Most of us have reduced fertilizer inputs as crop prices have gone down and this is documented by reduced phosphorus fertilizer sales. But still, I am often asked about issues of phosphorus from an agriculture and water quality standpoint, “Is rate the only concern?”
Rate, and more appropriately soil test level, is important from a water quality standpoint. If we are at a soil test level that does not require any fertilizer application, perhaps 35% of our fields, then risk of loss is reduced to whatever background levels are coming from the soil. If the soil test is in an agronomic range (15-40 ppm Bray P1), water concentration of Total P in runoff is 0.5 ppm or less. If soil test levels are four to five times agronomic levels, however we can see this produce a runoff concentration of 1 ppm or more.
Recently applied P is subject to loss based on the timing, source and placement during the nutrient application.