FORMS, DISTRIBUTION, AND POTENTIAL AVAILABILITY OF PHOSPHORUS FROM POULTRY MANURE

Phosphorus (P) availability to plants following soil application of poultry manure is influenced by manure composition, soil properties, and plant roots. As more than 42% of total phosphorus in poultry manure is not soluble, inappropriate application may result in accumulation of P that is not plant available, and eutrophic conditions in surface waters following runoff or erosion. Project objectives were to use sequential extraction by H2O, NaHCO3, NaOH, and HCl to (1) characterize inorganic and organic forms of phosphorus from poultry manure and determine the effects of laboratory drying on P fractions, (2) examine short-term changes in soil phosphorus fractions following manure application, and (3) identify changes in soil phosphorus fractions and soil phosphatase activities induced by both root processes and PM application. Results indicated that most of the inorganic phosphorus in poultry manure was in the labile (H2O + NaHCO3) or calcium-associated (HCl) fractions, representing 46 and 48% of total P, respectively. In addition, 53% of organic phosphorus was calcium-associated. Drying increased the amount of water-extractable P by 42-97% in the order of 65oC>22oC>freeze-dry. Therefore, freeze-drying is recommended as the method least likely to overestimate soluble P in manure. Incubation of soil with poultry manure added at 100 and 200 mg P kg-1 revealed that manure increased inorganic phosphorus in the H2O and HCl fractions; however, H2O-P was rapidly transformed to other fractions, whereas HCl-P remained elevated in soil that had received manure at 200 mg P kg-1. This was not observed in soil that received 100 mg P kg-1 of manure, suggesting that moderate application may not increase the proportion of soil phosphorus that is Ca-associated. Ryegrass grown with poultry manure at 112 mg P kg-1 had 30% higher labile-P in the rhizosphere than when grown in soil without manure. This was accompanied by increased soil phosphatase activity and 59% higher total P uptake by plants. In conclusion, both chemical and biotic processes work to transform mineral-associated, slowly available phosphorus from poultry manure into labile forms in response to plant uptake, and poultry manure application does not necessarily lead to accumulation of phosphorus that is not plant available.