Dairy Manure Impact on Soil Phosphorous, Nitrogen and Salt Accumulation in an Oat-Maize Rotation in Southwestern United States

The effect of manure applications on crop yield and the soil environment is becoming an ever more important issue in Arizona, where dairy herds are increasing and crop acreage is being lost to tile roofs. A field study was initiated in Central Arizona to evaluate the impact of dairy manure applications on soil phosphorus, nitrogen and salt levels after four seasons of an oat/maize rotation. Additionally, the applicability of Natural Resource Conservation Service's (NRCS) Phosphorus Index (P-Index) tool was assessed. An inorganic fertilizer treatment was used as a control. Nitrogen (N)-based rate applications of dairy manure took place semi-annually to an oat (fall) and maize (spring) rotation for four consecutive cropping seasons from 2002 through 2004. After two years and four cropping seasons, treatment effects on soil nitrate (NO3-N) levels were evident only in the top 0.3 m and concentrations never exceeded 10 mg kg -1. The salt content of the soil also increased, with electroconductivity (EC) values nearly doubling at many depths. Manured treatments, both composted and fresh, generally produced lower crop yields than control treatment, although differences were significant in only one season for each crop. Phosphate (PO4-P) levels in the soil (0 to 2.1 m depth) increased dramatically in the compost and manure treatments from 233 to 583 and 182 to 580 kg PO4-P ha-1 , respectively. Phosphate levels in the control treatment increased only 86 kg ha-1 during the same time period from 28 to 114 kg ha-1. From an agronomic standpoint, the amount of PO4-P in the manure and compost treatments was beyond the optimum requirement for crop growth and created a potential environmental hazard for the loss of phosphorus (P) due to erosion and runoff. However, when applying the NRCS P-Index tool to this field to determine the potential risk of the movement of P offsite, it was found that no restriction would be imposed on the application of any fertilizer (organic or inorganic) because the P-threshold level would not have been met. These results may indicate a need to adjust the P-Index presently used in Arizona.