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If a farmer is over applying phosphorus ... can it be tested to tell where it came from?

If a farmer is over applying phosphorus, theoretically, can you tell who is doing it so they can be "busted"? Can it be tested to tell where it came from?

Phosphorous can move in water two ways. It can either be dissolved in the water or attached to soil particles. Once it is in the water column, all dissolved phosphorous or all particulate phosphorous is alike.

There aren't practical or economical tests to determine its place of origin. Testing of runoff is very expensive due to the fact it has to be monitored 24 hours a day, 365 days a year to capture all that runs off. It can leave fields in many different ways and in many different directions.

Unlike a factory, where you can measure what is coming out of the pipe, it is impossible to measure non-point runoff from every single acre on a large scale. To test runoff from all farms is cost prohibitive. That is why we monitor larger watersheds and use research plots to determine what is coming off a watershed.
Farmers can monitor what is going on within their own farms by a combination of soil tests, manure tests, and crop yield measurements. Most farmers do this because phosphorous costs money and the best farmers don’t want to apply any more than needed. In addition, larger animal operations that are permitted are required to keep these records. Most farms are not losing large amounts of phosphorus per acre, but there are so many acres in the Maumee that a little bit per acre adds up to a lot. The timing and method of application is also very important in determining the amount leaving.

Steve Davis, USDA-NRCS

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