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Mulch is a protective cover placed over the soil, to retain moisture, reduce erosion, suppress weed growth and seed germination, and provide nutrients as they decay. Mulch in gardens and landscapes mimics leaf cover on forest floors.

Organic mulches decay over time and are temporary. The way particular organic mulch decomposes and reacts to wetting by rain and dew affects its usefulness.

Commonly available mulches include:

Leaves
Grass clippings
Peat moss
Wood chips
Bark chips
Straw mulch
Cardboard or newspaper

In temperate climates, the effect of mulch is dependent upon the time of year at which it is applied as it tends to slow changes in soil temperature and moisture content. Mulch, when applied to the soil in late winter/early spring, will slow the warming of the soil by acting as an insulator, and will hold in moisture by preventing evaporation. Mulch, when applied at the time of peak soil temperatures in mid-summer, will maintain high soil temperatures further into the autumn (fall). The affect of mulch upon soil moisture content in mid-summer is complex however. Mulch prevents sunlight from reaching the soil surface, thus reducing evaporation. However, mulch can absorb much of the rainfall provided during light rainfall, which will later quickly evaporate when exposed to sunlight, thus preventing absorption into the soil, whilst heavy rainfall is able to saturate the mulch layer, and reach the soil below.

Sour Mulch

Mulch should normally smell like freshly cut wood, but sometimes develops a toxicity that causes it to smell like vinegar or ammonia. This happens when material with ample nitrogen content is not rotated often enough and it forms pockets of increased decomposition. When this occurs, the process may become anaerobic and produce phytotoxic materials in small quantities. Once exposed to the air, the process quickly reverts to an aerobic process, but these toxic materials may be present for a period of time. If the mulch is placed around plants before the toxicity has had a chance to dissipate, then the plants could very likely be damaged or killed depending on their hardiness. Plants that are predominantly low to the ground or freshly planted are the most susceptible.

When to Apply

In order to maximize the benefits of mulch, while minimizing its negative influences, it is often applied in late spring/early summer when soil temperatures have rose sufficiently, but soil moisture content is still relatively high. Furthermore, at this point in the growing season, plants should be well enough established to be able to cope with the increase in the numbers of slugs and snails owing to the habitat provided for them by the mulch. Also, 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.

In the Mid-Ohio Valley, mulch should be applied in the spring a few weeks after bedding annuals become established. Many folks like to use weed cloth under their mulch, but I find that it is more trouble than it's worth. I like to apply a generous layer of wood mulch in the spring and then till it under in the fall so it can decompose an add nutrients to the soil. Also, when I plant my fall pansies and mums after tilling the beds, I like to mulch the beds with shredded leaves from my yard. By tilling my beds in the spring and fall before applying mulch, I have built up very rich soil over the years and have eliminated to need for excessive fertilizing.

Source: Wikipedia