Reduce Soil P to Lower Runoff Particulate P and Dissolved P
Reducing soil P concentration in a field will decrease both eroded sedimentP and dissolved P in runoff, thereby decreasing the P Index. The P Index equations use routine soil test P (Bray P1) to estimate runoff P concentrations.
Soil test P can be reduced by applying less P to a field than crops remove. Over the long term, this is an effective strategy to reduce P delivery to nearby surface waters. It may take a number of years, however, to lower soil test P enough to substantially reduce the rotational P Index.
An example of an alfalfa field with a soil test P of 100 ppm :
Soil test P should drop by approximately 1 ppm for every 18 lb P2O5 equivalent per acre removed by the crop and not replaced with manure or fertilizer. Alfalfa removes about 13 lb per acre P2O5 equivalent per ton of yield. A 5 T per acre alfalfa annual yield removes about 65 lbs of P2O5 equivalent per acre. Over the year, soil test P would be expected to drop by about 4 ppm, or 4%, if no fertilizer or manure P is added. Consequently, the concentration of dissolved P in runoff and in eroded sediments would be expected to drop about 4%, as would the soluble PI.Conversely, increasing soil test P in this field by adding more P than crops remove will cause the P Index values to rise in the same way. Although one year's excess P application may not make a big difference, applying excess P year after year will gradually increase soil P concentrations, leading to high PIs even in fields with low erosion and runoff rates.
Except where manure or other organic materials are applied to meet crop N needs, there is no reason to apply P in excess of recommendations. One way to reduce P buildup in soils is by reducing the P-to-N ratio in the manure by adjusting P levels in feed. Many farms have manure with a higher than necessary P concentration due to feeding more P than is needed.
Incorporating manure into the soil greatly decreases P concentrations at the soil surface. However, in some situations, incorporation results in disturbance that increases erosion significantly. If erosion increases, so will the loss of P in soil particles. Depending on the soil, cropping system, tillage, and manure type, it is possible for a field to have a higher P Index value if manure is incorporated than if it is left on the surface. The NRCS Tillage Equipment Pocket Identification Guide shows pictures of manure injectors that cause high or low levels of soil disturbance.
