Phosphorus release characteristics of manure and manure-impacted soils

Author: Vimala D. Nair ^*, Donald A. Graetz and Daniel O. Dooley

Received 3 January 2003, accepted 25 April 2003.

Abstract

Phosphorus (P) management for dairy and beef cattle operations can be improved by understanding P chemistry in manure and manure-impacted soils. This study characterizes the P forms in dairy and beef cattle manure, and evaluates P release characteristics of manure-impacted soils. Dairy and beef manure from 5 cattle was collected in two consecutive years and subjected to sequential P fractionation, using 1.0 M NH₄Cl (labile P), 0.1 M NaOH (Fe/Al-P) and 0.5 M HCl (Ca/Mg- P) for 2, 17, and 24 h, respectively. In the second year, manure samples were also extracted repeatedly with 1.0 M NH₄Cl until no more P was released. Phosphorus release was influenced primarily by Ca and Mg concentrations in the manure, and their P release potential varied with the manure source. The Ca:Mg ratios in the sequential water extracts of manure-impacted soils decreased from 1.0 in the first extract to 0.6 in the tenth extract for soils collected from active dairies with high rates of manure application. The ratio was a constant 2.0 for soils from abandoned dairies with high manure application rates. Ca:Mg ratios were even higher for beef pasture and forage soils. It also appears that Mg concentrations in the manure could have a detrimental effect on Ca-P stabilization in manure-impacted soils. To minimize the environmental impact of manure applications, it may be necessary to reduce the amount of Mg in the manure through diet or by pre-treatment prior to land application.