Michigan State University Extension
Ornamental Plants plus Version 2.0 - 00001799
01/01/98
Because organic mulches are derived from plant material, they will decompose and affect both the soil and the plants in several ways.
Physical Effects
If mixed in the upper soil layer, the mulch dilutes the soil and usually increases root growth. When a mulch such as sphagnum peat moss, bark or aged sawdust is used, it affects the soil almost immediately. On clay soils, bark or aged sawdust increases aeration. Sphagnum peat moss, increases the water-holding capacity is of sandy soil.
If the mulch is not well decomposed, it will promote granulation, or clinging together, of the soil particles. During decomposition of the organic material, soil micro- organisms secrete a sticky material that glues soil particles together to form granules. This is especially true of heavy soil types. Materials that decompose slowly, such as sphagnum peat moss, have little effect on granulation. Straw, fresh leaves or aged manure, which decompose rather rapidly, do promote granulation.
Mulches improve and stabilize soil structure, the arrangement of the soil particles. Because of the mulch layer, the soil structure is not disturbed by pelting rain or coarse streams or drops of water from irrigation devices. Some gardeners do not realize that cultivating when the soil is wet destroys good soil structure. Using mulches eliminates the danger of cultivation at the wrong time because very little, if any, cultivation is necessary. Another way to harm the soil structure is to walk on the soil when it is wet. A mulch on the soil serves as a cushion and reduces soil compaction.
Chemical Effects
The pH, the degree of acidity or alkalinity of the soil, may be slightly affected. Acid sphagnum peat moss will gradually lower the pH--make the soil more acid--while most other organic materials will raise the pH slightly--make the soil reaction more alkaline. Oak leaves may be acid when reasonably fresh, but as they finally decompose, the net result is a slightly alkaline reaction. Continuous use of oak leaves, however, will keep the soil acid.
Small amounts of plant nutrients become available during the decomposition of the mulch because it, too, was once plant material containing these essential elements. The amount of fertilizer added is usually very small, however, and may not have much effect on the nutrients available in the soil. As an example, manure has iron in it but not enough to correct iron deficiency in an azalea or a pin oak. Reasonably fresh animal manure could supply nitrogen but the ammonia in it could damage the foliage and/or roots. Dried animal manures that are packaged for sale are relatively high in nitrogen. If used at rates higher than those recommended on the container these products can cause problems from too much nitrogen. Broadcasting gypsum or superphosphate will reduce the problem from ammonia fumes.
Nitrogen starvation or deficiency may become apparent in plants mulched with certain materials. If organic mulches such as crushed corncobs, sawdust, wood chips or straw are used, micro-organisms decomposing the organic mulch remove appreciable amounts of nitrogen from the soil. The removal of this nitrogen, which is used by the micro-organisms, reduces the nitrogen reserve in the root zone of the growing plant. If additional amounts of nitrogen fertilizers are not applied regularly, nitrogen starvation occurs in the mulched plants, which become yellowish-green and grow slowly.
Biological Effects Organic mulches serve as food for many micro-organisms that maintain and promote soil granulation. The mulch also helps keep the temperature fairly constant so that activity of the micro-organisms can occur at a uniform rate.
Sometimes undesirable organisms--disease-causing fungi, bacteria and nematodes--may be added to the soil when mulches are applied. Molds often develop on cocoa bean or buckwheat hulls when these materials are kept too moist or are used in shady spots. Stirring them occasionally so the surface of the mulch will dry eliminates the mold. Insects and rodents may overwinter in some organic mulch. Poison baits can be used to eliminate rodents, which may damage plants.
Weed seeds may be introduced into the garden with hay or straw or strawy manure. If you use various kinds of hulls or corncobs, be sure they are free of the grain or seed. Grain mixed in with crushed corncobs can attract rodents. To control weeds, mulch needs to be sufficiently deep, or weeds from the soil below will grow through it.
Organic Mulch Materials
Grass Clippings
Grass clippings are readily available when the lawn is actively growing. Using grass clippings has several disadvantages, however. Weeds from the lawn can be introduced into the mulched bed. If the clippings are applied too deeply and packed too tightly they will release heat and a foul odor as they decay. Clippings from lawns that have been treated with weedkillers can cause herbicide injury in tender plants.
Leafmold
Leafmold can be made by composting leaves in the fall. The material should be partially decomposed by the following spring. It is a good mulch but difficult to apply evenly and may not be particularly neat looking. Leafmold collected from wooded areas may contain nematodes.
Leaves
Leaves, probably the most inexpensive material available, are used extensively in areas with many trees. Avoid leaves collected from streets that have been sprayed with oil or treated with calcium chloride--both oil and calcium chloride are toxic to plants. Leaves may be difficult to keep in place in windy locations. Very dry leaves can be a fire hazard, and packed, wet leaves interfere with air and water movement into the soil. Leaves should be mixed with some other light material, such as straw, to prevent their becoming a compacted, soggy mass.
Peat
Peat is often called "peat moss" but this is a misnomer. Moss peat comes from mosses, while other types of peat originate from cattails, reeds, sedges and other similar water plants. Sphagnum peat moss is acid and useful for mulching and amending the soil around azaleas, rhododendrons and other plants that grow best in acid soil. For other plants, apply limestone at 3 to 5 pounds per 100 square feet on top of a 2-inch Sphagnum peat mulch counteract the acidity. Sphagnum peat is very resistant to decomposition and has many fibers into which roots often grow profusely. Hypnum peat is usually neutral or slightly alkaline, breaks down more rapidly and may have few fibers.
Most types of peat are brown and serve as a neutral background in the landscape. If the surface of a peat mulch becomes very dry, it will shed water and require stirring. Carelessly tossed cigarettes can ignite peat, but it smolders rather than flames. Fine peat will often be blown away in windy weather, but chunky peat will hold. Self-seeding of some plants may occur in peat.
Pine Boughs or Needles
These are usually recommended for winter protection of newly set or somewhat tender ornamental plants but can serve as a mulch as well. Any needle-type evergreen is a satisfactory source.
Sawdust
Sawdust is very commonly used in areas where it is readily available. Nitrogen deficiency is almost certain if fertilizer is not applied regularly. The alleged toxicity of sawdust is usually nitrogen deficiency in the soil from the action of micro-organisms decomposing the material. Sawdust from walnut logs can stunt plants because of toxic materials in it.
Bark--Shredded, Chunked or Chipped
Bark has become popular in recent years. It makes an excellent mulch and is very attractive in landscape plantings. Shredded, chunked or chipped bark lasts as long or somewhat longer than peat moss and adds organic matter to the soil. Bark may also be used for walks or paths in the garden area. To prevent nitrogen deficiency in plants mulched with bark, apply nitrogen fertilizer.
Straw
Straw is used for winter protection and as a summer mulch. It is highly flammable, so do not use it where cigarettes or matches could be carelessly flipped into the material. Additional nitrogen must be applied to prevent starvation of the mulched plants. Weed seeds may be introduced with this mulch. Unless the straw is chopped into short lengths, it will be difficult to apply between plants growing close together.
Wood Chips And Shavings
Wood chips and shavings have become available in large quantities from utility companies. These materials vary in coarseness. They are sometimes rather fine or may be largely broken twigs or branches. Wood chips decompose slowly and may cause nitrogen deficiency if additional fertilizer is not applied. They make a very rustic-looking, satisfactory mulch. Wood chips should not be used in foundation plantings next to the house where termites are or could be a problem.