In the natural environment, soil is protected from the elements that can cause wind erosion, compaction, water erosion, extreme soil temperature fluctuations and the rapid evaporation of soil moisture. This protection is provided by living and decomposing vegetation. 
Root systems of plant material hold soil in place. Top growth reduces wind velocity that can cause wind erosion and drying.
It reduces raindrop velocity that can compact and erode soil and allows moisture to penetrate rather than run off. The crowns of plants also prevent evaporation by shading soil. In forested areas where heavy shade reduces ground vegetation growth, leaf litter provides a protective mat. As the leaves decompose, a rich humus layer is formed and nutrients are returned to the soil.
Whereas nature makes use of existing living and decaying plant material for soil cover, homeowners may prefer a more orderly appearance to their plantings. Whether your preference is for organic or inorganic material, mulching will protect soil from erosion, retain soil moisture, protect plant roots from soil temperature fluctuations in both summer and winter and prevent compaction of soil by rain and foot traffic.
There's a misconception that mulch will prevent the invasion of weeds. Laying landscape fabric under mulch will prevent seeds in the soil from germinating, but eventually, weeds will take hold. Organic mulches begin to decompose
and form a rich humus layer that is ideal for seed germination. Inorganic mulches, such as river rock and decorative stone, will eventually trap enough soil particles to provide a seedbed. Although inorganic material doesn't require topping up, extracting weeds can be difficult. Replenishing organic material on a regular basis will help with weed control, but weeding is always going to be part of gardening.
The stems of plants, shrubs and the boles of trees can be damaged if organic mulch is piled around them. The mulch will retain moisture and keep the stems and boles wet which can lead to crown rot.
So always leave a small area of soil around the base of any plant material. Initially, apply 10 to 15 cm (4 to 6") of mulch over the soil surface. Any greater depth than this can cause a disruption in the oxygen/ carbon dioxide exchange in the soil. This amount of mulch is sufficient for the first growing season. At the beginning of each new growing season, apply an additional 2.5 to 5 cm (1 to 2") of new mulch.
Preventing soil erosion on slopes requires the use of plant material rather than organic mulch or the two materials in combination.
Organic mulch alone can be washed to the bottom of the slope with heavy rains and you will end up with a heavy layer of mulch at the base of the slope and very little at the top. Consider planting native wildflowers and grasses that have deep fibrous root systems that will hold the soil in place. If you prefer shrubs, select low spreading material that will eventually cover the slope. Another option is a rock garden but be prepared for some pretty intensive weeding!
The ecological benefits of mulching are many. By retaining soil moisture, we can reduce supplemental irrigation. Mulching around trees and shrubs eliminates the use of gas trimmers. The rich humus layer produced by the decomposition of organic mulches enhances earthworm and microorganism activity. This improves the soil structure and fertility and reduces the amount of chemical fertilizers that are required. Thus mulching is just one more way that homeowners can do their part for our ecosystem.
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