A mulch is simply a soil covering material that is made of plastic film, bark, peat or other similar materials.
Composted mulches (commonly used in home gardens) are usually derived from decaying plant and animal waste. When composted mulches are completely decomposed, they improve soil structure and water infiltration rate. An example of these mulches includes the commercial compost — steer or chicken manure mixed with wood by-products or yard waste.
Composting materials can improve the physical characteristics of heavy soils and increase water-retention in sandy soils. Adding composted materials to heavy soil improves water penetration rate. In sandy soils, composts increase water-holding capacity, increase the availability of macro and micronutrients and save water. The benefits of composting are many, but the most important one is green waste can be recycled and volume of wastes going to landfills can be reduced.
Manure is an example of partially composted materials. This type of composted material is usually high in salts. If you suspect you have a high level of salts in your soil, you may want to limit the amount of manure you use in your garden. However, if your soil drains well, extra salts will wash away after a few irrigations. Most garden plants are sensitive to salts during their early stages of growth and developments.