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Field Testing Reveals Clues Toward a Proper Phosphorus Balance in Soil

Timing can be everything, especially when it comes to how much of the phosphorus applied to crop fields can be kept out of waterways.

That's why Agricultural Research Service scientists, in studying how plants capture phosphorus in manure, went to great lengths to recreate field conditions.

Soil scientists Thomas J. Sauer and John L. Kovar--who study nutrient management at the ARS National Soil Tilth Laboratory in Ames, Iowa--mixed several manures and soils together and changed the temperature and moisture periodically to simulate seasonal changes during a one-year incubation period. They did this because they believe it's important for farmers to know not just how much phosphorus is in manure, but when it is actually available to plants for uptake and use.

For farmers, readily available soil phosphorus is a must if they expect profitable yields. But since plants can absorb only so much of it, over-application leads to unused amounts that remain in the soil. There, it's susceptible to being lost with runoff into streams and rivers, where it can lead to algal blooms and oxygen depletion, among other problems.

However, under-application will make the soil phosphorus deficient, reducing yields.

The math that farmers must perform in order to strike a proper phosphorus balance is complicated by peaks and valleys in phosphorus availability that plants encounter during their growth.

Sauer and Kovar set out to find out how manure phosphorus availability to plants changes with time. Among their findings was that, on average, only 43 percent of the phosphorus applied in manure was available for plant uptake during the first year after application. That means that farmers may have to actually add more phosphorus than they remove with crops at harvest to totally replenish the soil.

Read more about this research in the January issue of Agricultural Research magazine.

ARS is the chief scientific research agency of the U.S. Department of Agriculture.