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Mulching materials and composts saves water

April 25, 2002

A mulch is simply a soil covering material that is made of plastic film, bark, peat or other similar materials.

Composted mulches (commonly used in home gardens) are usually derived from decaying plant and animal waste. When composted mulches are completely decomposed, they improve soil structure and water infiltration rate. An example of these mulches includes the commercial compost — steer or chicken manure mixed with wood by-products or yard waste.

Composting materials can improve the physical characteristics of heavy soils and increase water-retention in sandy soils. Adding composted materials to heavy soil improves water penetration rate. In sandy soils, composts increase water-holding capacity, increase the availability of macro and micronutrients and save water. The benefits of composting are many, but the most important one is green waste can be recycled and volume of wastes going to landfills can be reduced.

Manure is an example of partially composted materials. This type of composted material is usually high in salts. If you suspect you have a high level of salts in your soil, you may want to limit the amount of manure you use in your garden. However, if your soil drains well, extra salts will wash away after a few irrigations. Most garden plants are sensitive to salts during their early stages of growth and developments.


Shredded and decorative barks are two examples of non-composted mulches. Shredded bark decomposes slowly over time by soil microbes (tiny creatures that cannot be seen by the naked eye). These microbes are good for the soil; they love to decompose mulches. These microorganisms need oxygen and water to decompose organic material. The decomposition process generates carbon dioxide (CO2) and water vapor.

The composting process generates heat (thermophilic) and reduces the mass of the organic material by almost half. Most of the reduction is due to the loss of CO2 and water vapor. In most commercial operations raw materials are usually piled in windrow so it can be turned and mixed.

During the decomposition process, soil microbes need a lot of nitrogen fertilizer but most soils do not have enough nitrogen for both soil microbes and plant growth. Therefore, it is recommended to add 2-3 pounds of actual nitrogen fertilizer per cubic yard of non-composted mulch (one cubic yard is 27 cubic feet). Slow or controlled-release nitrogen fertilizer is recommended.

The use of mulching materials or composts on a soil surface reduces overall evaporation and soil erosion.

In general, soil mulches have five main advantages:

1. Reducing the rate of evaporation from soil surface and transpiration by plants.

2. Serve as a natural weed control method by preventing direct sunlight from reaching the surface of the soil and therefore reducing or eliminating weed growth.

3. Mulches ensure uniform application of water over the area.

4. Soil mulches serve as insulators. They prevent soil surface from becoming too hot in summer and therefore reduce the amount of water needed for plant growth and development.

5. Mulches control soil erosion and improve soil structure and aid in water infiltration and distribution.

>> Khaled Bali is the irrigation/water management adviser at the University of California-Imperial County Cooperative Extension.

>> The Cooperative Extension serves all residents of the Imperial Valley.

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