The method, amount, and timing of manure applications affect both the Particulate and Soluble P Index for a field. The equations that account for dissolved P losses from manure use the estimated water-soluble P in the manure remaining on the soil surface following application and also the runoff-to-precipitation ratio in the season-of-application. Dissolved P loss estimates from fertilizer applications use a similar set of equations. In Wisconsin, on average, a greater proportion of the precipitation that falls in the winter (when the soil is completely or partially frozen) runs off than for any other season. Therefore, winter manure applications lead to higher increases in the Soluble P Index than applications at other times of the year.
The effect of manure applications on the Particulate P Index can be negative or positive, depending on the P and solids contents in the manure as well as the field’s crop rotation and tillage. This is because manure applications increase the estimated P content of eroding sediment at the same time that the manure solids can have a dampening effect on estimated erosion rates. Manure applications are more likely to reduce estimated erosion rates when they have a high solids content and are applied to a field that is expected to have little residue at the time of application.
Figure 5 compares the annual P Index for three different methods and timing for applying 10,000 gallons of liquid dairy manure in the corn grain year of the corn-soybean rotation on the example field. A small amount of manure P is expected to be in runoff even after incorporation because chisel plowing leaves a small proportion of the manure on the surface. With this liquid manure, surface-applying increased both the estimated average particulate P and dissolved P losses over incorporation. Estimated average dissolved P losses coming directly from surface-applied manure are six times higher for winter than fall applications.
Figure – Annual P Index for the corn grain year in a corn-soybean rotation with 10,000 gallons fall-incorporated, fall surface-applied, or winter-surface applied liquid dairy manure for a chisel-plowed Loyal silt loam field with 4% slope, 47 ppm soil test P, and 2.5% organic matter. The P Index is shown by component: Particulate P I is P in eroded soil, Soluble P I soil is dissolved P from soil, Soluble P I manure is dissolved P from manure.
Phosphorus fertilizer was not applied in the above example because the soil test P is so high that no P fertilizer is recommended. (Note that a small amount of starter P fertilizer incorporated at planting does not usually create a noticeable difference in the P Index.)