PLSOIL 120 ORGANIC FARMING AND GARDENING

Maroon Divider
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Maroon Divider

  CHAPTER 5

MULCHES

A mulch is a covering layer of material applied over the surface of soil. The layer of mulch creates a buffer between the soil and aerial environment. Mulches are used for many different functions and may be composed of many different materials (Table 15). The functions of mulches include moisture conservation, weed control, temperature regulation, erosion control, and ornament, among other things. We commonly recognize straw, hay, sawdust, wood chips, and bark as materials for mulches. Plastics of varying colors also are well-known agricultural mulches. Lesser known materials are paint, gravel, stones, aluminum foil, paper, and carpeting.

Functions of Mulches

Moisture conservation

This practice is perhaps the most common reason for applying mulches. A mulch for this purpose should decrease the evaporation of water from soil and should increase the infiltration of water into soil. Water rises in the small pores and channels of soils through capillary action. A mulch breaks up the capillary rise and inhibits evaporation of water into the atmosphere. Also, the soil is protected from drying by direct sunlight and wind. Mulches decrease runoff over the soil surface and enhance penetration of water into the soil. On a bare soil, precipitation may not enter the soil as easily as it will run across the surface and off the land. Mulches absorb the energy of rainfall, impede runoff, and increase the time that water is in contact with a given area of soil, thereby increasing infiltration of water into the soil.

Mulches for moisture conservation should be loose materials which disrupt the upward flow of water, maintain a high humidity within the mulch, and allow water to percolate freely through the covering. These mulches should be 2 to 4 inches deep, being a little deeper on sloping land than on flat land. Thin mulches shift and decay and may not protect the soil over a growing season. The mulch should not be a water-absorbent material. Salt marsh hay is a highly preferred, although scarce, material. Its stiff stems resist packing and provide an ideal cover. Other good materials in descending order of quality are straw, hay, grass clippings, wood chips, composts, and manures. In dry climates, a dust mulch, created by tillage 1 or 2 inches deep, will impede evaporation. In humid regions, dust mulches must be created after each rainfall. Peat moss prevents evaporation from the soil, but peat moss absorbs water and does not allow precipitation to pass freely into the soil. Plastic mulches restrict evaporation but also inhibit movement of water into soil. Before plastic mulch is applied, it is critical that the soil moisture level be at field capacity. Water cannot be applied effectively by rain or overhead irrigation to crops growing under plastic mulch.

Weed control

In humid regions, this function may be the most important one. These mulches are particularly valuable when labor is in short supply or expensive. Mulching for weed control may make the difference in having a vacation or not having a vacation from the herb garden. Mulched crops require no tillage or at least a minimum amount of tillage. Weeds because of shallow rooting in mulched soil are easy to pull. In addition to labor savings, mulching helps to eliminate root or foliar injury that may occur during cultivation of unmulched crops.

Mulches for weed control should be thick enough or impervious enough so that weeds cannot grow through them. Straw or sawdust should be 3 or 4 inches thick. A similar mat of hay can be used, but it may contain plants that have gone to seed and that will infest the soil with weed seeds. Maintenance of a thick layer of hay helps to avoid the problem with weeds, but if the hay mulch decays or is turned under, weeds will be a serious problem on subsequently unmulched soil. Composts are excellent for weed control. Composts should contain few weed seeds unless weeds have grown in the compost pile or unless weeds were in the original materials entering the compost pile. Manures are not so good. They likely contain weed seeds from forages consumed by livestock. Manured plots usually become weedy, and mulching must be intensified by using thicker or impervious layers on manured land. If manures are used, composted manures or ones with a lot of bedding make a better surface for walking and for maintenance of crops than fresh manures low in bedding. Manures and composts are fertilizers, and a 2-inch thick layer may provide enough N and K nutrition to grow a crop.

Aggressive weeds, such as nutsedge, docks, perennial grasses, and Jerusalem artichoke, will grow through ordinary mulches. Impervious materials are needed to contain these weeds. Any affordable, sheets of impervious, nontoxic material, such as plastic or heavy paper, are effective. A layer of newspaper about 5 to 8 sheets thick placed underneath 1 or 2 inches of straw, compost, or manure will control most weeds. At the end of the season, the newspaper will be decomposed. No soil contamination results from the use of black-and-white news-paper. One, however, has to be careful in the use of plastic sheets under deep, permanent mulches on perennial plantings. Cases have been reported in which roots of perennials have grown into the layer of mulch over the plastic. These roots may constitute the major root system of the plant, and in dry weather they would be cut off from the soil by the plastic. Some people have almost lost their crops as a result.

Pulp, shredded paper, and shredded vegetable garbage harden into an impervious mat. For example, vegetable garbage can be ground in a garden shredder and poured onto the ground in a 2-inch layer, which will dry into a thin impervious mat with a texture somewhat resembling that of ceiling tiles.

A dust mulch has some effect in control of weeds. Sand, gravel, and stones give short-term benefits. In the long run, weed growth in the sand or gravel and around the stones destroys the effectiveness of these materials. These weeds can be controlled only by hand or by herbicides. One often regrets putting down stones or gravel as mulch to control weeds.

Temperature regulation

Temperatures in soils are stabilized by thick layers of organic matter. Plastic layers warm soils and facilitate early planting of crops in the spring.

For winter protection of crops, thick layers of organic matter are used. These materials should be applied after the ground has become cold after a few light freezes or after it has frozen. Their purpose is to keep the ground cold and to prevent the action of freezing and thawing in the soil. Freezing and thawing and the resulting heaving and subsiding movements may push plants out of the ground or snap their crowns from roots. Straw is the best readily available material for winter protection. Hay is also good. Avoid broad leaves, for these may pack and inhibit emergence of shoots in the spring. Conditions under the packed leaves may promote diseases that rot crowns of perennials. Mulch only up to the crown if broad leaves are applied. Pine needles or shredded leaves can be used more successfully than unshredded, broad leaves. Hardy plants that maintain some foliage over winter are often better left unmulched or should be mulched under their leaves.

It is not likely that a mulch can be applied deep enough to prevent the ground from freezing. In this respect, a deep cover of snow provides more protection than an organic mulch. The organic mulches may provide protection of shallow roots against dehydration from exposure to winter weather. A soil covered with organic mulch will warm slowly in the spring and may delay planting of annual crops. At 4-inch depths, soils under raw leaves will be as much as 20o cooler than bare soils. In this case, the mulch should be removed from areas to be planted. It can be replaced as soon as the soil warms. Mulches left on perennial crops in the spring will delay warming of the soil and will slow plant growth in the spring. An advantage of this action may be delaying early growth or flowering during unseasonable warm spells. In general, however, mulches applied for winter protection should be moved to the center of the rows after the danger of killing frosts has passed.

For enhanced warming of soils, use plastic mulches. Clear plastic mulches are the preferred ones for promoting earliness of crops. Clear plastics allow for greater warming of soils than black plastics. Although results vary among investigators, a clear plastic mulch may permit warming of 6 to 10oF to a depth of 6 inches, whereas black plastics permit warming of 1 or 2oF to 1 or 2 inches. Sunlight passes through the clear plastic and heats the soil. A layer of water on the underside of the plastic retains the radiant heat at night through what is known as a greenhouse effect. With black plastics, the mulch absorbs most of the sunlight and becomes greatly warmed, and little energy passes through to warm the soil. Shallow-rooted crops benefit from the warming permitted by black plastic and receive the added benefits of weed control and keeping fruit off the ground as the season progresses.

On a frosty night, the air above a plastic-mulched crop will be colder than that above bare soil, and foliar damage to the crop is likely to occur. Row covers or hot caps may be necessary to protect the crop. Ordinarily, to limit the costs of using plastic mulches, only the area around the plants is covered, leaving some bare ground between rows. Some warming of the air around the crop will occur from irradiation by the bare ground.

Light-colored plastic mulches, such as white plastic or aluminum-painted plastic reflect light. Soil temperatures under these mulches will be cooler than those under bare ground.

With perennial crops plastic can be left on site, but with annual crops, removal of plastic mulch at the end of a growing season often is a problem. All plastic mulches decompose in sunlight but do not decay in soil. Even biodegradable mulches require light for their breakdown. Problems with biodegradable mulches are that may not last for the season on the ground but portions under the ground will show little indication of decomposition. These fragments of mulch may be more difficult to remove and discard than nondegradable mulches. Plastic that is used on greenhouses is thicker (4 or 6-mil) than mulching plastic (1 or 1.5-mil) and is manufactured to be resistant to light-induced breakdown.

Experimentation with bituminous paints sprayed on the soil has shown that these materials are not effective in warming soils. Checking of the soil surface destroys these mulches, quickly dissipating any effectiveness that they may impart. Other black materials that might be used are soot, coal dust, and black paper. These black materials do not give warming equivalent to that of black plastic.

Other uses of mulches

Sanitation. Mulches will keep produce cleaner by keeping mud from splashing on upright or fallen plants. The mulch will lessen disease infection on foliage or fruits that come in contact with the soil. Organic mulches should not contain diseased plants or refuse from the plants that are being mulched.

Erosion protection. On sloping sites on which ground cover has not become established after tillage, mulches can protect soil against erosion. For densely planted crops, burlap or netting will help hold soil in place until plants become established. These materials are useful in applications in which the crop emerges through the mulch. The mulch must give no impedence to emergence while protecting the soil from erosion. Straw, hay, wood chips or shavings, or plastic may be used around or between rows of newly transplanted crops.

Ornament. Many mulches enhance the appearance of plots as well as provide other useful functions. Bark mulches are valued highly for their ornamental value. Wood chips because of their bright appearance when fresh are considered less desirable, but they soon weather into presentable mulches. Wood chips normally are much cheaper than shredded bark or chunks of bark. Cocoa hulls or cottonseed hulls make attractive mulches. Gravel, sand, crushed rock, and stones may provide walkways as well as attractive mulches. Weed control in areas covered with gravel, crushed rock, sand, or stone may be impeded by these materials. Tillage will be difficult to control weeds that grow through these mulches, and a lot of hand-weeding will be necessary.

Insect control. Reflective mulches, such as aluminum foil, foil-coated paper, or aluminum paint applied to plastic or directly to the soil have been demonstrated to repel insects. These materials are effective in repelling aphids that carry viral diseases that infect squash. Apparently, the reflective surfaces confuse the insects so that they do not land in the plots. Reflective white plastic mulches also have this capacity.

Plant nutrition. Manures, composts, hulls, leaves, and other plant residues contain plant nutrients, and sufficient nitrogen and potassium may leach from these mulches to fertilize a crop in a current season. Other nutrients generally are too low in concentrations in the mulch or too immobile in the soil to provide much nutrition to a crop in one season. By inducing shallow rooting, mulching promotes root growth in the top soil. This action permits plants to obtain nutrients in the richer zones of the soil and allows plant to obtain nutrients that have been applied to the soil surface and that have moved only a short distance into the ground.

Leaching control. On sandy soils, plastic mulches have been employed to inhibit leaching of fertilizers. In this use, mulch is applied over, sometimes elevated, beds in which concentrated bands of fertilizers are placed. The water table should be high in these soils; otherwise, the soil may become droughty.

Living mulches. Sometimes, plants are grown between rows of crops, sharing the same plots, as living mulches. Living mulches help to control soil erosion, crowd out weeds, aid in pest control, and, sometimes, add nitrogen. The trick is to grow the living mulch without harming the crop. Generally, the crop should grow in an 18-inch strip that is free of the living mulch. The mulches should be low-growing. Sometimes they are mowed to provide paths between rows of crops. They can be quite attractive in annual or perennial plantings.

Living mulches, even leguminous plants, may provide no nutrition to crops and may compete with the crops. Alfalfa and red clover for northern climates and subterraneum clover for southern climates are possible mulches for interplanting with row crops. Use of the subterraneum clover has been successful because it dies off in warm weather and does not compete with the crop. Living mulches should be used with caution in dry soils, for the mulch may dry the soil by transpiration. Deep rooted crops such as alfalfa could cause soils to become droughty. Aid in pest control come about from predators that may be in the living mulches. A potential worry from living mulches is that some pests may move from the mulch to the crop.

Problems with Mulches

Costs. Large amounts of materials are required for mulching. Polyethylene mulches cost about $200 per acre (4-ft wide strips placed on 5-ft centers). A typical cost might be $1,000 per acre to purchase organic materials. A bale of straw will cover about 25 to 50 sq. ft. of land. At least 20 tons of straw are needed to mulch an acre. Mulching with manures or composts may require 100 tons per acre. If one is to grow the materials, areas of land to grow the mulches need to be at least twice the area of land to be mulched. Labor to apply mulches is expensive.

Wrong application. Mulches tend to maintain soil conditions in the state that existed when the mulches were applied. A cold soil will remain cold; a dry soil will remain dry, and a wet soil will remain wet. Prolonging these conditions by mulching may not favor crop growth.

Packing of mulches. This problem was mentioned briefly above in discussions of including broad leaves in mulches. Care should be taken when mulching perennials to ensure that their crowns are not covered by mulch.

Decomposition of mulches. A mulched crop will have a shallower root system than an unmulched crop. If the mulch decomposes, the shallow roots, which may be practically at the soil surface, will be exposed. This problem is common in woody, perennial plantings such a blueberries. Sawdust and wood chips decompose slowly and make good mulches for woody perennials. Inspections should be made to ensure that roots in perennial plantings are covered with mulch, and additional mulch should be applied as needed.

Plant nutrition. Manures, composts, and hulls contain plant nutrients, primarily nitrogen and potassium. These nutrients ordinarilly are beneficial, but if a crop does not need any additional nutrients, harm may be done to the crop. Ornamental mulches of cocoa hulls have been known to cause foliar burning of rhododendrons. This burning is caused by the potassium that leaches from the mulch and that is then accumulated to excess by the plant. Bark, wood chips, and sawdust are so low in nutrients that toxicity from these materials is unlikely. One need not be concerned that decomposition of these mulches would affect plant nutrition. As long as the mulches are on top of the ground, little immobilization of nitrogen will occur in the soil, and this immobilization would be principally at the soil-mulch interface. If, for some reason, these mulches are turned into the soil, nitrogen deficiency may occur and last for up to 3 years. For every 100 lb of mulch turned in, 1 lb of nitrogen should be added to the soil at least during the first year. Generally, one should not apply fertilizers to mulches as long as they are on the surface, for this practice would hasten their decomposition. Nutrient deficiency may occur with plants in deep mulches of sawdust or wood chips. Roots will grow into mulches that are several inches thick (4 to 6 inches or deeper). These roots may become the principal feeder roots of the plants. Since the mulch might be nutrient deficient, roots in this zone would not be able to absorb sufficient nutrients, particularly nitrogen, to support plant growth. In this case, some fertilization will be necessary.

One does not need to be concerned with soil acidification under mulches. Some acidification may occur at the soil-mulch interface from organic acids produced in the mulch, but this acidity is dissipated soon after it enters the soil. A thin dusting of lime under the mulch will neutralize any acidity that may develop. With plastic mulches, all of the fertilizer necessary to grow the crop should be applied in the zone to be covered with mulch.

Mice. Rodents often live in the mulch and feed on the mulched plants and may creat problems particularly over winter. Woody plants may be girdled and killed. Plants should be protected by leaving space between them and the mulch or by somehow wrapping them with protective coverings.

Fire. Deep, dry layers of organic material, such as hay or straw, will burn if lit.

 

 

Table 15. Mulches and their uses.

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Function

Materials

Moisture conservation

Organic materials: straw, hay, sawdust, woodchips, composts, manures, dust

Weed control

Straw, sawdust, woodchips, leaves, black plastic, paper, thick layers of hay, composts, or manures

Temperature regulation

Plastic (soil warming), organic materials (soil insulation)

Sanitation

Plastic or organic materials

Erosion protection

Organic materials, plastic, burlap, stones, gravel

Plant nutrition

Composts, manures, hulls, leaves, plastic (to prevent leaching)

Insect repulsion

Light-colored mulches: white plastic, aluminum foil, foil-covered plastic or paper, aluminum-painted plastic

Ornament

Bark, woodchips, sawdust, peatmoss, hulls, sand, gravel, dust, stones, crushed rock

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Description | Syllabus | Notes |Guide | Internet | Lab Manual|Exams and Quizzes|Results|More|
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Produced and maintained by Your Name Allen V. Barker
University of Massachusetts, Amherst.
last updated - April 25, 1999