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 hori zon 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 conditi ons. Depth of cultivation affected the concentrations of the various P fractions. Although total P and phosphatase activity increased with m anure 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 ph osphatase activity of the control treatment. Concentrations of total P remained well above those of the control plots, even after 6 yr witho ut any additional manure loading. New approaches to the estimation of P fractions are needed to understand P in land used for manure disposa l.