Effect of animal manure composts on soil phosphorus chemistry
In manures and composts, the distribution of organic and inorganic phosphorus forms varies widely depending on its source, animal physiology and diet, and the method of storage and preparation. Generally, these organic amendments contain large amounts of calcium, which increase soil pH and give buffering effects of added bicarbonate and organic acids. Recent studies show considerable shifts from aluminum- and iron- to calcium-phosphorus reaction products, and the transformation of the relatively soluble brushite into the more crystalline octacalcium phosphate in animal manure- and compost-applied soils. Organic phosphorus can constitute up to 60% of the total phosphorus in manures and composts, but the dominance of organic phosphorus decreases in excessively manure-applied soils. The mechanism of transformation of phosphorus forms in soils by application of animal manures and composts is not well understood.
The objective of this study was to quantify the forms of phosphorus in animal compost and compost-amended soils, so as to understand the mechanism of their transformation in soils. The transformation of various phosphorus forms were studied in turkey litter compost and in an organically-managed calcareous soil amended with turkey litter compost by a sequential phosphorus fractionation method. Soil organic phosphorus pools were extracted using NaOH–EDTA method. The amount of phosphorus in each extract was measured and the organic P was characterized by solution 31P NMR spectroscopy.
This study showed that turkey litter compost is > 90% inorganic phosphorus, with a specifically high concentration of octacalcium phosphate accounting for 69% of the total phosphorus. Application of turkey litter compost to soil increased the proportion of the total phosphorus as octacalcium phosphate from 17% to 32% by the end of the incubation study (126 days). A model of soil phosphorus mineral species predicts that octacalcium phosphate will dissolve in soils amended with turkey litter compost and become plant available when pH and total phosphorus in soil solution decrease.
Proceedings of Organic Phosphorus 2013 Integration across Ecosystems. Smithsonian Tropical Research Institute, Republic of Panama. 4-7 February 2013.