Mulch is used for various purposes:
A variety of materials are used as mulch:
The way a particular organic mulch decomposes, and reacts to wetting by rain and dew, determines in great degree its effectiveness. Organic mulches can rot rapidly rather than slowly break down, require nitrogen to decompose, can mat into a barrier that blocks water and air, can wick water from the soil to the surface due to its porosity, all conditions that can be detrimental to crops and ornamental plants.
Living mulch may be considered a type of mulch. It involves sowing a fast-growing cover crop to grow under the main crop.
Grass mulching is a large part of keeping a healthy lawn. It causes proteins from old grass to soak into new grass and make it appear healthier the next time the grass is mowed. It can be done using a lawn mower by setting it to the mulching setting (usually by lifting a latch or removing a guard) before mowing. Mulching should only be done about every three weeks, depending on how poor the grass is.
Mulch is usually applied towards the beginning of the growing season, and may be reapplied as necessary. It serves initially to warm the soil by helping it retain heat. This allows early seeding and transplanting of certain crops, and encourages faster growth. As the season progresses, the mulch stabilizes temperature and moisture, and prevents sunlight from germinating weed seed.
Plastic mulch used in large-scale commercial production is laid down with a tractor-drawn or standalone layer of plastic mulch. This is usually part of a sophisticated mechanical process, where raised beds are formed, plastic is rolled out on top, and seedlings are transplanted through it. Drip irrigation is often required, with drip tape laid under the plastic, as plastic mulch is impermeable to water.
In home gardens and smaller farming operations, organic mulch is usually spread by hand around emerged plants. For materials like straw and hay, a shredder may be used to chop up the material. Organic mulches are usually piled quite high, six inches or more, and settle over the season.
In some areas of the United States, such as central Pennsylvania and northern California, mulch is often referred to as "tanbark", even by manufacturers and distributors. In these areas, the word "mulch" is used specifically to refer to very fine tanbark or peat moss.
Mulch made with wood can contain or feed termites, so care must be taken about not placing mulch too close to houses or building that can be damaged by those insects. Some mulch manufacturers recommend putting mulch several inches away from buildings.
If sour mulch is applied and there is plant kill, the best thing to do is to water the mulch heavily. Water dissipates the chemicals faster and refreshes the plants. Removing the offending mulch will have little effect, because by the time plant kill is noticed, most of the toxicity is already disappeared. While testing after plant kill will not likely turn up anything since the toxicity will have dissipated, a simple pH check may reveal a highly acid content, in the range of 1.8 to 3.6 instead of the normal range of 6.0 to 7.2. Finally, placing a bit of the offending mulch around another plant to check for plant kill will verify if the toxicity has departed. If the new plant is also killed, then sour mulch is probably not the problem.
Living mulches are plants sown to grow close to the ground, under the main crop, to slow the development of weeds and provide other benefits of mulch. They are usually fast-growing plants that continue growing with the main crops. By contrast, cover crops are incorporated into the soil or killed with herbicides. However, living mulches too may need to be mechanically or chemically killed eventually to prevent competition with the main crop (Brandsaeter et al. 1998, Tharp and Kells, 2001).