Evaluation of analytical methods for total phosphorus in organic amendments

Increased environmental concern regarding the build-up of soil phosphorus following application of organic amendments based on plant nitrogen (N) requirements will lead to an application strategy that is based on plant phosphorus (P) requirements. This will create demand for a rapid method of analyzing for total P in organic amendments in order to match application rates to plant P requirements. The wet oxidation method was evaluated in comparison with dry ashing technique for measuring total P in six organic amendments (hog, dairy cattle, beef cattle and poultry manures, municipal biosolids, and composted cattle manure) that ranged in P content from 0.25% to 4.5%. The wet oxidation method produced P concentration that was within 7% of that measured by the dry ashed method in 18 of the 24 samples analyzed and within 15% in the remaining samples. The experimental errors associated with both methods were small and the agreement between the two methods leads us to the conclusion that the wet oxidation method can be used for analysis of total P in a wide range of organic amendments. Each amendment was subjected to air drying, freeze drying, and oven drying in order to evaluate the effect of pre-treatment on the total P content of amendments relative to fresh sample. The effect of pre-treatment on the P content of organic amendments was small. In general, the P content of fresh sample was the highest with that of oven dry sample being the lowest. In studies where quantitative recovery of P is important, oven drying is not recommended as a method of removing water. In such cases, where samples cannot be analyzed in the fresh form, freeze drying or air drying is recommended.