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.