NITROGEN AND PHOSPHORUS FATE FROM LONG-TERM POULTRY LITTER APPLICATIONS TO OKLAHOMA SOILS

With the rapid growth of the poultry industry in eastern Oklahoma, information is needed on the impact of land application of associated litter on the area's soil and water resources. The effect of broiler litter application on the N and P content was investigated by sampling 12 soil profiles to 150 cm under 'Coastal' bermudagrass [Cynodon dactylon (L.) Pers.] to which litter had been continually applied for 12 to 35 yr. Litter application averaged 6 Mg ha-1 yr-1 (dry wt.), which contributed approximate N and P amounts of 270 and 90 kg ha-1 yr-1. Adjacent soil profiles under idle native grass, which received no litter or mineral fertilizer, were sampled at the same time to a 150-cm depth. On average, soil pH and organic C were 0.5 and 21 g kg-1 greater and bulk density 0.2 Mg m-3 lower in the surface 5 cm of treated than untreated soils. Below 25 cm, litter had little effect on these properties. The effect of poultry litter on N and P content was greatest in the surface 5 cm of soil, with NO3-N and Bray-I P (BP) averaging 49 and 188 mg kg-1 in treated and 13 and 9 mg kg-1 in untreated soils. Below 5 cm, N and P content decreased rapidly, with only slight NO3-N accumulations (5 mg kg-1) between 50 and 100 cm and no movement of P below 30 cm observed. Poultry litter application decreased P sorption to a depth of 30 cm. Average 1.2- and 2.5-fold increases in the total N and NO3-N content of treated compared with untreated 0- to 100-cm profiles were observed. For P, 2- and 13-fold increases in total P and BP were found, with little accumulation as organic P. Overall, BP increased 22 mg kg-1 for each 100 kg P ha-1 added in litter. The greater portion of litter P (72%) than N (44%) retained in the soil profile reflects the differing sorption of these nutrients by soil, and greater plant uptake of N than P. It also emphasizes the need to carefully manage continual land applications of poultry litter to minimize potential environmental impacts.