Lewis Gaston, Darren Cooper and Matthew Stephens

Controlling phosphorus loss from pastures fertilized with poultry litter is a nutrient management problem poultry farmers face. Some of the phosphorus lost from litter or soil enriched with phosphorus by years of poultry litter fertilizer winds up in surface water where it nourishes algae and aquatic plants and may increase their density. A shift in fish populations also may occur. From the standpoint of water-quality standards, this scenario constitutes impairment.

While there is no evidence surface-water eutrophication – or nutrient enrichment – like this is linked to the use of poultry litter fertilizer in Louisiana, research has demonstrated the usefulness of several management practices for reducing the loss of phosphorus from litter and soils amended with it. For example, the concentration of phosphorus in litter may be reduced by altering the poultry diet, thus reducing the buildup of phosphorus in soil as well as the direct loss of phosphorus from litter. In addition, various chemical amendments that contain aluminum, calcium or iron can be added to litter or soil to reduce the solubility of phosphorus and reduce its movement into surface water.

The movement of phosphorus from the soil into surface water also can be reduced by removing phosphorus from soil. This is done by growing and harvesting a crop without further phosphorus fertilization. For Louisiana pastures overly enriched with phosphorus, the crop would likely be Bahia grass, Bermuda grass or perhaps annual ryegrass. However, if a market for biofuel feedstocks such as miscanthus or switch grass develops, these grasses might be used to remediate high-phosphorus soil.

In LSU AgCenter research, plots approximately 33 feet by 10 feet on Ruston fine sandy loam soil were planted with common Bermuda grass at the Calhoun Research Station, and triplicate plots were fertilized with poultry litter at rates of zero, 2.2, 4.5 and 8.9 tons of dry matter per acre annually for six years. At the highest rate, this led to a soil phosphorus concentration of more than 400 parts per million in the surface 6 inches by 2001, and no litter was applied after 2001. Bermuda grass hay was cut an average of three to four times annually from 1997 to 2001, and dry matter yield and tissue concentration of phosphorus were determined.

From fall 2001 to 2004, annual ryegrass was seeded into the common Bermuda grass, and annual ryegrass and common Bermuda grass were harvested three to five times annually through summer 2005. Dry matter and tissue phosphorus were determined for each harvest. Small sub-plots of Bahia grass, common Bermuda grass, crab grass and switch grass were established within the main plots in 2002 and harvested three to five times annually for yield and tissue phosphorus concentration through 2005. From 2002 to 2005, plots were fertilized with 180 pounds of nitrogen per acre annually, and potash was applied as needed based on soil test results.

During the years litter was applied, average annual phosphorus uptake and removal ranged from 29 pounds per acre for control plots to 56 pounds per acre for the highest litter rate, with phosphorus removal tending to increase with increasing years of litter application (or soil phosphorus concentration). However, removal of phosphorus was less than the amount applied, ranging from an average of 56 percent applied at the 2.2-tons-per-acre rate to only 18 percent at the 8.9-tons-per-acre rate. Thus, phosphorus buildup occurred despite hay harvest, particularly at the highest fertilization rate.
 
Phosphorus uptake and removal with annual ryegrass and common Bermuda grass from 2002 to 2005 without phosphorus fertilization represent a potential reduction in soil phosphorus for this common north Louisiana soil. Table 1 shows little residual effect of poultry litter on average annual yields. On the other hand, phosphorus uptake and removal tended to increase with the increasing rate of previous fertilization, particularly for annual ryegrass, which showed an increase in tissue concentration of phosphorus with increasing concentration of soil phosphorus. Significantly more phosphorus was removed from the high-phosphorus plots than from the control plots by annual ryegrass in all years and by Bermuda grass in two of the four years.

This combination of summer and winter grasses had an average soil phosphorus removal rate ranging from 32 to 51 pounds per acre annually, depending on soil phosphorus concentration. Assuming essentially all phosphorus taken up came from the upper 6 inches of soil, this represents an average annual reduction in soil phosphorus of from 18 to 28 parts per million, or as a percentage of initial soil test phosphorus, from 18 percent to 6 percent. While it is evident that hay or silage harvest of these grasses can easily keep soil phosphorus in check where concentrations are moderate, research on the relationship between soil-test phosphorus levels and the movement of phosphorus in runoff indicates there is typically a break point in soil phosphorus above which phosphorus loss greatly increases. Thus, even though soil phosphorus removal in hay from a high-phosphorus soil may affect only a small reduction in the percentage of soil-test phosphorus each year, one or a few years of harvest removal may greatly reduce phosphorus movement into surface water.

While the combination of common Bermuda grass and annual ryegrass is clearly effective in reducing soil phosphorus, average annual phosphorus removal from control plots of common Bermuda grass from 1997 to 2001 was numerically greater than from these plots when they were overseeded with annual ryegrass from 2002 to 2005. The potential for soil phosphorus removal by common Bermuda grass and annual ryegrass may not be additive because annual ryegrass may delay regrowth of common Bermuda grass.

Results from the sub-plots from 2002 to 2005 are shown in Table 2. Trends in yields of Bahia grass, common Bermuda grass, crab grass and switch grass were similar to those for common Bermuda grass and annual ryegrass – generally increasing with increasing previous fertilization with poultry litter. Removal of soil phosphorus in the harvested crop likewise increased. Crab grass and switch grass removed significantly more soil phosphorus than Bahia grass and common Bermuda grass. The somewhat greater phosphorus removal by switch grass than crab grass was significant in four of five years and due to the much-greater yield of switch grass, which more than compensated for its lower tissue concentration of phosphorus. Besides greater biomass, another advantage of switch grass is that it is perennial. However, if harvested less frequently to preserve its long-term productivity, yield and phosphorus uptake removal by switch grass may be lower.

Lewis Gaston, Associate Professor, School of Plant, Environmental & Soil Sciences, LSU AgCenter, Baton Rouge, La.; Darren Cooper, Research Associate, Calhoun Research Station, Calhoun, La.; Matthew Stephens, County Agent, Jackson Parish, Jonesboro, La.

(This article was published in the spring 2010 issue of Louisiana Agriculture.)