Effect of 20 annual applications of excess feedlot manure on labile soil phosphorus

Concentrations were measured of various forms of soil P in the Ap horizon of a Lethbridge loam (Dark Brown Chernozemic) brought about by 20 yr of feedlot manure loading or 14 yr of feedlot manure loading + 6 yr of no further additions under both nonirrigated and irrigated conditions. Depth of cultivation affected the concentrations of the various P fractions. Although total P and phosphatase activity increased with manure additions, these increases diminished at triple the recommended loading rates. Increased phosphatase activities never rose above those of the soil from ungrazed native prairie. Irrigation increased the phosphatase activity of the control treatment. Concentrations of total P remained well above those of the control plots, even after 6 yr without any additional manure loading. New approaches to the estimation of P fractions are needed to understand P in land used for manure disposal.