Mulching

For arable soils, the most effective conservation practices for reducing water loss through surface evaporation are those that provide some degree of surface cover for the soil. A cover can be best provided by mulches or by tillage practices that leave plant residues on the soil surface (JAMIESON et al. 2001). Mulch is any material placed on a soil surface for the purpose of reducing evaporation, retaining moisture, reducing soil erosion, suppressing weed growth and providing plant nutrients as the material decomposes (INFONET BIOVISION). Mulches act as barriers to movement of moisture out of the soil. They can be either organic (e.g. straw, wood chips, peat) or man-made (e.g. transparent or opaque plastic). Besides keeping the moisture in the soil, mulches can also enhance soil temperature; reduce the spread of soil borne diseases; reduce weed growth; reduce soil erosion (see also soil degradation); and provide nutrients and organic matter ( see also soil amendment and soil cover and reforestation). Moreover, mulches support infiltration of runoff and irrigation water as the mulches protect the soil surface from the impact of raindrops preventing soil crusting (JAMIESON et al. 2001).

Plastic mulches are completely impermeable; they therefore prevent direct soil evaporation and limit water losses and erosion via the soil surface. The prevented evaporation also has a supplementary effect; it avoids the rise of water containing salt, which is important in countries with high salt content water resources. Furthermore, mulches can provide a barrier to soil pathogens and especially opaque plastic mulches prevent germination of annual weeds from receiving light. They also help to maintain warm temperatures at night, which enable seeds to germinate quickly and for young plants to rapidly establish a strong root growth system (TNAU n.y.). Inorganic mulches are often applied by machinery and holes or slits allow plants to grow through. These mulches are commonly used for vegetable and small fruit crops (JAMIESON et al. 2001).

Besides plastic sheets, inorganic mulches include stone, shredded rubber, crushed glass, and geotextile. Some stone mulches include crushed gravel and granite mulches, river rock or small stone mulches, lava rock mulch, decorative and colourful stone mulch, sand mulch, and crushed brick mulch (MULCH EXPERTS 2010). Geotextile used for mulching is an expensive blanket-like synthetic fibre. This mulch allows for great water penetration. It is almost always used in combination with bark mulch and wood chips used to cover the geotextile mulch. While the geotextile mulch does not need to be replaced as frequently as the wood chips, the mulch should still be replaced at some time. A single layer of the geotextile mulch should be used. In order for the mulch to effectively let irrigation and runoff water pass through, it should be properly penetrated (MULCH EXPERTS 2010).

Typical materials for organic mulching are straw, leaves, cut grass, wood chips, peat, cut branches or the left over of old harvested crops and various types of compost (see also large and small scale composting). Since organic mulches are derived from plant material, decomposition does occur, and several undesired effects on the soil and on plant growth will be apparent to gardeners. The addition of organic mulches such as leaves or shredded bark brings almost an immediate positive effect. These effects are physical (infiltration, improved water-holding capacity, soil structure and if decomposing organic material is correctly incorporated into the soil it also has a positive effect on aeration), chemical (soil pH, release of small amounts of nutrients, decomposing) and biological (temperature regulation for increasing microorganism activity). However, if organic mulches are kept to wet disease-causing pests fungi and other undesired microorganism may develop (WILLIAMS n.y.).

Proper mulching can reduce the water needs because of reduced evaporation and enhanced water holding capacity of the soil. In general, plastic mulch is more expensive than organic mulch. It must be fabricated and disposed of. It might not be available in some countries. Organic material is almost available everywhere. It can be collected by the farmers (e.g. straw, leaves, cut grass) and does not necessarily need to be bought.

Inorganic Mulches

To apply plastic mulching, a lot of knowhow is required. It depends on the crop, climate, soil, precipitation, and water quality what kind of plastic mulch is used. The colour influences temperature of the soil and airflow around the plants. Deteriorated plastic mulch might be difficult to remove if it has fallen apart in little pieces and is mixed with the soil. In case the mulch film needs to be used for more than one season (thicker film) the plant is cut at its base near the film and the film is removed and reused (for more detailed information please check TNAU n.y.). In the table below, find some general instructions for selecting the correct plastic mulch (TNAU n.y.):

The selection of mulches depends on the ecological situation. Source: TNAU (n.y.)

A common practice to irrigate plastic mulched fields is the drip irrigation system where the laterals are laid under the mulch. With this technique the water can be distributed without moving the plastic mulch. Furthermore there is no evaporation or water loss.

Organic Mulches

Organic mulching has to be carried out carefully as well. The organic mulching material should be applied around established plants in the garden in mid-spring, when the soil has warmed up sufficiently for active root growth. If the mulch is applied too early, it will keep the ground cool and root development will be delayed. With newly planted material, the mulch should be applied after the plants are set in place and watered in well. If planting is done in late summer or early fall, the mulch should be applied immediately after watering the plants so that the soil temperature will be kept warm during the cool nights. It is important for fall-planted stock to have sufficient root growth so that the plants don't heave out of the ground during the winter months because of alternate freezing and thawing. Organic mulches such as leaves, sawdust, or shredded bark should be moist when applied to the soil. Extremely dry mulches act as a blotter and remove moisture from the soil (adapted from WILLIAMS n.y.).

As beneficial as organic mulch is, too much mulch can be harmful. A thick layer of organic mulch can be effective in suppressing weeds and reducing maintenance, but it often causes additional problems. Deep mulch can lead to excess moisture (root rot, mould fungus); it can create a habitat for rodents, which damage the plant. Furthermore, thick blankets of fine mulch may prevent penetration of water and air (TREE SPECIALISTS n.y.).

• Some organisms can proliferate too much in the moist and protected conditions of the mulch layer. Slugs and snails can multiply very quickly under a mulch layer. Ants or termites, which may cause damage to the crops also may find ideal conditions for living. When crop residues are used for mulching, in some cases there is an increased risk of sustaining pests and diseases (INFONET-BIOVISION n.y.).
• Damaging organisms such as stem borers may survive in the stalks of crops like cotton, corn or sugar cane. Plant material infected with viral or fungal diseases should not be used if there is a risk that the disease might spread to the next crop. Crop rotation is very important to overcome these problems (INFONET-BIOVISION n.y.).
• When carbon rich materials such as straw or stalks are used for mulching, nitrogen from the soil may be used by microorganisms for decomposing the material. Thus, nitrogen may be temporary not available for plant growth (risk of N-immobilisation) (INFONET-BIOVISION n.y.).