Phosphorus losses from agricultural fields can be divided into three categories:
- Flash losses of soluble phosphorus soon after application of fertilizer or manure.
- Slow leak losses of soluble phosphorus.
- Erosion events.
Flash Losses Of Soluble Phosphorus
Manure, like fertilizer, has a vastly higher concentration of soluble phosphorus than soil does. If a rainfall runoff occurs soon after a surface application, the concentration of soluble phosphorus in the runoff can be more than 100 times higher than normal in the runoff.
Over time, highly soluble manure and fertilizer phosphorus on the soil surface will react with the soil, reducing soluble phosphorus in runoff back to initial levels. Normal levels return over the course of a month in warm soils, longer in cold soils. Manure and fertilizer application is not recommended on frozen or snow-covered soils because phosphorus never has a chance to react with the soil before runoff occurs.
Research from Arkansas on poultry litter and swine manure applied to pastures shows that soluble phosphorus concentrations increase in direct proportion to increasing application rate in these flash phosphorus loss events.
Flash losses of soluble phosphorus have high concentrations of phosphorus in a form that is readily available to aquatic organisms. These events occur only if rainfall runoff occurs soon after a surface phosphorus application or when phosphorus is surface applied to frozen or snow-covered fields. However, one ill-timed application can contribute more phosphorus to surface water than is lost by all other processes over the course of a year or more.
To minimize flash losses of soluble phosphorus:
- Apply phosphorus sources below the soil surface in a manner that does not increase soil erosion.
- Surface-apply phosphorus sources during periods of the year when runoff is unlikely.
- Surface-apply phosphorus sources only on fields with a low potential for runoff.
- Do not surface-apply phosphorus sources to frozen or snow-covered soils.
- Maintain low-phosphorus buffer strips around water resources.
- Add alum or a similar treatment to manure to reduce the availability of phosphorus.
Slow Leak Losses Of Soluble Phosphorus
All soils naturally release some soluble phosphorus into surface runoff. The concentration of soluble phosphorus in runoff is affected by the soil test phosphorus level of the soil.
Soil tests for phosphorus were developed to help estimate phosphorus fertilizer requirements for crops. Research on soils from other states indicates that soils near optimum soil test levels for growing crops typically support soluble phosphorus concentrations of 0.5 ppm or less.
There is substantial evidence that soluble phosphorus concentration in runoff will increase linearly with increasing soil test phosphorus levels. This linear relationship changes from soil to soil. Tripling soil test phosphorus above the high soil test category may increase soluble phosphorus in runoff to 0.5-2.5 ppm.
Slow leak phosphorus losses are important because they occur in every runoff event. Because of the cumulative effect of multiple runoff events, this mechanism can be the most important source of phosphorus loss. To minimize slow leak losses:
- Apply phosphorus only to fields that have an agronomic need for phosphorus.
- Reduce the amount of annual runoff from agricultural fields through crop selection and soil conservation practices.
- Maintain buffer strips where no phosphorus is applied around water resources.
When runoff water gains sufficient energy to cause soil erosion, the amount of phosphorus lost from the field increases dramatically. Controlling erosion losses by reducing or eliminating tillage on corn or wheat can reduce total phosphorus losses by 50 percent or more.
In soil, the total phosphorus is much higher than the soluble phosphorus content. Soil particles have a tremendous capacity to fix soluble phosphorus, allowing only a small proportion of the plant-available phosphorus to exist in the soluble form.
The sorting of soil particles that naturally takes place during erosion results in the soil particles with the highest phosphorus concentration being carried with runoff. Soils with higher soil test phosphorus levels will have higher phosphorus content in eroded particles. To minimize erosion losses of P:
- Adopt soil conservation practices to minimize soil erosion.
- Maintain buffer strips around water resources where no phosphorus is applied.
Source: University of Missouri Extension