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.
How quickly eliminating or reducing P will reduce P Index values will depend on the particular field and cropping system. For example, a high yielding field in a corn silage alfalfa rotation could be expected to remove enough phosphorus to reduce soil test P (Bray P1) by 3-5 ppm per year if no additions are made. A field on a Loyal soil in Marathon County in this rotation with fall tillage and a starting soil test P value of 300 ppm and initial P Index value of 6 would require 10-12 years of no additional P applications to reduce soil P concentrations enough so that the P Index was 5.
Conversely, increasing soil test P in any field by adding more P than crops remove will cause the P Index values to slowly rise. 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 P Index values even in fields with low erosion and runoff rates.