Phosphorus Leaching in Soils Amended with Animal Manures Generated from Modified Diets

New dietary modifications for dairy (reducing P content in feed) and poultry (addition of feed additives such as phytase) aim to reduce P excretion in manures. Our objective was to investigate if dietary changes were effective at reducing P leaching loss on land application of manures. We used 54 undisturbed lysimeters (30 cm diameter, 50 cm deep) collected from three typical mid-Atlantic soils. Lysimeters received 85 kg total P ha from fertilizer (superphosphate), dairy manures generated from low- or high-P diets, or broiler litters generated from normal diet or reduced P- and phytase-amended diets. Lysimeters were irrigated with 50 mm of water each week for 9 wk. The major forms of P in the leachate were dissolved (dissolved unreactive > dissolved reactive P [DRP]) rather than particulate (total particulate P). The higher P solubility (100%) in superphosphate resulted in greater leaching of DRP, whereas the lower P solubility (<30%) in dairy manures or broiler litters resulted in lower DRP leaching from soils. Preferential flow in two soils caused greater DRP leaching; this effect was more pronounced in the superphosphate-amended than in the manure/litter-amended lysimeters. The dairy and poultry dietary modification was effective at reducing the amount of P in manures and litters. However, the application of treatments at similar P rate (85 kg ha) resulted in the addition of a higher amount of manure (54-66%) in lysimeters that received low-P dairy manure-amended and phytase-amended broiler litter, which then controlled P leaching from soils.