Many organic materials are used as mulches including farmyard manure, leaf mould, bark, compost, lawn clippings and spent mushroom compost.
Organic mulches increase earthworm activity at the surface, which promotes better and more stable soil structure in the top layers. Soil compaction by water droplets is reduced and, as the organic mulches are incorporated, the soil structure can be improved. If thick enough mulches can suppress weed growth, but it is counter-productive to introduce a material that contains weeds. Likewise, care should be taken not to introduce pests and diseases or use a material such as compost where slugs can be a problem.
When organic matter is added as a mulch it is acting, in effect, as an extra layer of loose soil. Thus, water loss from the soil surface is reduced because it is covered with a dry layer (see evaporation). Soil temperatures lag behind the surface temperatures because of its insulating properties, with the greater lag at greater depth. They tend to reduce soil temperatures in the summer, but retain warmth later in the autumn.
Manufactured materials, such as paper, metal foil or, most commonly, polythene, are also used. In response to the demand for this type of material, woven polypropothene mulches are available. Whilst these have very little insulating effect they are particularly effective in reducing water loss by evaporation at the surface (see water conservation). The colour of the mulch is important because lightcoloured material will reflect radiation whereas dark material will absorb it and can thus lead to earlier cropping by warming up the soil earlier.
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