Linda F. Benedict, Breitenbeck, Gary A., Boquet, Donald J.
Gary A. Breitenbeck and Donald J. Boquet
Beneficial use in agriculture of organic wastes generated by municipalities, industry and agricultural commodities is receiving considerable attention as an alternative method of disposal because of the rising costs and environmental concerns associated with present disposal methods. Application of these wastes directly to soil or after composting can provide crop nutrients and improve soil structure and waterholding capacity. Fields that are droughty, depleted in organic matter or subject to erosion are ideal candidates. Farmers must weigh the potential benefits against the costs and risks of including these wastes in their production systems.
One way to use organic wastes in agriculture is direct application. After transport, the material can be broadcast with manure spreaders or specialized applicators and incorporated into soil using available tillage equipment. Slurries of organic wastes such as sewage sludge can be injected into soil with equipment specially designed for this practice.
The principal advantage of direct application is that it simplifies handling of the material. Also, addition of readily available organic matter generally results in greater improvements in soil tilth than when a more stable organic material such as compost is applied.
The principal disadvantages of this practice are that it requires transport of greater volumes of material that must be applied to allow sufficient time for degradation and avoid crop nutrient deficiencies. Readily degradable wastes can induce temporary immobilization of crop nutrients by soil microorganisms similar to that commonly observed when cover crops or crop residues are incorporated into soil.
Composting is a waste treatment often considered more benign than land filling or other disposal practices. Between 70 percent and 80 percent of most municipal waste streams consist of paper, food wastes, grass clippings and other biodegradable organic materials. The principal advantage of composting is that it can greatly reduce the volume of waste and thereby reduce transportation and handling costs. Composting of most organic wastes produces a clean, safe organic material suitable for use as a soil amendment around homes and on farms.
Some municipalities have constructed sophisticated composting facilities that receive their entire waste stream including their sewage sludge. Glass, metal, plastic and other nondegradable items are separated for landfill disposal. The finished compost produced by these facilities has the appearance of potting soil and is free of contaminants. The high temperatures generated during a properly managed composting process destroy pathogenic microorganisms and viable weed seeds. Most municipalities recognize the merits of composting. They also are aware of the additional costs of implementing and operating a compost facility. The economic viability of composting as an alternative to landfilling depends, to a large degree, on the development of suitable markets for the finished compost. Home gardeners and landscapers represent a ready but limited market. Clearly, if a significant portion of our nation’s waste stream is to be composted, production agriculture must become the principal recipient of the huge volumes of compost that will be generated.
Compost derived from urban and farm waste has been used successfully in crop production for thousands of years. The primitive agriculture of the past, however, differs from today’s intensive agriculture. LSU Agricultural Center research shows that benefits of compost use in production systems may be limited for many crops, including sugarcane, because of economic considerations Many farmers conclude that the possible benefits of amending their soils with municipal solid waste compost do not offset the costs and additional risks posed by including this practice in their production systems.
Industrial waste research
During the past several years, LSU Ag Center scientists have evaluated several industrial organic wastes for potential for plant nutrients and soil amendments. Louisiana’s Department of Environmental Quality generally requires companies considering beneficial use of their waste in agriculture to first obtain an exemption permit to demonstrate the benefits. This exemption permit usually requires an initial evaluation of the material in the laboratory and greenhouse.
Scientists at the LSU Ag Center have developed a battery of laboratory and greenhouse tests useful in identifying the value as well as the potential problems associated with various organic wastes. The material is analyzed to determine pH, organic matter content, carbon to nitrogen ratio, and content of nitrogen, potassium, phosphorus and other plant nutrients as well as heavy metal contaminants. Industrial organic wastes can contain appreciable amounts of sodium and other salts, and therefore the amounts and types of salts are measured. A nitrogen mineralization study is performed to assess the potential of the material to immobilize and release crop nutrients. The laboratory tests of various amounts of material on seedling germination reveal potential toxicities. A soil column study is performed to determine the effects of the waste on water infiltration, bulk density and soil structuring. We have found that although some organic wastes can improve water infiltration and holding capacity, others can effectively seal off the soil, preventing entry of water and air. Also, many organic wastes can have an offensive odor or develop an odor during transport or storage. A simple test is performed to score the odor nuisance potential of the material.
After these tests are performed, a greenhouse trial is conducted using bermudagrass to assess the potential of the organic waste to supply plants with nutrients and to cause phytotoxicities. The results of these tests are then used to determine optimum and maximum loading rates to be used in a small field plot evaluation. Not all wastes require a permit before field evaluation. The Solid Waste Division of the Louisiana Department of Environmental Quality offers guidance on permitting requirements.
Municipal solid waste compost research
Safe, environmentally sound and cost-efficient disposal of municipal solid waste is a primary concern for Louisiana’s municipalities, both small towns and large cities. Most municipal solid waste is sent to landfills. Wellmanaged landfills offer a relatively safe and inexpensive solution. Many older landfills have become full and are closed or in the process of closing, however. Locating sites for new landfills is difficult. Citizens near those sites object to the potential decrease in their propertyvalues and concerns about long-term effects of landfills on water supplies. Communities forced to haul their wastes great distances are seeking alternatives.
Curbside recycling programs recover a portion of the glass, plastic, metals and other recyclables. The bulk of municipal solid waste, largely organic materials of plant and animal origin, is not recovered by these programs. Technology is available to compost the entire waste stream, reducing the volume of municipal solid waste that must be landfilled by as much as 75 percent. The costs of composting municipal solid waste are high, and this approach has greatest appeal where landfill tipping fees exceed $35 per ton.
Another concern is the safety of the compost generated from municipal solid waste and biosolids. Biosolids are the organic solids, largely of microbial origin, that accumulate in wastewater treatment facilities. Land application of biosolids is carefully regulated to ensure that pathogens and pollutants, such as heavy metals and organic contaminants, do not adversely affect people, livestock and crops.
Health and safety concerns
Extensive research in Louisiana and elsewhere suggests that health and safety concerns about the use of municipal solid waste in agriculture are often exaggerated. For example, several field experiments involving municipal solid waste compost in sugarcane production in Louisiana showed no increase in heavy metals in soils, sugarcane plants or milled sugar. These findings are not surprising considering the low amounts of heavy metals present in the compost used in the studies. Populations of fecal and total coliform bacteria, indicators of the presence of pathogenic microorganisms, were well within established limits. In fact, more of these organisms were detected in unamended soils than where compost was applied.
Despite the apparent safety in using compost generated by well-managed facilities, farmers who routinely apply large quantities of compost should submit annual soil samples to state or private laboratories for analysis. State departments of environmental quality can provide information on local regulations. Also, compost can contain high numbers of fungal spores that can cause respiratory disorders. While such diseases rarely occur, the use of a dust mask by compost applicators is an appropriate safeguard.
The principal limitation in using municipal solid waste compost in agriculture is its inability to enhance yields sufficiently to offset shipping and handling costs. As a rule, municipal solid waste compost does not supply appreciable quantities of nitrogen or other essential nutrients to crops. The major benefits of applying municipal solid waste compost may result from improved physical properties related to increased organic matter content rather than its value as a supply of plant nutrients.
Compost typically contains 25 percent to 70 percent organic matter depending upon the composition of the waste material used to produce the compost. During the composting process, a substantial portion of the initial organic matter is released to the atmosphere as carbon dioxide as a result of microbial degradation. Once the process is complete, the organic matter that remains is similar to soil humus, the more stable fraction of soil organic matter. For that reason, addition of compost has a much greater impact on organic matter accumulation in soil than does incorporation of an equivalent amount of crop residue or other type of readily decomposable organic material.
Building better soil
Municipal solid waste composts can enhance the productivity of problem soils by increasing water infiltration and holding capacity, improving soil aeration and ensuring a more dependable supply of crop nutrients. Incorporation of municipal solid waste compost can improve aggregate stability and increase resistance of soils to erosion. This overall improvement of soil tilth often results in fuel savings during field operations. Improving the physical condition of the soil can increase access to fields following periods of heavy rainfall, which is important in humid areas such as Louisiana.
In addition to the overall benefits of increasing soil organic matter, compost has other properties that can be exploited to overcome specific production problems. For example, sugarcane production in Louisiana is usually limited to a fouryear cycle because of a decline in yield of each subsequent stubble crop. Stubble crop decline results from many factors including damaged root systems caused by winter cold, poor aeration and increased root disease. We have several studies under way to identify management practices that use compost to offset stubble crop decline to allow a profitable third stubble crop.
The ability of compost to suppress root diseases and nematodes is receiving attention. In general, experimental results have been inconsistent. While the prospect of controlling root disease with compost is intriguing, further research is needed to identify the mechanisms by which composts can suppress plant pathogens and to develop optimum control practices.
The chief economic and agronomic benefits of using municipal solid waste compost or other organic wastes in agricultural production are most likely to be obtained when the soil conditioning properties of these wastes are used to overcome specific production problems. Our society, in general, stands to reap the greatest rewards of using our organic wastes to preserve and enhance the productivity of our agricultural lands. By cautious redirection of our nation’s waste stream, we avoid the costs and environmental concerns associated with conventional disposal systems while contributing to the sustained productivity of our soil.
Gary A. Breitenbeck, Professor, Department of Agronomy, LSU Agricultural Center, Baton Rouge, La., and Donald J. Boquet, Professor, Northeast Research Station, Macon Ridge Location, Winnsboro, La.
(This article was published in the spring 1999 issue of Louisiana Agriculture.)