If the same amount of P is applied over a four- to five-year period, the total risk of loss is probably similar to that with an annual application compared with a heavier rate applied once during this period. More important is the time of application, or specifically the risk of a runoff event within a few days after application. More frequent application is likely to take more time, maybe reducing the flexibility of time of application and the ability to avoid application at times when risk of runoff is relatively great.

Risk of P loss in runoff decreases significantly with the time following manure application. Non-runoff precipitation events are helpful in reducing risk. These events provide moisture for soil and manure to bind and react to improve soil aggregate size and stability.

For more information on this topic, see the Extension publication Phosphorus Management and Water Quality Protection in the Midwest.

Charles Wortmann, Soil Scientist, University of Nebraska