Measurement of the maximum amount of water-extractable phosphorus in animal manure compost by continuous and sequential water extraction

Abstract

Measuring the phosphorus (P) solubility in animal manure compost (AMC) is important to estimate both the risk of P loss from agricultural land and the P availability for crops in agriculture that use AMC as fertilizer. Water-extractable phosphorus (WEP) has most often been used to estimate P solubility in AMC. A single water extraction with a high ratio of compost to water is usually used to determine WEP, but in many cases, this may not evaluate the maximum WEP. In this study, we extracted the P included in AMC by using a continuous extraction method with a large amount of water, and tested this approach for 16 AMCs: four cattle manure composts, five swine manure composts, five layer (chicken) manure composts, and two broiler (chicken) litter composts. The P dissolution patterns were fitted to kinetic models, and the maximum WEP (WEP_max) was determined by the coefficient of the non-linear regression equation. The WEP_max values corresponded to the sum of H₂O- and sodium bicarbonate (NaHCO₃)-extractable P measured using the modified Hedley sequential extraction method. The result also suggests that the maximum amount of WEP from AMC can be estimated rapidly using a sequential extraction (the Hedley method) that has been widely used to characterize P in manure or AMC. The maximum water-extractable magnesium (Mg) (WEMg_max) was significantly positively correlated with WEP_max (r = 0.854, P < 0.01). This suggests that WEP_max in the AMCs is affected by water-soluble magnesium compounds.

Key words:

• Animal manure compost
• phosphorus fractionation
• water-extractable phosphorus
• continuous water extraction
• nutrient availability