Using Landscape Position and Soil
Information to Optimize Manure Application

Thomas J. Sauer
USDA-ARS

Sebastian M. Braum
University of Minnesota

In spite of marked changes in the form, collection, and storage of animal waste, little has changed in terms of attitudes and perception concerning land application of the waste material. Land application of animal waste is often reluctantly approached as a necessary yet unproductive activity. Consequently, growers fail to fully appreciate the waste material as a valuable nutrient and organic matter resource that can sustain and/or revitalize their soil. Failure to utilize animal wastes effectively can lead to over application and offsite impacts that encourage increased government regulation and costs of production (Govindasamy et al., 1994; Copeland, 1995; Morse, 1995; Martin, 1997).

Here we present some concepts for improving manure utilization by considering the role of landscape position and soil properties in designing land application strategies. A team of researchers, agency personnel, and growers has been organized to develop practical solutions and recommendations for effective animal waste management. The focus of this work concerns application of poultry and swine waste to grazing lands in the Ozark Highlands but the principles behind this effort are applicable to any region with concentrated livestock production.

Optimizing plant uptake of nutrients contained in animal waste should be one of the primary objectives of any animal waste management program. Profitable crop production can be sustained only by maintaining optimum levels of essential plant nutrients in the soil root zone. Excessive amounts of nutrients, when applied as commercial fertilizer or animal waste, create the potential for offsite movement to surface or subsurface water resources. In particular, transport of phosphorus (P) in runoff water or attached to eroded sediment and nitrate (NO₃) into groundwater are often observed after excessive levels of P and nitrogen (N) have been allowed to build up in the soil. Too much P in lakes and streams causes accelerated eutrophication (algal blooms, aquatic weed growth, and low dissolved oxygen). High levels of NO₃ in groundwater cause methemoglobinemia (Wetzel, 1975; U. S. Environmental Protection Agency, 1976). Therefore, effective manure management that promotes uptake of nutrients by plants is good for both crop production as well as the environment.

Most Best Management Practices (BMPs) offered for animal waste management relate to manure generation and handling or cropping considerations (Council for Agricultural Science and Technology, 1996). Clearly, there is no single solution to animal waste management problems. It is important to note that in most instances appreciable amounts of land-applied nutrients will not leave the site of application except when dissolved in water or attached to eroded sediment. Knowledge of how rainfall and snowmelt are partitioned between runoff and infiltration is key to improving the nutrient utilization efficiency of land-applied animal waste. This aspect of animal waste management has received little attention but is one of the central objectives of this research/education program.

The Savoy Experimental Watershed (SEW)

The Ozark Highlands are a center of poultry and swine production and the majority of the poultry litter (solid bedding material plus manure) and liquid swine waste is applied to pastures. The soil types and topography of the Ozarks in combination with increased poultry and swine production have led to concern over possible animal waste impacts on regional water resources.

The SEW is being developed as a cooperative research/education site between scientists, agency personnel, and producers to address issues of animal waste management in the Ozarks. The goal is to take a holistic approach to the management of nutrients derived from applied animal wastes and the excreta of the animals grazing the pastures. An interdisciplinary team of researchers with backgrounds in soil science, hydrology, animal nutrition, forage production, grazing management, and hydrogeology has assembled to initiate comprehensive studies of nutrient fate and movement within the watershed. An advisory committee composed of commodity group, producer, and extension representatives will provide input on the studies undertaken.

Figure 1 shows a soil map of the SEW which is located in northwestern Arkansas adjacent to the Illinois River. In general, the upland and less-steep side slopes of the SEW are in tall fescue and bermudagrass pasture while the steep side slopes and valley bottom are in hardwood forest. Compared to pastures outside the SEW, animal waste and commercial fertilizer applications in the watershed have been limited in recent years. The watershed is 363 acres in size and contains 6 soil types (Table 1).

As part of an initial site assessment, several soil properties were measured on surface soil cores (0 to 4 in.) taken from 4 soils (Rg, Ca, Gu, and Na). A complete discussion of these data is available in Sauer et al. (1998). A brief summary is presented here. Each of the soils sampled had comparable amounts of sand, silt, and clay (approximately 20, 70, and 10%, respectively) but the Rg soil had significantly more gravel (43% as compared to 26, 22, and 12% for Gu, Na, and Ca, respectively). As a result, the Rg soil had a much higher hydraulic conductivity than the other soils indicating a greater ability to transmit water. When ponded infiltration measurements were made the differences were much smaller. This is likely due to the presence of rodent burrows in the Ra soil and layering within the Rg soil. Water draining from the SEW must cross the Razort silt loam soil (Ra, Figure 1) in the alluvial plain before entering the Illinois River. Measurements on this soil showed that it had the highest infiltration rate of any of the soils measured (2.1 in h^-1). In addition, it also had the highest clay content (20%) and cation exchange capacity (an index of a soil's ability to retain some forms of nutrients).

This study indicates that soil types in the valley bottom and along the Illinois River have higher infiltration rates and potential to absorb P and N forms (i.e. higher clay content). Thus, nutrients transported in runoff water from animal wastes applied to upland pastures could be adsorbed when the runoff water reaches these soils. Even though the Ca, Gu, and Na soils have lower infiltration rates and clay content and are located on steeper slopes, application of animal wastes to these soils may not represent an unreasonable risk of nutrient runoff to the river due to the presence of soils with greater infiltration and nutrient adsorptive capacity downhill.

Recognizing that water is the primary carrier of nutrients from animal waste across and into soils, the varying ability of soils to infiltrate water and absorb nutrients present in the water are important considerations in devising a land application strategy. Preliminary data from a site in Arkansas suggest that changes in infiltration rate, nutrient content, and absorptive capacity with landscape position could be used to identify preferred locations for animal waste applications. Such an approach could supplement existing methods for assessing nutrient runoff potential (e.g. Lemunyon and Gilbert, 1993) and/or be incorporated into site-specific farm management programs (Vanden Heuvel, 1996) to both optimize plant uptake of nutrients contained in the animal waste and to reduce offsite impacts.

Table 1. Soil types of Savoy Experimental Watershed.

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