Lewis Gaston, Terry Clason and Darren Cooper

Poultry is the leading animal agricultural industry in Louisiana. The industry is concentrated in the hilly, northern Coastal Plain area where land is used mostly for pasture and timber production. The grains used for poultry feed create a large influx of plant nutrients that accumulate in poultry litter. The litter is generally applied to nearby pastures and provides nutrients and organic matter to the soils, the two soil components that have historically limited crop production in Coastal Plain soils.

The commercial pine plantations in Louisiana generally have not been fertilized, but high prices for timber and recent research demonstrating growth responses to added nutrients have made fertilization of pine plantations a viable management option. The abundant supply of poultry litter would provide an economical source of nutrients for this purpose.

It is important in using poultry litter, however, to minimize nutrient losses to surface waters. Nutrient enrichment may degrade water quality, limit its intended use and offset benefits of poultry litter as a soil amendment. Loss of phosphorus to surface water is a particular concern because elevated levels may cause excess algae growth in freshwater systems.

Studies on the benefits of poultry litter fertilizer on pasture and forest soils (Ruston fine sandy loam) were initiated at the LSU AgCenter’s Calhoun Research Station

in 1995. Litter was applied to test plots of bermudagrass or loblolly pine from 1996 to 2001. See Table 1. This research was expanded in 1997 to test plots of bahiagrass on pine silvopasture soil (Wolfpen fine sandy loam) at the LSU AgCenter’s Hill Farm Research Station. See Table 2.

Yield of Forage Grasses, Growth of Pine

In the Calhoun study, the high litter rate treatment has given significantly higher yields than the low rate and check treatments in most years. Similarly, yields from the medium rate plots have generally been higher than from check plots. Effects of fertilization and increasing rate of litter on bahiagrass yields from the silvopasture plots have been similar.

Average relative increase in basal area of loblolly pine from 1995 to 2002 (Calhoun plots) increased with increasing litter rate, but this trend was not statistically significant until the most recent measurement (Figure 1). In part, this may reflect the expected lag between application of nutrients to established pine and growth response. Also, ice storm damage to trees on several occasions probably affected results. The trend of increasing growth with fertilization is less clear and nonsignificant with the older pine used in the silvopasture study.

Soil Quality

Recently, soil scientists have begun to examine the overall fitness of a soil to function in its surroundings. Besides the capacity of a soil to support plant and animal productivity, its capacity to maintain or improve water and air quality and support human health and habitation are included in the concept of soil quality. Many soils in the Louisiana Coastal Plain were degraded by erosion and nutrient depletion when these were row-cropped years ago. Therefore, amending such soils with a nutrient-rich, organic material like poultry litter may improve their quality.

The best evidence for this is seen in the accumulation of nutrients like calcium. See Figure 2. Addition of litter to pasture and forest soils increased the level of soil organic matter, particularly in the forest soil. See Table 3. However, the long-term effect of litter application on biological measures of soil quality such as earthworm populations and microbial activity is not as clear.

Buildup, Movement of Phosphorus

Six annual applications of litter at the high and medium rates greatly increased the concentration of phosphorus in the surface (zero to 6 inches) of pasture and pine soils. See Figure 3. Phosphorus buildup caused by five annual applications of litter to the silvopasture soil is similar, but there is no evidence of appreciable leaching of phosphorus in any of these soils. Even at application rates of 9 tons per acre, the level of soil phosphorus below 2 feet is not higher than in check plots. See Figure 4.

Runoff Water Quality

Elevated concentrations of phosphorus in surface runoff and subsurface seepage may degrade surface water quality. Both poultry litter and soil are sources of phosphorus loading into surface water. Clearly, the higher the rate of litter application and the higher the level of soil phosphorus, the greater is the potential for phosphorus loss.

Limited mobility of phosphorus in the pasture and pine soils, despite high rates of application, suggests that lateral subsurface loss of phosphorus to surface water may be minimal except where a shallow, impervious clay horizon exists. Whenever runoff occurs, however, dissolved and particle-bound phosphorus from soil or freshly applied litter will be transported downstream.

Results for phosphorus in edge-of-field runoff from the pasture and pine plots show highest concentrations following litter application and positive correlation with soil phosphorus level. After a few rains, the effect of freshly applied litter is gone, and subsequent loss of phosphorus is controlled by the level of soil phosphorus.

The total amount of phosphorus lost in runoff (loading) depends on runoff volume as well as concentration of phosphorus in the runoff. Although the concentrations of dissolved phosphorus in runoff from pasture and pine plots amended at the same rate with poultry litter are similar, total runoff volume from the pasture plots has consistently been greater. Thus, edge-of-field phosphorus loading from the pasture plots is greater than from the pine plots. However, within a soil type and land use, pasture or forest, there can be substantial variability in infiltration rate and runoff amount. Consequently, soil management objectives for minimizing nutrient losses in runoff should include improving rainfall infiltration rate.