In contrast, JPR was the least effective fertilizer.