Donald J. Boquet, Gary A. Breitenbeck and Christopher B. Coreil Jr.
Certain soils in the southern states may have low pH, low organic matter content and natural shallow hardpans that limit root development. The Macon Ridge, a geological formation that extends from central Arkansas to Baton Rouge, includes a soil, called loess, with these unfavorable characteristics. More than 300,000 acres of row crops, including large areas of cotton, are grown on this problem soil in Louisiana, which, because of its droughty nature, requires one to two weekly irrigations in summer to produce a profitable crop. Crops grown without irrigation usually do not produce profitable yields.
Extensive research has shown that longterm use of winter cover crops, such as wheat and hairy vetch, and reduced tillage practices are effective, but slow, ways to increase productivity of loess silt loam. Both eventually increase the soil organic matter. A faster and more economical way to improve soil productivity is to use waste or byproduct organic materials produced in municipal sewage treatment plants and agricultural processing facilities, such as paper mills and cotton gins, as soil amendments.
Each year in Louisiana, municipalities and manufacturing facilities produce more than two million tons of organic wastes, which are stored in lagoons and landfills or incinerated. Beneficial use as agricultural soil amendments would be a better method of disposal. This would recycle thousands of tons of plant nutrients that would otherwise be lost, possibly enhancing the productivity of land used for cotton production and protecting the environment from potential sources of pollution. Soil amendments
Research was conducted for three years at the LSU Agricultural Center’s Macon Ridge Research Station near Winnsboro to determine whether cotton yields could be increased by applications of organic and other waste materials to the soil. The experiments evaluated municipal biosolids from the Winnsboro treatment facility, composted sewage sludge from United Soils in Vidalia (using sewage sludge from New Orleans), and paper mill sludge (primary, dewatered) and paper mill boiler ash from the International Paper Company mill in Natchez, Miss., as soil amendments for cotton production. To ensure that the materials were safe for agricultural use, the sewage products were tested for fecal coliform bacteria, plant nutrients and heavy metals. All materials were determined to be safe for application to cropland.
The waste products were applied using two methods of application: 1) broadcast on the soil surface and incorporated, and 2) buried under the row in a 6-inch-wide by 24-inch-deep trench. This latter method is called vertical mulch. All treatments were applied in May 1996 either as a single material or in various two- and three-way combinations. For comparison, control treatments–standard production practices with inorganic fertilization of 80 pounds of nitrogen per acre–without any amendments were included.
After harvest in September 1996 and 1997, the plots were left undisturbed, except for stalk cutting, and the experiment was planted in May 1997 and April 1998 without re-applying amendments. Treatments that became nitrogen deficient were supplemented with inorganic nitrogen fertilizer. Supplemental nitrogen fertilizer was required by cotton in the papermill sludge and boiler ash treatments in each year. The experiments were grown without supplemental irrigation. To determine the effects of the treatments, data were collected on soil properties and plant growth and yield. Taller plants
In the first year, broadcast application of municipal biosolids increased plant height 16 percent and fiber yield 22 percent compared with standard practices. The vertical mulch was somewhat less effective, increasing yield only 10 percent. Composted sewage sludge was a less effective soil amendment than municipal biosolids. It increased yield when applied as a vertical mulch but not when applied broadcast. Application of papermill sludge decreased plant growth and yield. Cotton plants in these treatments were nitrogen deficient throughout the growing season even with the addition of inorganic fertilizer nitrogen. Two- and three-way combinations of the amendments were not better than applications of a single material except for the combination of municipal biosolids and boiler ash, which increased yield 12 percent above that obtained with municipal biosolids alone.
In the second and third year after application, residual effects of the soil amendments increased plant growth and fiber yield to a greater extent than in the first year. With both methods of application, the highest-yielding treatments were those containing municipal biosolids, which increased yield 50 percent to 55 percent above that of the standard production practices (Figure 1). Composted sewage sludge was not as beneficial to yield as municipal biosolids but did increase fiber yield 46 percent when applied broadcast.
Paper mill sludge increased yield in some treatments in 1997, a vast improvement over the 70 percent yield decrease of 1996. In 1998, the vertical mulch paper mill sludge again increased yield, but the broadcast method did not affect yield. The overall results with paper mill sludge as an incorporated soil amendment were not favorable. As an average of three years, application of paper mill sludge decreased cotton fiber yield. Boiler ash
Boiler ash applied alone increased fiber yield. But in the second and third year the benefits of adding boiler ash to other amendments were less than in the first year. This was likely because the amendment treatments contained high amounts of calcium, the primary nutrient supplied by boiler ash.
Much of the benefit from the amendments was from the nutrients they contained, especially nitrogen and calcium. This was evident from plant analysis that showed up to a 1400 percent increase in plant nitrogen from applying municipal biosolids. Additionally, some of the amendments increased soil pH and the soil levels of phosphorus and potassium. Greater yield increases in the years following application, rather than in the year of application, were related to the increases in availability of nutrients and in soil pH that occurred during the two years the amendments underwent decomposition by soil microorganisms. Soil-incorporated paper mill sludge treatments, especially, required a reduction in the carbon-to-nitrogen ratio to release the nitrogen immobilized in the first year. For plants to use the nitrogen, the ratio of carbon to nitrogen must be less than 30 to 1. If it is higher, the microorganisms within the soil use the nitrogen, preventing its use by plants.
The inorganic boiler ash was, as expected, an effective liming material and raised the soil pH. This was particularly beneficial in the vertical mulch treatment because increasing the pH of the subsoil reduced the availability of aluminum and manganese, making these elements less toxic to cotton plants. In addition to the nutritional benefits of the amendments, the organic components reduced the soil density. This increased water infiltration. Vertical mulching also eliminated the shallow hardpan directly under the row for three years, which allowed additional water storage and deeper root development. Root development out of the mulched area into the undisturbed subsoil was still limited, however. The interface between the mulch and subsoil was the area of greatest root development. Increased growth, yield
A one-time application of waste materials to cropland increased cotton plant growth and yield per acre and improved soil properties for three years. Application of organic materials with high amounts of nitrogen and narrow carbon to nitrogen ratio increased plant growth and crop yield. Paper mill sludge, with its wide carbon-to-nitrogen ratio, decreased yield in the year of application. In the year after application, the carbon to nitrogen ratio in the paper mill sludge treatments was narrowed by mineralization processes, and cotton plant growth and yield increased. Because of the immobilization of nitrogen and resulting yield decreases in the year of application, paper mill sludge is suitable for use as an incorporated soil amendment only if applied in combination with another material high in nitrogen. Using high-nitrogen organic materials as soil amendments is a faster, and perhaps more economic, means of improving soil productivity and cotton yield than other methods.
Donald J. Boquet, Professor, Northeast Research Station, Macon Ridge Location, Winnsboro, La.; Gary A. Breitenbeck, Professor, and Christopher B. Coreil Jr., former graduate student, Department of Agronomy, LSU Agricultural Center, Baton Rouge, La.
(This article was published in the spring 1999 issue of Louisiana Agriculture.)