A rapid and simple technique for digestion and determination of total phosphorus in animal manures and herbage

Abstract

This study describes a simple, innovative, acidified persulfate method of autoclave assisted digestion coupled with molybdenum blue spectrophotometric analysis for determination of total phosphorus (TP) in animal manures and herbage. Reagent addition was optimized to give maximum estimations of TP from four manure types of unknown phosphorus (P) content and a certified herbage material of known P content. This method was then compared with the established procedure, an aqua-regia digest followed by Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) determination. The persulfate method gave TP estimations of 18.95 mg g⁻¹ for cow manure, 18.90 mg g⁻¹ for broiler litter and 22.82 mg g⁻¹ for pig slurry. These were significantly greater (P<0.01) than the values of 16.10 mg g⁻¹, 17.48 mg g⁻¹ and 19.16 mg g⁻¹ for cow manure broiler litter and pig slurry respectively, determined by the aqua regia/ICP-OES method. No significant difference in TP estimation was found for layer manure or the certified herbage material between methods. The persulfate digestion was simpler, faster, used less hazardous reagents and did not require the continuous attention of the operator as recommended for the aqua-regia method. This work highlights the need for the implementation of a reliable standard method for TP estimation in manures and the need for certified reference manure materials against which laboratories can gauge the reliability of their procedures.

Acknowledgments

The authors would like to thank John Driver and associates of Cherub Environmental and Educational Projects LTD for their foresight in funding research into the recovery of phosphorus from animal manure of which this work forms a small part. The Institute of Grassland and Environmental Research (IGER) is supported by the Biotechnology and Biological Sciences Research Council (BBSRC), Swindon.