Use of mulches

The term mulch, as used here, refers to any naturally-formed, undisturbed soil covering any material added to serve as a soil cover and to crop residues left in part on the surface as dead or dying materials. This definition is, therefore, broad enough to include essentially all organic and inorganic materials that serve as soil covers. The main emphasis here, however, is on conventional mulches of organic materials, where the soil is usually completely covered to various depths by plant materials added from outside sources. Reference will also be made to stubble- or trash-mulches, where the soil surface is usually only partially covered. Whether the soil is completely or only partly covered, most of the effects produced are very similar, but differ in degree. 

Mulching materials consisting of various kinds of plant materials have long been used by gardeners, fruit growers, nurserymen, and those interested in ornamental shrubs and flowering plants. These materials commonly include such materials as peat, manures, straw, cornstalks, corncobs, sugar-cane trash, leaves, peanut hulls, wood products, and various other kinds of litter. Since such materials must be applied to a depth of at least two inches if weeds are to be kept under control, it is obvious that they are too expensive (at least in terms of labor) for use as full mulches on field crops. 

Sawdusts from various species of trees are often available in quantity for use in flower 

Mulches are used primarily to increase water infiltration, reduce evaporation, modify soil temperatures, control weeds, prevent evaporation, and increase crop yields. They also increase the biological activities in the soil, modify the level of available nutrients, and help to maintain or increase the level of soil organic matter. Under conditions of no tillage, or minimum tillage of fine-textured soils, there is also likely to be a favorable effect on the physical conditions in the soil. These effects are discussed in the following pages. 

Mulched soils usually (Verma and Kohnke, 1951), but not always, supply more moisture to growing crops than do bare soils, or those that are cultivated frequently. All mulches serve to break the impact of the falling raindrop, usually increase the intake of water, anu decrease the turbulent transfer of water vapor to the atmosphere (Lemon, 1956). The chief factors involved include the kind and depth of mulch, soil texture, amount and frequency of the rainfall, and amount of weed growth. 

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There are no approved chemical methods to control weeds in organic crops. Growers have to rely primarily on preventive or physical measures to maintain weed control. These measures include rotation, timing of cultivation, undersowing and the use of mulches. Physical methods may also be used and these include hand weeding, the use of mechanical techniques such as brush hoes and thermal destruction. 

Montagnini, F., K. Ramstad, and F. Sancho, 1993- Litterfall, litter decomposition and the use of mulch of four indigenous tree species in the Atlantic lowlands of Costa Rica. Agroforestry Systems 23 39-61. 

An organic mulch also protects the soil against crust formation, but aside from the scarcity of mulching materials and costs, the use of mulches presents other problems that are discussed in detail in Chapter 25. Many types of mulches, if properly managed (Army et al., 1961), are very effective in minimizing runoff. Plant residues on the surface also markedly decrease the rate of drying of soil in the surface half-inch layer, but under field conditions Army et al. observed that the soil moisture content below 2 inches was not materially increased by surface residues. 

Soil or crop management practices include crop rotation where a different crop is grown on the same area of land in successive years in a four or five year cycle. This avoids exhausting the soil and can improve the texture of the soil. The use of mulches, that is. covering the ground with plant residues, affords protection to the soil against raindrop impact and reduces the effectiveness of overland flow. Reafforestation of slopes, where possible, also helps to control the flow of water on slopes and reduce the impact of rainfall. 

The use of mulching films presents the grower with an additional expense and unless this extra cost gives an adequate return it will not be used. Huang [5] and Yang [26] have calculated the ratio of extra cost to increased income, and this is shown in Table 10.8 for six crops. 

Opaque mulch in comparison to clear film ensures an earlier ripening of the crop and a significant suppression of weed growth thus the use of chemical weed killers is reduced. The combined use of mulch and ripener must be avoided as this causes undesirable over-ripening even at the lowest dosage. 

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