Mulch

In agriculture and gardening, mulch is a protective cover placed over the soil, primarily to modify the effects of the local climate. A wide variety of natural and synthetic materials are used.    

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Purposes

Mulch is used for various purposes:

• to adjust temperature by helping soil retain more heat in spring and fall, and by keeping soil cool and even out temperature swings during hot and variable summer conditions
• to control weeds by blocking the sunlight
• to retain water by slowing evaporation
• to add organic matter and nutrients to the soil through the gradual breakdown of the mulch material
• to repel insects
• to incrementally improve growing conditions by reflecting sunlight upwards to the plants, and by providing a clean, dry surface for ground-lying fruit such as squash and melons.
• for erosion control - protects soil from rain and preserves moisture
• for sediment control - slows runoff velocity

Materials

• organic residues - grass clippings, leaves, hay, straw, shredded bark, whole bark nuggets, sawdust, shells, wood chips, shredded newspaper, cardboard, wool, etc. Many of these materials also act as a direct composting system, such as the mulched clippings of a mulching lawn mower. There are many differing opinions on what to use.
• compost - This relies on fully composted material, where potential weed seed has been eliminated, or else the mulch will actually produce weed cover.
• Rubber mulch - Made from recycled tire rubber.
• plastic mulch - Crops grow through slits or holes in thin plastic sheeting. This method is predominant in large-scale vegetable growing, with millions of acres cultivated under plastic mulch worldwide each year (disposal of plastic mulch is cited as an environmental problem).
• organic sheet mulch - Various products developed as a biodegradable alternative to plastic mulch.
• rock and gravel can also be used a mulch. In northern climates the heat retained by rocks will extend the growing season.

The way a particular organic mulch decomposes, and reacts to wetting by rain and dew, determine 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 porocity, all conditions that can be detrimental to crops and ornamental plants.

Living mulch may also be considered a type of mulch, or as a mulch-like cover crop. This technique involves undersowing a main crop with a fast-growing cover crop that will provide weed suppression and other benefits associated with mulch.

Mulching is an important part of any no-dig gardening regime, such as practiced within permaculture systems.

Application

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 plastic mulch layer. 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, northern California, more specifically 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.

Sour mulch

Mulch should normally smell like freshly cut wood, but sometimes will develop a toxicity that will cause it to smell like vinegar, ammonia, sulfur or silage. This happens if the material is not rotated often enough and it forms pockets where no air is circulating. When this occurs, the decomposition process become anaerobic and produces these toxic materials in small quantities. Once exposed to the air, the process quickly reverts back to an aerobic decomposition, but these toxic materials will 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 severely damaged or killed depending on their hardiness. Plants that are predominantly low to the ground or freshly planted are the most susceptible.

If sour mulch is applied and there is plant kill, the best thing to do is just water the mulch heavily. Water will help the chemicals to dissipate more quickly and refresh the plants. By the time plant kill is noticed, most of the toxicity will have already disappeared anyway, so removing the offending mulch will have little effect. 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, perhaps in the 1.8 to 3.6 range instead of the normal 6.0 to 7.2 range. 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.

Straw mulch

Mulch made from straw is generally lighter and easier to use than bark mulches, with the added advantages of being biodegradable and neutral in pH. Straw mulch tends to additionally have higher moisture retention and weed controlling properties than other mulches.

Living Mulches

Living mulches differ from cover crops in that plants continue growing with the main crops whereas cover crops are incorporated into the soil or killed with herbicides. However, living mulches might need to be mechanically or chemically killed at some point to prevent competition with the main crop (Brandsaeter et al. 1998, Tharp and Kells, 2001).

• Wikipedia site Living mulch

References

• Beware of Sour Mulch

• Ironing out the mulch