Mulch |
In
agriculture and gardening, mulch is a protective cover placed over
the soil to retain moisture, reduce erosion, provide nutrients, and
suppress weed growth and seed germination. Mulching in gardens and
landscaping mimics the leaf cover that is found on forest floors.
Materials used as mulches vary and depend on a number of factors.
Use takes into consideration availability, cost, appearance, the
effect it has on the soil — including chemical reactions and pH,
durability, combustibility, rate of decomposition, how clean it is —
some can contain weed seeds or plant pathogens.
A variety of materials are used as mulch:
Organic residues: grass clippings, leaves, hay, straw, kitchen
scraps comfrey, shredded
bark, whole bark nuggets, sawdust, shells, woodchips, shredded
newspaper, cardboard, wool, but also animal manure, etc. Many of
these materials also act as a direct composting system, such as the
mulched clippings of a mulching lawn mower, or other organics
applied as sheet composting.
Compost: This should be fully composted material to avoid possible phytotoxicity problems, and the weed seed must have been eliminated, otherwise 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).
Rock and gravel can also be used as a mulch. In cooler climates the heat
retained by rocks may extend the growing season.
Organic Mulches: Organic mulches decay over time and are
temporary. The way a particular organic mulch decomposes and reacts to
wetting by rain and dew affects its usefulness.
Organic mulches can negatively affect plant growth when they are
decomposed rapidly by bacteria and fungi, which require nitrogen that
they remove from the surrounding soil. Organic mulches can mat down,
forming a barrier that blocks water and air flow between the soil and
the atmosphere. Some organic mulches can wick water from the soil to the
surface, which can dry out the soil.
Available organic mulches include:
Leaves from deciduous trees, which drop their foliage in the fall.
They tend to be dry and blow around in the wind, so are often chopped or
shredded before application. As they decompose they adhere to each other
but also allow water and moisture to seep down to the soil surface.
Thick layers of entire leaves, especially of Maples and Oaks, can form a
soggy mat in winter and spring which can impede the new growth lawn
grass and other plants. Dry leaves are used as winter mulches to protect
plants from freezing and thawing in areas with cold winters, they are
normally removed during spring.
Grass clippings, from mowed lawns are sometimes collected and used elsewhere as mulch. Grass clippings are dense and tend to mat down, so are mixed with tree leaves or rough compost to provide aeration and to facilitate their decomposition without smelly putrefaction. Rotting fresh grass clippings can damage plants; their rotting often produces a damaging buildup of trapped heat. Grass clippings are often dried thoroughly before application, which mediates against rapid decomposition and excessive heat generation. Fresh green grass clippings are relatively high in nitrate content, and when used as a mulch, much of the nitrate is returned to the soil, but the routine removal of grass clippings from the lawn results in nitrogen deficiency for the lawn.
Peat
moss, or sphagnum peat, is long lasting and packaged, making it
convenient and popular as a mulch. When wetted and dried, it can
form a dense crust that does not allow water to soak in. When dry it can
also burn, producing a smoldering fire. It is sometimes mixed with pine
needles to produce a mulch that is friable. It can also lower the pH of
the soil surface, making it useful as a mulch under acid loving plants.
Wood chips are a byproduct of the pruning of trees by arborists, utilities and parks; they are used to dispose of bulky waste. Tree branches and large stems are rather coarse after chipping and tend to be used as a mulch at least three inches thick. The chips are used to conserve soil moisture, moderate soil temperature and suppress weed growth. The decay of freshly produced chips from recently living woody plants, consumes nitrate; this is often off set with a light application of a high-nitrate fertilizer. Wood chips are most often used under trees and shrubs. When used around soft stemmed plants, an un-mulched zone is left around the plant stems to prevent stem rot or other possible diseases. They are often used to mulch trails, because they are readily produced with little additional cost outside of the normal disposal cost of tree maintenance.
Bark chips, of various grades are produced from the outer corky
bark layer of timber trees. Sizes vary from thin shredded strands to
large coarse blocks. The finer types are
very attractive but have a large exposed surface area that leads to
quicker decay. Layers two or three inches deep are usually used, bark is
relativity inert and its decay does not demand soil nitrates.
Straw mulch or field hay or salt hay are lightweight and normally sold in compressed bales. They have an unkempt look and are used in vegetable gardens and as a winter covering. They are biodegradable and neutral in pH. They have good moisture retention and weed controlling properties but also are more likely to be contaminated with weed seeds. Salt hay is less likely to have weed seeds than field hay.
Cardboard or newspaper can be used as mulches. These are best used as a base layer upon which a heavier mulch such as compost is placed to prevent the lighter cardboard/newspaper layer from blowing away. By incorporating a layer of cardboard/newspaper into a mulch, the quantity of heavier mulch can be reduced, whilst improving the weed suppressant and moisture retaining properties of the mulch. However, additional labor is expended when planting through a mulch containing a cardboard/newspaper layer, as holes must be cut for each plant. Sowing seed through mulches containing a cardboard/newspaper layer is impractical. Application of newspaper mulch in windy weather can be facilitated by briefly pre-soaking the newspaper in water to increase its weight.
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