Are Mulches a Good Idea?

DANA O. PORTER, P.E.
WVU Extension Service
Agricultural Engineering Specialist

As a result of restrictions on placing organic materials in landfills, recycling and disposal alternatives are being sought for yard wastes, paper wastes, newsprint, and other organic wastes. Yard wastes frequently are composted or recycled for mulch. Paper wastes and newsprint have been tested in many recycling applications, including mulch.

Perhaps you are considering using paper mulch on your farm or garden, but are not sure whether the material would be beneficial, or even detrimental to your production system. The following discussion may help in your decision-making.

1. Types of mulches: A mulch, in general, is a material placed on or spread over the soil surface. Mulches are used to protect the soil from erosion, conserve soil moisture, and suppress weed growth. Materials commonly used for mulch include plastic sheeting, compost, grass clippings, wood chips or bark, and nut shells (where available).

2. Effects of mulches: Mulches serve as physical barriers that dissipate erosive energy from raindrops, thereby protecting the structure of the soil at the surface (and thus improving permeability of the soil and reducing soil erosion). Mulches may serve as vapor barriers, thus reducing evaporation of soil moisture. Depending upon the type of mulch used, it may shade the soil, reducing weed growth and possibly reducing soil temperature increases due to solar radiation. In some cases, dark-colored materials or clear plastic may be used to increase soil temperature to allow for early planting or to encourage early seedling emergence.

3. Soil fertility: Some mulches are tilled into the soil before planting a new crop, and therefore may have an effect upon soil fertility and soil chemistry. In the short term, mulches may decrease nitrogen available for a given crop. A material that has a high carbon content and is very low in nitrogen and other nutrients may actually "bind" or immobilize plant-available nitrogen temporarily. This occurs because soil microorganisms use available nitrogen to metabolize and decay the organic material. The immobilized organic nitrogen can be made available (mineralized) later as the organic matter continues to decompose.

There may be some positive water quality implications associated with incorporation of mulch materials, provided no significant additional hazardous substances are introduced with the mulches. Organic matter can increase Cation Exchange Capacity (CEC). It also may increase availability of phosphorus in soils (Tisdale and Nelson, 1975). Organic matter and a higher CEC can reduce leaching of some organic and cationic pesticides (Brown, et al., 1983). In some cases, the temporary decrease in nitrogen availability may reduce nitrogen leaching losses below the root zone.

4. Soil structure: The effects of organic matter on soil structure include increased aggregation, increased infiltration (permeability), and greater water-holding capacity. In very coarse, sandy soils, addition of organic material can improve water retention, thereby reducing water losses to deep percolation and leaching losses of nutrients.

Mulches can reduce surface sealing and/or crusting in some soils with unstable aggregates, as energy from raindrop impact is dissipated. Accordingly, using mulch may reduce soil loss (erosion) of these soils.

5. Soil conditions and plant production: Mulches generally tend to shield the soil from solar radiation effects. Since evaporation from the soil surface is reduced, soil moisture likely will be higher in a mulched soil. Because of the increased moisture content and reduction of incoming solar radiation energy, a mulched soil also will be cooler. Further, the difference between day and night temperatures will be lower in a mulched soil. Some exceptions are: (1) dark mulch can adsorb more solar radiation and may actually increase soil surface temperature; (2) transparent plastic mulches may increase temperature through a "greenhouse" effect; and (3) plastic mulches can prevent water from entering soil, thereby decreasing soil moisture and increasing water runoff.

Whether increased soil moisture and decreased soil temperature are beneficial or detrimental to your crop depends upon the climate, your soil, and your crop. Low soil temperatures can retard growth of some crops, such as corn (Tisdale and Nelson, 1975). Increased soil moisture usually is good for supporting crop growth. However, excessive moisture in poorly drained soils may promote some types of plant pathogens. Whether you will benefit from applying a specific type of mulch, therefore, depends upon whether your crop will perform better with increased soil moisture, or higher or lower soil temperature.

6. Pathogens, insects and weeds: Mulches tend to decrease weed problems because they shade the soil surface and/or provide a barrier to limit weed emergence. Disease and insect pressures can be affected by the modified soil environment (temperature and moisture).

Research involving use of recycled paper in crop production

Benefits of surface-applied recycled paper mulches seem to depend upon the crop type and whether soil moisture is deficient or excessive. A multicounty, cooperative project was conducted in West Virginia to study recycling newspaper for mulching vegetable and small fruit crops (Selders, et al., 1994). In this project, several depths of shredded and chopped newspaper mulches were tested against a control (unmulched) soil and a black plastic mulch. Findings included: (1) higher tomato yields occasionally were observed with the use of paper mulches, but yield differences were reduced or insignificant in the absence of water stress; (2) paper mulch was credited with significant yield improvements for cucumber and bell pepper; (3) weed control in tomato plots was greater with greater depths of newspaper mulch, but excessive soil moisture in one study negatively affected plant vigor and soil temperatures, resulting in no marketable yield; (4) weed control in mulched bell pepper plots was significantly better than the control; and (5) paper mulches did not increase harvestable strawberry yield.

Recycled newsprint incorporated into the soil does affect soil fertility, and the effects may depend upon the type of fertilizer used. A field study in Alabama investigated newsprint and nitrogen source interactions and their effects on corn growth and grain yield (Lu, et al., 1994). The project involved application and incorporation of ground newspaper with various nitrogen sources to achieve a C:N ratio of 30:1. When the ground newsprint was applied to soil in combination with inorganic nitrogen sources (ammonium nitrate, urea, and anhydrous ammonia), corn seedlings were stunted during the first four to six weeks after emergence, compared to fields where poultry litter was used as the nitrogen source. When poultry litter was applied as the nitrogen source, ground newsprint increased the grain yield by 37%; when anhydrous ammonia was applied as the nitrogen source, ground newsprint reduced grain yield by 78% (Lu, et al., 1994). Application of ground newsprint had no effect on grain yield when ammonium nitrate and urea were used as nitrogen sources. Nutrient disorders and elevated levels of aluminum and iron were observed in corn treated with ground newsprint and inorganic nitrogen sources. The authors indicated that ground newsprint applied with poultry litter to balance the C:N ratio enhanced corn growth and grain yield.

Summary

In determining which type of mulch, if any, to apply to your crop, you should take into account the specific conditions in your field. Will your crop benefit from increased soil moisture, or will excessive moisture conditions promote pathogens or reduce plant vigor? Will lower soil temperatures help or harm plant growth and development? Will increased soil temperatures (from use of dark mulch) help or harm plant growth and development? Do you plan to remove the mulch from the field after the growing season, or do you plan to till it into the soil? If you incorporate the mulch material into the soil, can you observe and accommodate possible changes in crop nutrient requirements?

References

Brown, K.W., Gordon B. Evans, Jr., and Beth D. Frentrup. 1983. Hazardous Waste Land Treatment. Butterworth Publishers. Boston.

Lu, Ningping, J.H. Edwards, R.H Walker, and J.S. Bannon. 1994. "Newsprint and Nitrogen Source Interaction on Corn Growth and Grain Yield." In: Environmentally Sound Agriculture, Proceedings of the Second Conference, Orlando, FL., April 22-24, 1994. American Society of Agricultural Engineers, St. Joseph, MI.

Selders, Arthur W., Rodney M. Wallbrown, John E. McCutcheon, Michael T. Kubina, and Edsel Redden. 1994. "Recycling Newspaper for Mulching Vegetable and Small Fruit Crops." NABEC Paper No. 94356. Presented at: 1994 Northeast Agr/Bio-Engineering Conference, Guelph, Ontario, Canada, July 24-27, 1994.

Tisdale, Samuel L. and Werner L. Nelson. 1975. Soil Fertility and Fertilizers. Macmillan Publishing Co, Inc., New York.