Factors for Phosphorus loss from land to surface water

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. The key to reducing loss is for the applied phosphorus to quickly equilibrate and stabilize in the soil before the next runoff producing event. Surface applications are most at risk. Soil test levels on the surface can be higher than the eight-inch soil test sample result under reduced tillage systems. Surface water, or water preferentially flowing to a tile, can result in an elevated water concentration of P. The same rate of nutrient applied can have different water quality results based simply on placement — surface verses incorporated for example.

Erosion is a factor that should not be forgotten. Loss of sediment bound nutrients 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 structures 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.

If you are concerned about runoff potential; a good source of a printable forecast can be obtained from NOAA through the website http://weather.gov. A zip code close to the application site can be entered on the website. A detailed hourly forecast graphic can be reviewed and printed. Rainfall can be totaled from the graphic to obtain the needed 12 or 24 hour predicted rainfall. A short video presentation showing how to obtain the forecast can be found on the OSU Agronomic Crops Team YouTube Channel at https://www.youtube.com/watch?v=Z7Ip8hsL4bA.

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