Reveals Clues Toward a Proper Phosphorus Balance in Soil
By Luis Pons
January 15, 2003
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
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
ARS is the chief scientific research agency of the
U.S. Department of Agriculture.