DETERMINING PHOSPHORUS IN BIOSOLIDS: USING A NEUTRAL AMMONIUM CITRATE EXTRACTION

More frequent occurrences of major fish kills and more focus on watershed management have recently increased efforts to better control non-point sources of pollution. Land applied materials, such as biosolids and manure, are major sources of agricultural nutrients. A major focus is being placed on phosphorus (P) management, since it is often applied in excess of crop requirements via organic P sources. Some states are implementing, or at least considering, Pbased land application rates, instead of the traditional nitrogen (N) based application rates. With N-based application rates, P is typically over-applied. If P-based (limiting) application rates are used, biosolid land application programs could be significantly restricted due to P limitations. Additionally, the N-needs of the crops would not be met. Farmers would be forced to apply supplemental N fertilizers to meet crop needs.The P Index tool is being developed by the Natural Resources Conservation Service. It is being considered by more than 20 states as a way of evaluating P risk. The purpose of the P Index is to provide a method of assessing various landforms and management practices for potential risk of P movement to water bodies. One component of the P Index is the organic P source application rate. Not all of the P contained in organic P sources (i.e., biosolids and manure) represents an immediate threat to the surrounding environment. Therefore, identifying a method of P determination (other than total P measurement) in organic P sources is becoming critical.An appropriate method of P determination in biosolids must be identified. Because of the varying characteristics of biosolids, a “one-size-fits-all” measure, such as total P, is not appropriate. Some measure of plant available (bioavailable) P is preferred. Overestimating plant available P with P-based application rates could result in increased land area requirements for biosolids disposal as well as supplemental N fertilizer requirements. Underestimating plant available P could compromise the surrounding water quality. Unfortunately, the P issue is more complicated than considering only one parameter. This illustrates the need for a tool such as the P Index. By improving the accuracy of each of the parameters, the P Index could become a very powerful tool for biosolids managers.This paper presents the results of recently completed work and the implications of its findings. Thirty-seven biosolids, representing a range of treatment processes were analyzed for extractable P using the neutral ammonium citrate (NAC) procedure. Various manures and inorganic P fertilizer samples were also tested for comparison. This extraction is used by Pennsylvania in its official (inorganic) fertilizer testing program, and is commonly applied to other materials (due to its simplicity and broad acceptance as a P test method). For all materials tested, the NAC- extractable P (NAC-P) varied from 75% to 144% of the total P as determined by EPA method 3051. The mean NAC-P for all biosolids was 101.5% of the total P, suggesting that the NAC procedure extracted nearly equal amounts of P from biosolids as the strong acid EPA method. Because uptake studies reported in the literature indicate that plant availability of biosolids-P can vary from 0% to 100% of total P, NAC-P was shown to be an unreliable guide to biosolids-P bioavailability. Further, NAC-P results did not compare well to actual plant uptake results in studies conducted using the same biosolids used in this study. Using a general linear statistical model, no significant relationships were found between NAC-P and biosolids treatment method, various single biosolids chemical parameters, or a combination of treatment method and single chemical parameters. Since NAC-P closely approximates total P in biosolids, significant relationships are not expected. Based on these results, it was concluded that the state-approved method of inorganic P fertilizer analysis would be a poor predictor of the amount of biosolids-P available to crops. Other chemical analyses should be evaluated for P determination in biosolids. This will be necessary to accurately assess this parameter for use in determination of organic P source application rates, such as required in the P Index.