I am often asked in the conversation about issues of phosphorus from agriculture and water quality standpoint, “Is rate the only concern?” If it were only that simple, we would likely already have seen reduced non-point source loads from a water quality monitoring standpoint. Most of us have reduced fertilizer inputs as fertilizer costs have increased and this is documented by reduced phosphorus fertilizer sales.
Rate, or more appropriately soil test level, is important from a water quality standpoint. If we are at a soil test level that does not require a fertilizer application, risk of loss is reduced to whatever background levels are coming from the soil. If the soil test is in an agronomic range (15 to 40 parts per million Bray P1), water concentration of Total P in runoff are 0.5 ppm or less. If soil test levels are four to five times agronomic levels, we can see this become a runoff concentration of one ppm or more. As a point of reference concentrations of P in lakes at 0.01 to 0.04 ppm will support algae growth.
Risks of edge of field loss of phosphorus are greater when the nutrient is applied. Recently applied P is subject to loss based on the timing, source and placement during the nutrient application. The key to reducing loss is for the applied phosphorus to quickly equilibrate and stabilize in the soil before the next runoff producing rain event. Surface applications are most at risk. Soil test levels on the surface can be higher than the “0-8 soil test sample result” under tillage systems with reduced mixing. Surface water, or water preferentially flowing to a tile system, can result in an elevated water concentration of P that mimics the water quality results of the high legacy soil test situation. The same rate of nutrient applied can have different water quality results based simply on soil placement.
Erosion is a factor that should not be forgotten. Sedimentation and loss of sediment bound nutrient play an important role in water quality. It is still important to address surface runoff of water that results in soil erosion through conservation tillage, cover crops, waterways, structures and other practices that reduce water velocity or filter sediment.
Since runoff producing events have been mentioned a couple of times, “are there practices that will reduce these events?” Compacted soils shed water resulting in ponding and/or increased surface flows versus allowing infiltration. Practices that improve soil structure such as no-till/reduced tillage, optimal soil pH, drainage and other soil improvement practices will encourage water infiltration and retention of water. Tile drainage control structure can be added to subsurface drainage to retain water in the field. Retention of water is one factor that directly reduces edge of field nutrient loss.
The risk of P entering surface waters increases as the distance to concentrated water flow and/or the water body decreases. Fields where concentrated flow occurs within 100 feet of a channel have a higher risk of P loss to the connected water. The Ohio P index is currently under review by The Ohio State University and USDA-ARS. This research will provide a greater understanding of surface and subsurface water flows and BMP’s needed to reduce losses.
The issues of phosphorus and water quality effects are complex and contributions vary from field to field. It is not all about rate but includes a systems approach that includes 4R nutrient principals of rate, source, placement and timing along with tillage, soil health and water management.