Effects of Dairy Manure on Phosphorus Concentrations and Losses in Tile Effluent

Dissolved phosphorus concentrations and losses in effluent from a tile‐drained silt loam were compared before and after manuring. Effluent samples were collected from 0.33‐ha tile‐drained plots for 4 years before and 3 years during consecutive annual dairy manure applications of 35 or 200 wet metric tons/ha. Plots had received fertilizer P in years prior to manuring. More than 95% of the effluent samples had dissolved molybdate reactive P (DMRP) concentrations < 30 ppb before manuring. During the 3 years the manure was applied, 53 and 92% of the effluent samples contained < 30 ppb P for the 200‐ and 35‐metric‐ton applications, respectively. The flow‐weighted DMRP concentrations in tile effluent were 116, 441, and 97 ppb for 200 metric tons of manure/ha per year, and 22, 10, and 9 ppb for 35 tons/ha per year, respectively, during the 3 years manure was applied. The tile flow accounted for about 40% of the drainage water lost from the plots, but the P contents of tile flow were 10% of total dissolved P losses in surface runoff. P additions from 30 to 40 wet metric tons/ha of manure would be adequate to meet crop needs but should not produce excessively high P concentrations in tile flow.