Changes in release characteristics and runoff phosphorus for soils amended with manure

Application of manure on agricultural land can introduce considerable amounts of phosphorus (P) to natural water resources. The objectives of this study were to evaluate the effect of dairy manure application on 1) P released from surface soil by rainfall, 2) P removed from surface soil by runoff, and 3) soil P available for plants. A technique implementing a Soil Survey Laboratory method and USDA Runoff Model was applied on four Texas and three Utah soils. The application of manure (100 Mg/ha) considerably increased the amount of P released from the surface soil by rainfall, but there was no significant change in the pattern of P release (phosphorus release characteristics). Manure application increased both the runoff and available P for soils. For the Blanket soil (Texas), P released from surface soil by rainfall increased from 1.06 to 30.8 kg/ha/yr. The runoff P (kg/ha/yr) increased from 0.18 to 5.15 for fallow, from 0.16 to 4.71 for cropland, and from 0.13 to 3.88 for grassland. Soil P available for plants increased from 0.88, 0.90, and 0.93 to 25.7, 26.1, and 26.9 kg/ha/yr for fallow, cropland, and grassland, respectively. Similar effects of manure application were noticed for other Texas and Utah soils. The data suggest that manure could provide substantial amounts of available P for crop production in these soils. However, irrigated cropland amended annually with manure could contribute to nonpoint source pollution of surface freshwater bodies. The technique provides a tool to quantify the impact of manure application to agricultural land on water resources.