Nutrient seepage from turkey litter operations
In recent years, a growing concern has been voiced by concerned citizens and environmental groups over nutrient contamination of groundwater supply from poultry production systems. To address this issue, a study funded by Minnesota Department of Agriculture purporting to investigate the nutrient status in the soil profile for deep-bedded litter systems used in turkey barns. In this project, nine turkey farms located in areas of different soil types in southern Minnesota were selected and the soils underneath the bedding materials sampled. The farm information is presented in Table 1.
Table 1. General information on the turkey barns sampled
|Barns||Age (year)||# birds/m2/stocking period||Soil texture|
|6||40 - 50||2.604||Silt loam|
|7||40 - 50||2.721||Sandy loam|
Six composite core soil samples were collected from each barn using a truck mounted, hydraulic powered soil sampling probe and a total of five soil samples were generated for each core at one-foot increments to a depth of five feet. The sampling was conducted over a period from early April to August 2000. All soil samples were analyzed for ammonia nitrogen (NH4-N), nitrate nitrogen (NO3-N), Olsen-phosphate, and pH. In addition to chemical analysis, for each site, one core sample collected from the barn center was also analyzed for soil textural components.
According to the pH analysis, for barns built on clay loam soils, the deep-bedded turkey litter has the potential to reduce soil pH by about one unit (from 7.94 to 6.98) and, thus, may impact soil chemical characteristics and groundwater quality located 4 feet below the surface if the barns are in continuous operation for 45 years or longer. For barns built on loam soil, the manure effect on soil mineral N up to 5 feet sampling depth may occur in less than 30 years. For turkey barns built on either a sand or a silt loam soil, the current data are insufficient to make legitimate conclusions on the rate of downward leaching of nutrients in the soil.
Data indicate that the manure phosphorus tends to stay in the topsoil for both loam and clay loam soils because of a big difference observed in phosphorus level between the top two sampling depths (114 vs. 23 ppm for loam soil and 45 ppm vs. 28 ppm for clay loam soil). Therefore, the phosphorus pollution of ground water resource due purely to seepage may not constitute a concern. In addition, the data indicate that clay loam soil is able to keep phosphorus from moving down by at least ten to twenty years, implying that the downward movement of phosphorus into clay loam soil could be a relatively slow process. It follows that the chance of phosphorus leaching to the groundwater through clay loam soil is unlikely if the water table is 5 feet below the ground surface. Again, data from the barns built on silt loam and sandy loam soils are insufficient to generate legitimate conclusions.
Speaking of nitrogen, the downward movement of NH4-N in loam soil covered with turkey litter is limited to the topsoil only. Thus, the potential pollution of groundwater by NH4-N leaching may not be a problem for barns in continuous operation of 30 years. Although the data are quite variable resulting in no statistical differences in most instances, the average concentrations are higher inside than outside, suggesting the need for extra caution when determining the possibility of NH4-N polluting groundwater. For the clay loam soil, it seems that the saturation of NH4-N in the topsoil is reached in 20 years of continuous operation (at 2178 ppm). Therefore, it may be suggested that the topsoil layer in turkey buildings be replaced every 20 years for areas of high groundwater (within 3 feet or so), and every 40 years for areas with low groundwater (greater than 5 feet). Obviously, this suggestion may not be practical for most turkey operations. However, if the concern of groundwater pollution becomes an issue that threatens the growth of turkey industry, the measures recommended herein may offer an option to the growers. Silt loam soils may not be suitable for turkey litter sites because of the poor retaining capacity of nutrients, while sandy loam soil needs further research.
For the clay loam soil, the NO3-N can leach about 2 feet deep in ten years. For barns older than ten years, it is observed that the NO3-N concentrations are significantly higher inside than outside of barns throughout the 5 feet sampling depth. Therefore, it is recommended to replace the topsoil layer, in instances where the barn is located on a clay loam soil, every ten years to obviate the potential pollution of groundwater resource by nitrate leaching. Similar measures may also be needed in cases where the barn exists on a loam soil. Further research on barns located on silt loam and sandy loam soils is warranted before definite conclusions can be drawn.