Knowing When Plants Capture Phosphorus
Soil scientist John Kovar
plants corn seedlings into
a silt loam soil amended
with turkey litter. Kovar
and soil scientist Thomas
Sauer will compare the
effectiveness of the litter
and commercial fertilizer
as phosphorus sources for
corn and evaluate depletion
of available soil phosphorus
under intensive cropping.
ARS research into
how and when plants use the phosphorus in manure may aid farmers as
they try to stem nutrient runoff into waterways.
"A future challenge," says soil scientist Thomas
J. Sauer, "will be not only to avoid over-application of phosphorus
to soil, but also to ensure that in doing so a farmer does not make
the land phosphorus deficient."
Sauer and soil scientist John L. Kovar focus on phosphorus
as they study nutrient management of animal manure at ARS' National
Soil Tilth Laboratory in Ames, Iowa.
"Farmers are encouraged to reduce nutrient runoff
from their fields because of its effects on waterways," says Kovar.
"Aquatic plants absorb nutrients like phosphorus, grow more, and
consume oxygen in the water when they die and decompose. Also, the decomposition
process often gives drinking water a bad taste."
Kovar says that when soil is saturated with phosphorus,
costs and labor involved in applying more are wasted, as it goes unused
and becomes vulnerable to loss in runoff or movement into groundwater.
The scientists compared 27 manure-soil mixtures, which
included 15 soils from 7 states and manure from turkey, beef, swine,
and dairy farms.
Each manure-soil combination was divided into containers
and incubated for 1 year. During the year, phosphorus was extracted
from the soil eight times by three methods commonly used by farmers
and regulators. Sauer says all three tests were used with each manure-soil
"The study was unique in how we simulated field conditions,"
says Sauer. "We mixed the manure and soil together, then we varied
the incubation temperature so that it was cool in the morning and warmer
in the afternoon. We also changed the temperature and moisture periodically
to simulate seasonal changes."
"We set out to find when plant availability of the
phosphorus peaked," says Sauer. "Farmers who know when and
what amount of phosphorus is available to plant roots have an advantage
in making sure they don't over- or undercompensate when they replace
the phosphorus removed at harvest."
The results of the experiment can be used to predict plant
availability of phosphorus for various soils and crops.
The scientists found that, on average, 43 percent of the
phosphorus applied in manure was plant-available during the year after
application. Sauer says this is important to farmers trying to replenish
phosphorus removed at harvest with the same amount of the nutrient in
"If a soil has low to moderate levels of available
phosphorus, the grower may need to apply more than the amount of phosphorus
removed at harvest to meet crop needs," he says.By Luis
Pons, Agricultural Research Service Information Staff.
This research is part of Water and Quality Management,
an ARS National Program (#201) described on the World Wide Web at http://www.nps.ars.usda.gov.
Thomas J. Sauer
and John L. Kovar are with the USDA-ARS
National Soil Tilth Laboratory, 2150
Pammel Drive, Ames, IA 50011-4420; phone (515) 294-3416 [Sauer], (515)
294-3419 [Kovar], fax (515) 294-8125.
"Knowing When Plants Capture Phosphorus" was published in the January 2003 issue of Agricultural Research magazine.