Soil Cover and Reforestation

With a reduction of moisture loss in the soil, groundwater resources are protected, soil quality and agricultural production can be improved, and water use for irrigation can be decreased, thus optimising the local water cycle. Soil moisture loss (and also soil degradation) can be minimised through different techniques of soil moisture conservation: soil cover (with living plants) and reforestation, mulching, different tillage techniques, and soil amendments. Soil cover is common on agricultural land. Reforestation is common to protect water protection areas and groundwater resources, or to prevent soil degradation in non-agricultural sites.

As a result of soil cover, agricultural production subsequently improves because soil cover both promotes and maintains optimum soil conditions for plant growth (nutrient availability) and water infiltration (water availability). Soil cover with living plants (soil can also be covered with mulch) protects the soil surface from rain, wind and sun (FAO 2005):

• It reduces soil erosion and protects the fertile topsoil, thus preventing the silting of rivers and lakes.
• It stops the soil surface from sealing, and reduces the amount of precious rainwater that runs off.
• It suppresses weeds by smothering their growth and reducing the number of weed seeds. This reduces the amount of work needed for weeding.
• It increases the soil fertility and the organic matter content of the soil.
• It increases soil moisture by allowing more water to sink into the ground and by reducing evaporation.
• Decomposing vegetation and the roots of cover crops improve the soil structure and make the clumps and lumps in the soil more stable – making it harder for rain to break them up and wash them away.
• Earthworms and other forms of life can prosper in the cover as well as in the soil.
• Soil cover stimulates the development of roots, which in turn improve the soil structure, allow more water to soak into the soil, and reduce the amount that runs off.

Water Management

As mentioned above, soil cover can reduce soil moisture loss. In contrast, some growers use cover crops such as rye as a way to reduce soil moisture in early spring. The crop takes up the soil moisture and uses it within the plant. The water is incorporated in the plant tissues or is transpired. In both cases, the water is removed from the soil. This use of cover crops requires very good management and understanding of plant growth habits. Under warm wet conditions some cover crops can become very difficult to control either with tillage or chemically. Also, under dry spring conditions the cover crop should be controlled earlier to prevent excessive moisture loss, which may interfere with crop establishment and growth (adapted from OMAFRA 2011).

There are two main types of soil cover:

• Living plant material: crops, cover crops and forest structures.
• Dead plant material such as crop residues and prunings from trees and shrubs (see also mulching).

In conservation agriculture, a combination of mulch and living plants to keep the soil covered is common. To obtain a good soil cover, crop residues (e.g. maize stalks) can be left in the field or additional dead plant material from outside the field (e.g. cut grass or leaves) can be added (see also mulching and soil amendment).

In addition, cover crops can be planted, either during the cropping season (to cover the area in between the crop rows), or afterwards to cover the whole field. During the cropping season, the crops themselves act as soil cover. An intercrop of tall plants (such as maize) and low-growing plants (such as beans) make a good cover. It can be difficult to keep the soil covered, especially in semi-arid areas. It may be necessary to compromise: some cover is better than none (FAO 2005).

How to Choose a Cover Crop

Growing cover crops is a common soil management practice. There are a lot of good reasons to grow cover crops but it is often hard to put a dollar value on the return from growing them. Cover crops are an important part of a system of soil maintenance - particularly important on the lighter soils with lower organic-matter or on fields with short rotations and little return of crop residue or manure. It is important to know the goal when planting a cover crop and to select the best one for that job. Small-scale farmers prefer a cover crop which fits into their normal cropping system and which has multiple purposes (FAO 2005):

• Edible seeds and vegetables
• Soil fertility
• Animal fodder
• Firewood/fencing material
• Weed suppression
• Medicines.

First, check which cover crops grow well in your area. This depends on the soil type, rainfall, temperature and altitude. If you live in an area with little rainfall, select a cover crop that grows quickly, such as cowpea, desmodium, lablab, lucerne, mucuna, or pigeonpea. Then check how much work each cover crop will need: for land preparation before planting, weeding, and producing and harvesting the seeds. Species with big pods and grains (such as pigeonpea and mucuna) are easier to deal with than species with small pods (such as vetch and grasses). Most farmers prefer species that cover the soil quickly and completely, and which can also be used for food and fodder. As a support to decide which cover crop is suitable and when it should be planted, FAO (2005) recommends answering the following questions:

Questions to help deciding when to plant the cover crop:

• How much moisture will the cover crop need to grow and produce enough mulch and seed?
• At what time of year will there be enough moisture for the cover crop to grow?
• What other factors (temperature, frost) will favour or hinder its growth? Is it sensitive to the day length? To frost? To drought?
• How easy will it be to manage the cover crop while it is growing and afterwards (mulch management)?

Questions about the cropping system:

• When will cover be most needed?
• How fast will the cover crop grow and provide the required cover?
• How long will the cover last?
• Might anything (grazing by animals, burning, etc.) reduce the amount of cover?
• How to stop the cover crop and main crop from interfering with each other?

As already mentioned, it is important to know the goal when planting a cover crop and to select the best one for that job. The following table looks at the various reasons for including cover crops in a rotation (see also crop rotation) and the potential cover crops that best meet those goals. These may vary for other climates and countries.

Matching cover crop choices to function. These suggestions are matched to the climate of Ontario, Canada. It may change in other countries or climates. Source: OMAFRA (2011)

Generally it is accepted that tree removal by logging, forest fire or wind damages increases runoff (ZOMER et al. 2006). Deforestation, especially in tropical nations, leads to extensive areas of degraded land. In many of these places, even firewood has become so scarce that certain foods requiring cooking have been eliminated from the diet. People must use crop residues and dried dung for fuel, which robs the soil of organic matter and nutrients and accelerates erosion. Soil eroded from degraded lands fills riverbeds, reservoirs and lakes, increasing the severity of floods and causing water scarcities (OTA 1983).

First of all, inappropriate land-use practices on forest lands should be prevented. If not possible, reforestation is one way to improve the productivity and protection of many degraded lands, and provide useful products for the people. Trees have positive effects on human beings (e.g. food, fuel, fodder) and protect soil from heat, rain and wind. Soil temperatures are lower under tree canopies, permitting reaccumulation of organic matter that restores soil structure and microbiota and enhances moisture- and nutrient-holding abilities. Bacteria on the roots of some trees fertilise soil with nitrogen, while fungi on tree roots can convert soil minerals to useful forms. In dry areas, trees can help to prevent the rise of saline groundwater. Where surface soils are dry or infertile, deep tree roots can tap underground reservoirs of nutrients and water and bring them to the surface (OTA 1983). Furthermore, planted trees absorb carbon, which is an important factor when we discuss about climate change. Ten key points on plantation impacts on stream flow can be read in KEENAN et al. 2006.

Example: The Neem Tree

The neem tree is native to India and Southeast Asia, but it is now grown in the warm lowland tropics as well as in arid and semi-arid areas of the world. Neem seems to be a good candidate for planting in most warm parts of the world. It grows vigorously in many semiarid and tropical areas and is regarded as a valuable forestry species in many parts of Africa; but even there it could become more widely employed. It is a multipurpose species that provides villagers with various products from which to derive an income during the years when the trees are maturing. It is also promising for planting in areas suffering desperate fuel wood shortages. It is useful as a windbreak, exceptional as a city tree, and it can grow in (and may even neutralise) acid soils in the tropics. On the farm and around the house, neem is useful not only as a windbreak and a welcome source of shade, but its seedcake is a good fertiliser, as it contains nitrogen, potash, phosphorus, calcium and magnesium (INFONET BIOVISION 2011).

There are additional costs above and beyond normal cropping practices that must be considered in systems that include cover crops. Extra expenditures include the cost of the cover crop seed as well as labour and time for planting. Also, special or alternative equipment may be needed to handle the greater amounts of residue present in no-till systems. Cover crops must be managed like any other crop produced in the farm operation (adapted from PENNSTATE 1996).

Reforestation also costs money, e.g. for labour (growing, planting) and seedlings. On the other hand, there are many advantages and money can be saved (fire wood, food, enhanced soil conditions).

Soil cover management needs expert knowledge. The following list outlines some challenges you may encounter for maintaining soil cover and some ways to overcome them (adapted from FAO 2005):

• Semi arid areas: In semi-arid areas, where there is little rain and most of it falls in one season, establishing a cover crop may be difficult. Crops, shrubs and trees produce few residues, and farmers often need them for feed or building materials. Cover crops use precious water.
• Diseases and pests: Diseases and insect pests might attack the cover crop and will require special attention. Farmers often use fire to destroy pests and diseases. But this leaves the soil bare and destroys valuable organic matter.
• Rats: A dense cover crop may encourage rats, which may attack the crop. Slash cover crop as close to the ground as possible, use traps (poison could kill other animals as well), or use rotating crops (interrupt food supply).
• Termites: Many farmers fear that soil cover will attract termites. Only a few types attack crops, most of them are important (break down of soil organic matter, aeration).
• Fire: Bushfires or uncontrolled fires on neighbouring fields can spread into a conservation agriculture field and destroy its soil cover. To prevent this, you can leave a buffer zone around your field.
• Livestock: Livestock need to be fed. Farmers often allow them to graze on stubble or on fallow fields, and other livestock owners may not keep their animals out of a field planted to a cover crop. This may especially be a problem in dry years or in semi-arid areas, where few alternative sources of feed are available. Fence crop field or find alternatives.

Trees also need to be maintained, especially at the beginning. During growth and irrigation, additional nutrients may need to be provided. Trees must be protected from the above-mentioned impacts as well.