Stacy Faulkner, Wes Wood, Wayne Reeves, and Randy Raper

Alabama ranks third in the nation in broiler production, marketing 921.8 million birds in 1999. The industry processes about 18 million broilers a week, and these broilers produce more than 2 million tons of litter each year.

Broiler litter is most often disposed of by application to local agricultural land, especially pasture and hay fields, due to hauling expenses. After long-term application of litter, phosphorus (P) accumulates in surface soil and is susceptible to losses through runoff and erosion. Although litter is a beneficial source of plant nutrients, long-term application can lead to degradation of surface water quality.

Phosphorus is considered to be the limiting nutrient causing freshwater eutrophication. Eutrophication occurs when nutrient input to surface water causes increased algae production. Overabundance of algae depletes oxygen from water and increases turbidity, or cloudiness, which, in turn, increases the potential for fish kills, growth of toxic cyanobacteria, and poor drinking water quality. The waters used for fisheries, industry, drinking, and recreation may all be negatively affected.

Many watersheds in Alabama are at risk for surface water degradation due to long-term application of manure with high amounts of P. Tillage practices have been shown to have a significant impact on surface water quality, affecting both P concentrations and losses. An AAES study was initiated to look at the impact of tillage on P in soils overloaded with broiler litter.

The study was conducted at the Sand Mountain Research and Extension Center (SMREC) in Crossville to determine the effectiveness of various tillage systems in diluting P into the soil profile. Tillage treatments of no-till, chisel plowing followed by disking, moldboard plowing followed by disking, and each of these in combination with paraplowing, a form of deep non-inversion tillage, were applied in 1998 to a pasture at the SMREC having a history of broiler litter application and an extremely high soil test P. Soil samples were collected using a tractor-mounted probe immediately following tillage. Soils were analyzed for total and inorganic extractable P.

Conservation tillage, which leaves a significant amount of residue and fertilizer on the soil’s surface, is promoted for purposes of reducing soil erosion. Although sediment P losses are decreased with this type of tillage, soluble P may accumulate at the soil surface. Conventional tillage by moldboard plowing involves the turnover of soil and soil amendments into the plow layer. Moldboard plowing does stimulate P losses through soil erosion, but without the incorporation of broiler litter, P will continue to accumulate on the soil surface, thus contributing to long-term P loss to surface waters.

Tillage primarily affected P concentrations in the top few inches of the soil (see figure). Moldboard plowing greatly reduced surface soil P concentrations compared to chisel plowing and no-till. Para-plowing further reduced P concentrations in some instances, although the decrease is not necessarily of environmental significance. Both no-till and chisel plowing are considered types of conservation tillage.

Mehlich-1 extractable inorganic P as affected by tillage at each soil sampling depth.

Phosphorus accumulation occurs because the N: P ratio for broiler litter is approximately 3:1, while plants take up these nutrients at a ratio of 8:1. Because litter is usually applied at nitrogen fertilizer recommendation rates, P application exceeds crop removal rates. The soils undergoing no-till and chisel plowing management had extractable inorganic, or plant available, P concentrations of 300 to 400 parts per million (ppm) at the surface (see figure). This is much greater than the optimum soil test P level of 25 ppm, above which P fertilizer should no longer be applied. Moldboard plowing reduced P concentrations to as low as 110 ppm at the soil surface. Ideally, application of litter by P rather than N recommendation rates would eliminate surface soil P build-up. Unfortunately, in many areas of Alabama, soils are already heavily loaded with P. It is unlikely that plants alone could ever be able to remove such an accumulation of P, even with no additional input. Deep tilling the soil may be the only way to remediate a build-up of this magnitude.

Reducing risks of P loss from surface soil is of utmost importance when considering surface water quality. Results of this study indicate that tillage is quite effective in reducing P in surface soil by incorporating litter into the soil profile, mixing high P concentration soils with lower P concentration subsoils. Researchers have suggested that deep tilling of P overloaded soils every five to 10 years may be sufficient to maintain reasonable soil test P levels. Further research on this subject will be required. So far, moldboard plowing soils with extremely high soil test P concentrations appears to be the best method for redistributing P and preventing long-term losses.