Because organic mulches are derived from plant material, they will decompose and affect both the soil and the plants in several ways.
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.
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.
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. is not sufficiently deep, weeds from the soil below will grow through it.