Phosphorus Leaching Differs in Dairy Manures
Perry May 28, 2009
Agricultural Research Service
(ARS) scientists have found that solid dairy manure is better than commercial
fertilizer in mitigating the amount of phosphorus that can accumulate in water
percolating through the soil. But using liquid dairy manure can make it worse.
These findings could help farmers in the semiarid western United
States protect local watersheds from agricultural pollutants. Idaho is now the
second-largest milk producer in the western United States, and farmers there
are using substantial amounts of dairy manure for fertilizing irrigated crop
fields. Phosphorus can fuel the excessive growth of algae and other plant
matter in freshwater ecosystems.
ARS soil scientists
Leytem used manure they obtained from two dairy farms in Idaho to study
phosphorus leaching in the fine sandy loam soils typically found in the region.
In laboratory tests they amended 24 soil columns with either liquid dairy
manure, solid dairy manure or monoammonium phosphate (MAP), a commercial
Then the researchers "irrigated" the soil columns 13 times over nine
weeks and collected the leachate-the liquid that drained out of the soil,
carrying substances picked up along the way-from each irrigation event. The
leachate was analyzed for total organic carbon and total phosphorus. After the
irrigation testing, they also analyzed the soil in each column for phosphorus,
carbon, calcium, iron and manganese.
Tarkalson and Leytem found that the largest quantities of phosphorus
moved through soils that had been amended with liquid manure. They also found
that the phosphorus in MAP was more mobile in the soil than phosphorus in the
The scientists also observed that liquid manure and solid manure
differed significantly in their carbon compound makeup, which may contribute to
the resulting variations in the manure leachates. Other factors may also play a
part in the dynamics of phosphorus leaching, including microbial activity and
metal content in the soil and the ability of clay particles in the soil to
attract and hold onto phosphorus.
Tarkalson and Leytem work at the
Northwest Irrigation and Soils Research Laboratory in Kimberly, Idaho.
ARS is the principal intramural scientific research agency of the
U.S. Department of Agriculture.