Combining nitrogen fertilizer with poultry litter is an effective method for reducing soil phosphorus buildup and potential environmental problems, according to four years of Texas Agricultural Experiment Station research.
Reducing phosphorus buildup and runoff can be as simple as adding 50 to 100 pounds of nitrogen fertilizer in combination with poultry litter, the research shows. Forage yields increased too.
“The focus of this research is really on water quality,” said Gerald Evers, Experiment Station forage researcher based at the Texas A&M University Agricultural Research and Extension Center at Overton.
“In fields fertilized with animal manure, excess phosphorus runs off into streams, ponds and lakes during heavy rainfall. Wherever there is a confined animal feeding operation, you have the potential for problems with phosphorus,” Evers said.
In the eastern third of Texas, “confined animal feeding operation” usually means broiler houses. Approximately 600,000 tons of poultry litter are produced annually by Texas poultry operations.
“That's a lot of manure, and its most beneficial and environmentally safe use is as a nutrient source for plants,” Evers said.
Poultry litter has several advantages over nitrogen fertilizer produced from natural gas or other nutrients that are mined. If it doesn't have to be trucked too far, it can be more economical than commercial fertilizers. It contains organic matter — which commercial fertilizers do not — increasing the soil's ability to hold water and nutrients. And the nitrogen present is released slowly and is less likely to leach into groundwater.
But for every pound of nitrogen, poultry litter has about a pound of phosphorus. Forage crops typically use 4 pounds of nitrogen for every pound of phosphorus, so this means there's a lot of phosphorus left over each growing season. Also, a large part of the poultry litter phosphorus is bound to organic matter, whereas phosphorus from commercial fertilizers is largely water soluble.)
The water soluble phosphorus will percolate into the soil and bind with clay particles, but the microbial action slowly releases the organically-bound phosphorus via a process called “mineralization.”
The mineralization process is both a good and bad thing. During mineralization, the organically bound phosphorus is available to plants as if it were in a time-release capsule rather than in a jolt. Also, as it is mineralized, the phosphorus becomes water soluble. On the bad side, the organically bound material can sit on the surface and be washed off by heavy rains before mineralization can occur.
Applied in moderation, 2 to 4 tons per acre, and on fairly level ground, poultry litter can be environmentally friendly. The problems arise from using litter as the only nutrient source and in heavy applications — 5 tons or more per acre — year after year. Eventually, continued use of poultry litter can result in high soil phosphorus levels, 200 parts per million or more, building up in the top few inches of the soil. So where poultry litter is heavily used, the imbalanced proportions of nitrogen to phosphorus — the leftover phosphorus forage crops can't use — becomes a serious issue.
Phosphorus isn't toxic in itself, but because it is a plant nutrient if it enters surface water, it can cause runaway aquatic plant growth. The flush of growth can clog waterways, and some aquatic plants, such as blue-green algae, can produce toxins. When the excess plant material dies, oxygen levels drop, resulting in fish deaths.
Evers' research provides a blueprint of sorts of how poultry litter can be used over long periods of time without endangering Texas water.
“With poultry litter, nitrogen becomes the limiting nutrient for plant growth because all the nitrogen is taken up, but only about 25 percent of the phosphorus,” he said.
Evers compared several treatments during the four-year period on coastal bermudagrass pastures overseeded with annual ryegrass. He applied 4 tons per acre of poultry litter in October 1998 and October 1999 and 2 tons per acre in October 2000 and October 2001. Fifty-pound nitrogen applications were applied one to four times each year in December, March, May, and/or July.
“The idea was to increase plant growth with the extra nitrogen and take up some of the excess phosphorus,” Evers said.
After four years, he took soil samples. For comparison, Evers soil-tested plots where neither commercial nitrogen fertilizer nor litter was applied. He also had plots where he only applied poultry litter and no nitrogen.
The core samples from the plots with no fertilizer at all — neither litter nor nitrogen — showed base phosphorus levels of about 7 parts per million. The litter-only (without extra nitrogen) had phosphorus levels of more than 44 parts per million.
All the treatments with added commercial nitrogen to poultry litter showed a reduction of 25 percent to 50 percent of the residual phosphorus in the top 6 inches. In most instances, excess soil phosphorus was reduced as much as with a single 50-pound treatment as with two, three or four nitrogen applications.
“From the residual soil phosphorus standpoint, there was little advantage to applying more than one nitrogen application,” Evers said. “Because ryegrass has a higher nutritive value than bermudagrass and grows at a time livestock are fed hay, the best time for this single nitrogen application is during the ryegrass-growing period.”