Soil and water conservation

Irrespective of the extent of damage by soil erosion, land and soil can still be rehabilitated. If the topsoil is lost and erosion is severe, it will take much effort and considerable time to rehabilitate the land. Usually it takes the commitment of the entire community in order to improve the situation. However, even at the individual household level, there are many measures a farmer can establish to control soil erosion. These measures will at the same time control erosion and conserve soil moisture.

The easiest way to protect the soil from being eroded by water or wind is to keep it covered by a plant or mulch cover as much as possible. The soil can be covered with living plants (cover crops), especially within perennial crops, or dead mulching material. Erosion due to rains is more a problem in annual crops where the land is regularly tilled for planting and when soil preparation coincides with rainfall. To avoid erosion during this period, the soil is best covered with residues from the previous crop and other dry plant material, and soil cultivation is reduced by tilling strips only.

Cover crops are usually low-growing perennial plant species, which are sown with the main intention to protect the soil, prevent weed growth and maintain soil fertility. They are also called ‘green’ or ‘living mulch’. Cover crops are used in a similar way as green manures with the main difference that green manures are grown to produce maximum biomass mainly and are usually slashed before flowering and incorporated into the soil. Cover crops may also require regular slashing, mowing or grazing to avoid competition with the main crop.
Some information sources do not distinguish between cover crops and green manures, as both can include the same species, and differences are little depending on their management. But it makes sense to approach them separately due to the different functions they can have in a cropping system.
The primary strength of cover crops is to rapidly cover the soil and to maintain it permanently covered. Cover crops benefit both short and long term productivity of a cropping system. They improve physical properties of the soil, reduce runoff and erosion, suppress weeds and, if the cover crop is a legume, transfer nitrogen to the main crop, when mulched. Soil organic matter levels are usually maintained under a cover crop from a combination of increased input of residues, reduced soil organic matter decomposition due to reduced exposure as a result of reduced or zero tillage, and decrease of soil temperature.
The benefits of cover crops can be limited by the competition of the cover crop for water and nutrients and the very slight increase of soil organic matter level. Slashing, mowing or selective cultivation temporarily reduce competition from the cover crop. While leguminous cover crops supply some nitrogen to the main crop, pure grass-based cover crops require nitrogen for proper growth.
The following characteristics make an ideal cover crop:

• It is low-growing and not climbing;
• It grows fast and covers the soil in a short time;
• It is resistant against pests and diseases, and does not transmit any to the main crops;
• It tolerates drought;
• It fixes nitrogen from the air;
• It develops a deep root system which is able to loosen the soil and contribute to regeneration of degraded soils;
• It is easy to sow and to manage, and can be slashed, grazed or cut for fodder;
• The seeds are cheap, easy to purchase or can be easily reproduced on the farm.

The residues of cover crops are usually not harvested, but are left to decompose in place. This explains, why they are also called living mulch. Some cover crops can be grazed, provide food or can be used to produce forage. Growing a cover crop limits possibilities of soil tillage. Therefore cover crops are usually grown in cropping systems, which include reduced tillage. Cover crops can be grouped into legumes, grasses, and other groups such as cucurbits, which also cover the soil well. For a permanent soil cover, a mixture of legumes and grasses is best, as their root systems usually complement each other well in their growing depths and together they provide a balanced source of feed for livestock.

Mulching is the process of covering the topsoil with plant material such as leaves, grass, twigs, crop residues or straw. Sometimes artificial mulches, such as plastic cover, are used (for weed control mainly; they do not provide the same advantages as organic mulch). Mulching has many advantages, including protecting the topsoil from being washed away by strong rain and from drying out by the sun. Protection reduces evaporation of water and thus keeps the soil humid. As a result the plants need less irrigation or can use the available rain more efficiently. A humid soil also enhances the activity of soil organisms such as earthworms, and microorganisms as rhizobia and mycorrhiza.
Organic mulch material is an excellent food source for soil organisms and provides suitable conditions for their growth. As the mulch material decomposes, it also releases its nutrients, while part of the mulch material is transformed to stable humus, contributing positively to the soil’s organic matter content. A thick mulch layer further suppresses weed growth by inhibiting their germination. For all these reasons mulching plays a crucial role in preventing soil erosion.

Building contour bunds and digging ridges

An effective measure to limit above soil movement of water is digging ridges along the contour lines of a slope. Or natural drain ways are identifi ed and planted with grass to allow storm water to move smoothly over the fields without breaking into gullies. Waterways need dense vigorous vegetation and water should be directed with diversions. Ditches may be dug along the contour to trap the running water and encourage infi ltration into the soil. Contouring is often combined with strip cropping furrows between the contour ridges, in which strips of a crop such as maize alternate on the side of the hill with strips of denser vegetation like legumes. Much of the soil washed downhill from each strip of row crops is trapped by the strip of denser vegetation growing below it. Laying trash lines of stalks and other crop residues, instead of the dense vegetation, is also helpful.
The A-frame is a simple, cheap and easy-to-construct tool for marking contour lines along a slope, making it readily accessible to African farmers. The A-frame is made from three poles, some rope, a stone and a supply of stakes.

How to build and use an A-frame:

1. Fix three poles of about 2.5 meters long each in a position forming an even ‘A’. If rope is not sufficient to tie the ends, use nails.
2. Tie one end of a piece of cord to the top of the A and fix a stone tied to the other end so that the stone is at some distance from both the ground and the crossbar.
3. Put the A-frame upright and mark the position of both legs. Then, mark the point where the string passes the crossbar of the A.
4. Turn the A-frame so that the placement of the legs is reversed. Again mark the point where the string passes the crossbar. If the two marks are not at the same point, mark a third point with a knife exactly halfway between the first two.
5. Drive the first stake at the edge at the top of the field. Place one leg of the A-frame above and touching the stake. Place the other leg in such a position that the string passes the level position point on the crossbar.
6. Drive another stake into the ground just below the second leg. Move the A-frame and continue in the same way across the field.
7. The next contour line is placed 3 to 6 meters below the first line. The steeper the slope, the closer the lines should be.

Placing grass strips
Fodder grasses such as vetiver grass (Vetiver zizanioides), napier grass (Pennisetum purpureum) and guinea grass (Panicum maximum), Bahia grass (Paspulum notatum) can be planted in strips at intervals across the slope to slow down runoff of water. In addition to reducing soil erosion, the grasses provide feed for the animals. The grass strips can be mixed or replaced with a hedge row of leguminous fodder trees such as Leucaena diversifola, Calliandra calothyrsus, Sesbania sesban, Gliricidia sepium.

Terracing

Mechanical measures for controlling erosion are usually more costly than those that depend primarily on vegetation. They require more labour, materials and skill to install. Terracing includes many different types of terraces, diversions and grade stabilization structures. Terraces break up a long slope into a series of short ones. Each terrace collects and controls the excess water from a definite area of the slope above it. Water collected in a terrace channel may be connected to protected outlets such as man-made ponds or natural water ways where it will cause no damage. If the soil in the field is permeable enough, terraces may be built level and water allowed to stand and soak into the ground. Even well-constructed terraces need continuous repair in order to be effective. Unless kept in good condition, terraces may cause more erosion than if they were not built.

• Bench terraces. They are found on medium slopes and transform the steep slope into a series of level shelves or beds running across the slope on which crops are grown. The steps are separated by almost vertical risers (walls or bunds) of rock or earth protected by a heavy growth of vegetation. The risers need to be kept covered with grass and continuously repaired to maintain their stability. Although bench terraces take a lot of labor and time to construct, they can last a long time if well maintained
• Stone lines. Use of stone lines is most applicable when stones are easily accessible in the area. Here stones are piled across the slope, breaking it into small sections where crops are grown. They slow down runoff and soil eventually builds up behind them, forming nearly leveled beds.
• Fanya juu (Converse) terraces. Fanya juu (‘throw it upwards’ in Kiswahili) terraces are made by digging trenches along the contours and throwing the soil uphill to form embankments (bunds), which are stabilized with fodder grass such as Napier (Pennisentum purpureum) and multipurpose agroforestry trees. The space between the embankments is cultivated with crops and over time, the fanya juu develop into bench terraces. They are useful in semi-arid areas to harvest and conserve water.

Conserving vegetation

Plant roots hold soil particles together and protect the soil from being washed away by water or wind. Land that is covered with vegetation is less susceptible to erosion than unplanted land. This can be achieved by maintaining natural grass cover in perennial crops or by growing a cover crop. On the other hand, very steep slopes should be planted with trees instead of cultivating them for crops.
Growing trees in rows (alley cropping) and hedges in the fi elds or around the fields will reduce wind speed. They also create a micro-climate, which reduces evaporation and protects the soil and crops from the drying effects of wind. When trees are planted in rows in the fi eld, they often compete with the crops for water, and in drier areas, this will reduce the yields of the fi eld crops. In such areas, planting hedgerows is recommended or if planted with crops, they should be heavily pruned at the beginning of the growing season of the fi eld crop to reduce competition. During the dry season the trees will have grown again and be able to shade the soil.

Reduced tillage is shallow soil tillage or loosening of the soil by a chisel without deep soil cultivation or making furrows or holes where seed is planted. Minimum tillage promotes build-up of organic matter in the soil, activity of soil organisms and contributes to more stable soil aggregates resulting in better water infiltration. Minimum tillage also implies reduced labour and about half as much energy and effort for land preparation. The greater the part of the soil surface that remains undisturbed and covered, the more positive the impact is. Ideally the seedbed is prepared only where the seeds are planted and the residues remain on the topsoil and are not buried.
Minimum tillage involves techniques such as scraping out shallow planting holes with a hoe, planting with a dibble stick or digging narrow furrows with a chisel-shaped ripper pulled by animals or a tractor. The distance between the furrows results from recommended spacing for the crop. Compared to a conventional plough a ripper is smaller, lighter and easier to operate, and also cheaper to buy and maintain. As a ripper requires about half of the draught force of that of a plough, farmers can use weaker and smaller animals also. For making planting holes with a hoe a long string with knots or bottle tops indicating the planting distance and pegs are helpful.
Reduced or minimum tillage is well suited to many tropical soils, in which intensive tillage leads to rapid breakdown of the soil structure and loss of water and organic matter. However, the adoption of reduced tillage also involves some challenges. The most important is weed control. Farmers who renounce the use of herbicides depend on mechanical weed control or on a thick mulch cover or on cover crops and proper crop rotation to prevent weed growth.
In systems, however, where the inter-row is never ploughed, weed pressure decreases over time, as weeds are not allowed to germinate.