Phosphorus mobility in calcareous soils under heavy manuring

Confined animal production systems produce large amounts of manure that are usually disposed on limited land areas in proximity to the source. There is concern that continuous heavy manuring may saturate the soil's capacity to retain manure-P and result in groundwater contamination with excessive P. The objectives of this research were to determine the fate of waste-P in heavily manured soils and assess the risk of soil and water quality degradation in turkey (Meleagris gallopavo) and beef (Bos taurus) feed lot production areas. All soils were calcareous throughout the profile. Samples were collected by 30-cm depth increments to a depth of 210 cm or a limiting layer and were analyzed for NaHCO3-extractable P (including inorganic and organic), and total soil P. Large amounts of extractable organic P in the surface soil layers decreased to background levels within 2 to 3 yr after manuring ceased. In heavily manured fields extractable inorganic P was well above background concentrations to as deep as 210 cml apparently because of mo-cement and subsequent mineralization of organic P, Organic matter in these soils was about twofold higher than background levels. There appeared to be no practical limit to the P-retention ability of these calcareous soils. Under these conditions the risk of groundwater contamination bg organic or inorganic P from waste P disposal to land is negligible.