Jim A. Entry, Brenda H. Wood, James H. Edwards, and C. Wesley Wood

Safe disposal of large amounts of municipal solid wastes (MSW) is a major problem for cities and large industries in the United States, but properly applying MSW to agricultural lands may provide a safe and beneficial way to dispose of these by-products. It may also help reduce the risk of negative environmental impacts from the application of animal waste products.

An average of 67% of municipal solid waste in the U.S. goes to landfills, 23% is recycled, and 10% is incinerated. Landfill disposal of MSW is expensive—ranging from $8 per ton in New Mexico to $75 per ton in New Jersey (1994 figures). Disposal of MSW also is under scrutiny by the U.S. Environmental Protection Agency, which has mandated a reduction in the nation's dependence on landfill disposal. The expense and problems associated with MSW disposal has caused many municipalities to look for alternative disposal methods for MSW. One potential method is applying organic MSW to agricultural lands.

Continual agricultural production can gradually decrease the organic matter content of soils, which can decrease soil fertility and crop yields. Applying animal wastes to farm land has been one effective way to improve the physical, chemical, and microbiological properties of soils, which ultimately results in increased crop yields. But animal wastes also run the risk of nitrate contamination to surface and groundwater. An AAES study was conducted to determine if organic MSW could provide benefits similar to animal wastes for soils and also mitigate nitrate problems associated with animal waste applications.

The study focused on carbon-nitrogen ratios. Carbon (C) and nitrogen (N) are soil nutrients that are important components in soil fertility. A proper balance of C and N is needed for nitrogen to be available to plants and for organic matter to break down in the soil. The study examined the influence on soil nutrient status of four organic by-products that were used as a source of C when combined with organic and inorganic sources of N.
The field study was initiated in spring 1992 at the E. V. Smith Research Center in Shorter. Treatments included: C sources from noncomposted organic by-products (newsprint, woodchips, yard trimmings, cotton gin trash) and N sources (poultry litter as an organic source and ammonium nitrate as an inorganic source). Noncomposted refers to organic by-products that have not undergone significant microbial degradation. Control plots also were established using no added C or N in the treatments. Plots were 30 feet long by six feet wide.

Organic by-products were applied annually in the spring and fall of 1992, 1993, and 1994. By-products were immediately incorporated into the top six inches of soil. When adjusted for ash and moisture content, each waste was applied at rates equivalent to newsprint applied at 12,400 pounds per acre of total C. Poultry litter was applied at 2,000 pounds per acre to supply 100 pounds per acre P₂O₅ (phosphorus) and 120 pounds per acre of K₂O (potassium). The final C:N ratios of all plots were adjusted to 20:1 with ammonium nitrate.

Plots receiving inorganic N were fertilized to bring the final P and K applications in the plots to 100 pounds per acre P₂O₅ (0-46-0) with concentrated superphosphate and 120 pounds per acre K20 with muriate of potash (0-0-60). Soil in all treated plots was then adjusted to a pH of 6.3 with lime (CaCO₃). Cotton was planted in the spring of each year; a cover crop was not planted. Soil samples were collected before application of organic by-products and from each plot in winter 1992 to a depth of six inches. Soil was analyzed for total C, total N, phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), manganese (Mn), iron (Fe), zinc (Zn), and pH.

Prior to addition of organic by-products and N sources, concentrations of soil C and N and soil pH levels did not differ among soils in treatment plots. After three annual applications of organic by-products and N sources, the pH of the soil did not vary among treatments but increased by at least one pH unit (a factor of 10) when compared to soil pH prior to amendments because of the addition of lime to treated plots (see the table). The addition of organic by-products and N sources increased pH, C, N, P, K, Ca, Mg, Mn, Fe, and Zn concentrations in all soils. In general, the addition of carbon sources with poultry litter resulted in higher concentrations of extractable nutrients in the soil than the addition of carbon sources and ammonium nitrate. The addition of cotton gin waste with poultry litter resulted in a higher concentration of nutrients in the soil than the other treatments.

Results of this study suggest that organic MSW by-products can be used to enhance agricultural lands and work especially well when mixed with animal wastes or other organic sources of nitrogen. It appears that a C:N ratio greater than 150:1 mixed with animal manure that has a C:N ratio of 10:1 provides a proper balance that may help reduce water pollution by nitrates while improving soil quality.

Entry is an Assistant Professor, B.H. Wood is a Research Associate, and C.W. Wood is an Associate Professor of Agronomy and Soils. Edwards is an Adjunct Associate Professor of Agronomy and Soils and a Research Scientist with USDA's National Soil Dynamics Laboratory.