Linda F. Benedict, Breitenbeck, Gary A., Boquet, Donald J.
Donald J. Boquet and Gary A. Breitenbeck
Previous studies with paper mill sludge have shown that this byproduct of paper manufacturing decreases cotton yields when it is applied either soilincorporated or as vertical mulch. The yield decrease is caused by severe nitrogen deficiency of the cotton plants, which is associated with the rapid decomposition of the sludge by soil microorganisms.
Soil organisms must have carbon to manufacture carbohydrates and nitrogen to make the protein they need to grow. Paper mill sludge is high in carbon but lacks nitrogen, which means it has a wide carbon to nitrogen ratio. Soil microorganisms use the readily available carbon in the sludge and, in the process, use any available nitrogen in the surrounding soil. This process is referred to as immobilization of nitrogen because the nitrogen consumed by the soil microorganisms is not available for plants to use. In time, the decomposition of the sludge is completed. Then the immobilization process is reversed, and nitrogen is released for plants to use. With paper mill sludge, this process usually takes several months to a year.
Immobilization of soil and fertilizer nitrogen can be compensated for by adding nitrogen or nitrogen-containing material along with the paper mill sludge. This may be impractical, however, because of the large quantity of nitrogen needed.
Adding organic materials such as paper mill sludge to these soils improves water infiltration and water-holding capacity and reduces the need for irrigation.
Using paper mill sludge as a soil amendment and, at the same time, avoiding excessive nitrogen immobilization was the subject of studies in 1996 and 1997. Soils in the Macon Ridge area of Louisiana are extremely droughtprone because of low organic matter and shallow hardpans. Frequent irrigation is needed to produce any summer crop, but irrigation is expensive and impractical in many areas because of uneven, sloping land topography and high salt levels of the aquifers used for irrigation water. Adding organic materials such as paper mill sludge to these soils improves water infiltration and water-holding capacity and reduces the need for irrigation. Organic materials buffer the crop against short-term drought.
The studies, conducted at the LSU Agricultural Center’s Macon Ridge Research Station in Winnsboro, had two objectives: (1) to determine precisely how much fertilizer nitrogen is needed to overcome the immobilization process when paper mill sludge is applied to a cotton field, and (2) to determine if paper mill sludge and fertilizer nitrogen could be applied in a way that kept them separate so that, during the decomposition of the sludge, bacteria could not use the fertilizer nitrogen.
For the first objective, paper mill sludge was soil-incorporated at 10 tons or 20 tons per acre with nitrogen rates of 0, 60, 120, 320 or 640 pounds per acre. The same rates of sludge also were applied as surface mulch with the standard fertilizer nitrogen rate for cotton of 80 pounds per acre injected 4 inches below the soil surface. The yields from these treatments were compared to yields obtained from normal cotton production and fertilization practices without sludge applications.
The quantity of nitrogen needed to compensate for immobilization was determined in 1997. A nitrogen rate of 320 pounds per acre was needed to supply enough nitrogen for both sludge decomposition and the cotton crop. This is clearly an excessive fertilizer nitrogen rate from both an economic and environmental standpoint. It would not be a practical approach to solving the nitrogen immobilization problem.
In contrast, where the sludge was applied as a surface mulch and fertilizer nitrogen was protected from immobilization by injection below the mulch, the highest cotton yields were produced with only 80 pounds of nitrogen per acre (Figure 1). This treatment yielded 30 percent more cotton than using the normal production practices without mulching. Thus the cotton crop was able to benefit from the surface mulch and did not require excessive nitrogen fertilizer.
The experiment was continued into 1998 for a second year without reapplying paper mill sludge. In 1998, the treatments with surface-mulched paper mill sludge and injected fertilizer nitrogen yielded 35 percent more than the control, confirming the results of the first year and also demonstrating that, once applied, the mulch benefit lasted for two growing seasons.
Second study in 1998
A second study was conducted in 1998 to verify that this surface mulch and to gain more information about paper mill sludge and nitrogen application rates. This study omitted the incorporated treatments that required excessively high nitrogen levels to produce acceptable yields and included only surface mulch treatments with injected fertilizer nitrogen. The rates of paper mill sludge were 0, 10, 20 and 30 tons per acre, and the fertilizer nitrogen rates were 80 pounds and 110 pounds per acre.
The 1998 growing season from May through August was one of the hottest and driest on record, with significant rainfall only twice. Even so, the benefits of the paper mill surface mulch in conserving the small amount of water available was obvious in yield increases. The cotton surface-mulched with 30 tons of paper mill sludge per acre yielded 30 percent more than where no mulch was applied. There was no yield increase from an increase in the fertilizer nitrogen rate from 80 pounds per acre to 110 pounds per acre, indicating that isolation of sludge and fertilizer nitrogen occurred and very little of the fertilizer nitrogen was immobilized. This conclusion is tentative, however, because of the very dry conditions in 1998 that may have limited the growth of soil bacteria and decomposition of the paper mill sludge.
Separation leads to success
Paper mill sludge applied as a surface mulch with fertilizer nitrogen injected into the soil increased cotton yield 30 percent to 35 percent. The key to successful use of this material with its wide carbon-to-nitrogen ratio and capacity to immobilize up to 300 pounds of fertilizer nitrogen per acre is to maintain separation between the sludge and fertilizer nitrogen. This can easily be done by applying the sludge as a surface mulch and injecting the fertilizer into the soil so that soil microorganisms do not have access to both the mulch and fertilizer nitrogen simultaneously.
Donald J. Boquet, Professor, Northeast Research Station, Macon Ridge Location, Winnsboro, La., and Gary A. Breitenbeck, Professor, Department of Agronomy, LSU Agricultural Center, Baton Rouge, La.
(This article was published in the spring 1999 issue of Louisiana Agriculture.)