Tillage and Manure Timing Effects on P Index Loads

Soil, topography, and climate influence how changes in tillage or manure application increase or decrease runoff P losses. Shown below are graphs of comparing P Index loads for ten fields with three different managements. All of the fields have a soil test P of 70 ppm and have a rotation of three years of corn silage followed by four years of alfalfa. Liquid dairy manure was applied at 7,000 gallons per acre to the second year of corn , 14,000 gallons per acre before the third year of corn, and 14,000 gallons per acre before the alfalfa was seeded.

Management 1: Fall chisel plowing for corn and alfalfa seeding with manure incorporated by this tillage

Management 2: No-till all years with manure surface-applied in fall

Management 3: No-till all years with 7,000 gallons per acre manure applied in winter, remainder in fall

As you can see, the P Index values vary a lot from field to field under the same managements. Site susceptibility to erosion and runoff influence P losses. For these fields, the comparatively steep and erodible Ashdale silt loam in Green County has the highest particulate P loads for all managments. The Pella silt loam has a high potential for rainfall runoff, and therefore has the highest soluble P losses for no-till. Soluble P Index loads from manure applications increase for all fields when manure is applied in the winter insead of the fall.

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