Conserving soil and water

Soil erosion happens when particles of soil come loose and are carried away by water or the wind. When it rains so much that the water cannot seep into the soil fast enough, the extra water flows down the slope, carrying soil particles with it.

Many agricultural soils are easily eroded. The erosion problem is likely to be more severe on certain types of soils, on steep slopes, where there is intense rainfall, and where the vegetation is removed.

Soil conservation

Soil conservation means reducing the amount of soil erosion and maintaining soil fertility. It relies on increasing the amount of water seeping into the soil, reducing the speed

Common types of erosion Rain-splash erosion Occurs when raindrops fall on unprotected ground. The impact on the soil splashes away soil particles and digs a crater. Rain-splash erosion Sheet erosion Occurs when thin layers of the topsoil are moved by the force of the runoff water, leaving the surface uniformly eroded. Sheet erosion Rill erosion Caused by runoff water when it creates small, linear depressions in the soil surface. These are easily removed during land tillage. Rill erosion Gully erosion Unlike rill erosion, gullies are too deep to be removed during normal cultivation with ordinary farm implements. They are formed from small depressions, which concentrate water and enlarge until several join to form a channel. The deepening channel undermines the head wall, which retreats upslope. The gully then widens as the side-walls are worn back. Gully erosion

and amount of water running off, and keeping enough vegetation to protect the soil surface and to bind the soil together.

For any form of land use to be sustainable, production must be combined with conservation of the resources it depends on. Soil conservation efforts should:

• Be able to control erosion sufficiently.
• Maintain the organic matter of the soil.
• Maintain soil physical properties.
• Maintain the appropriate level of nutrients in the soil.

Water conservation

Soil conservation is closely related to water conservation. In arid and semi-arid areas, rain falls only during a few months of the year, but is unreliable even during those months. It typically comes in a few, heavy storms, and much of the water runs off the surface, causing flooding and erosion.

Water conservation relies on trapping as much of this water as possible and storing it on the surface (in tanks or reservoirs) or allowing it to sink into the soil in order to raise the water-table and increase the soil-moisture level. More water can seep in if it is spread over a large area of soil rather than being concentrated into fast-running streams. So water-conservation efforts focus on stopping the water from becoming concentrated in the first place (by ensuring a protective cover of vegetation on the soil surface), slowing down the flow of running water (for example, with pits and dams), and spreading the water out over a large area (for example, with contour ditches).

Advantages

• Conserving water makes water available for crops, livestock and domestic use over a longer period.
• Controlling soil erosion improves crop or pasture yields.
• Conservation measures improve the supply of fuel and forest products.
• They increase the value of the land.
• Terraces make cultivating steep slopes easier.
• More and better livestock fodder is available, for example from grass strips, hedge barriers and terrace embankments.
• Employment opportunities in soil- and water-conservation work increase.

Disadvantages

• Fragmented land ownership makes it difficult for farmers to invest optimally in soil and water management systems.
• Conservation structures need a lot of labour to build and maintain.
• Crop production in semi-arid areas involves a lot of risks, including flooding. This makes it difficult for farmers to realize the full benefits of conservation

• Many farmers lack the skills to design and build conservation structures; sub-standard and poorly constructed structures often result.
• Land-tenure systems determine the ownership of the structures and influence farmers' interest in conservation and in maintaining the structures.
• Irregular rainfall reduces the effectiveness of vegetative erosion-control practices.

Agronomic/vegetative conservation measures

There are many ways of conserving the soil and water, some of which are described elsewhere in this book. They can be divided broadly into agronomic and physical conservation measures. Agronomic or vegetative measures (listed below) include the use of vegetation and soil tillage practices; physical measures (described later in this section) involve building permanent structures, usually of soil or stone, to control the flow of water.

Crop management

Good crop management reduces soil erosion by water and wind to tolerable levels and can improve soil fertility. Select appropriate crops for the soil and slope, plant early, and use suitable cropping systems and rotations to keep the soil covered.

Tillage methods

Tillage aims to optimize soil physical and biological conditions for crop production, and to ensure timely seedbed preparation, planting and weed control. Use a tillage method that does not make the surface soil too fine and powdery. Break up the hardpan if necessary (see the sections on Deep soil preparation and Minimum tillage).

Applying organic matter

Adding manure and fertilizers to the soil provides the required plant nutrients for vigorous crop growth. This covers the ground quickly (protecting it from erosion and allowing water to seep in) and produces higher yields. See Part 6 on Soil fertility in this book.

Agroforestry

Agroforestry involves planting trees or shrubs in the farm, or keeping those that are already there. Trees can conserve the soil in many ways. They cushion the impact of raindrops on the soil, so reducing the amount of rain-splash erosion. Their roots bind the soil. Planted along contours,

they can interrupt the flow of water running off the surface. They shade the soil, reducing the soil temperature and cutting the amount of water that evaporates into the air. They break the wind, reducing the amount of wind erosion. They recycle nutrients from deep in the soil, and leguminous trees fix nitrogen that can benefit food crops.

Apart from helping conserve soil, agroforestry can provide many other ecological, economic or social benefits: fodder, fuelwood and charcoal, timber and building poles, and fruit.

Contour farming

Contour farming involves ploughing, planting and weeding along the contour, ie, across the slope rather than up and down. Experiments show that contour farming alone can reduce soil erosion by as much as 50% on moderate slopes. However, for slopes steeper than 10%, other measures should be combined with contour farming to enhance its effectiveness. See the sections on Marking contours and Contour tree-planting for more information.

• Contour ridges are used mainly in semi-arid areas to harvest water, and in higher rainfall areas for growing potatoes.
• Trashlines made by laying crop residues or "trash" in lines along the contour. They slow down runoff and trap eroded soil, eventually forming terraces. However, the contour line can be destroyed by termites eating the trash.
• Grass barrier strips planted along the contour. They are planted with fodder grass such as Napier, or are left with natural grass. They are effective soil conservation measures on soils that absorb water quickly, and on slopes as steep as 30%.

Physical soil conservation structures

Physical soil conservation structures are the permanent features made of earth, stones or masonry. They are designed to protect the soil from uncontrolled runoff or erosion, and to retain water where it is needed. They supplement agronomic or vegetative measures but do not substitute for them. The appropriate type of physical structure depends on:

• Climate and the need to retain or discharge the runoff.
• Farm sizes.

• Soil characteristics (texture, drainage, and depth).
• Availability of an outlet or waterway.
• Labour availability and cost.
• The adequacy of existing agronomic or vegetative conservation measures.

Below is a list of some common physical conservation structures.

Cutoff drains

Cutoff drains are dug across a slope to intercept surface runoff and carry it safely to an outlet such as a canal or stream. They are used to protect cultivated land, compounds and roads from uncontrolled runoff, and to divert water from gully heads (see the section on Gully reclamation).

Retention ditches

These ditches are dug along the contour. They catch and retain incoming runoff and hold it until it seeps into the ground. They are an alternative to cutoff drains when there is no nearby waterway to discharge the runoff into. They are often used to harvest water in semi-arid areas.

Infiltration ditches

Infiltration ditches are one way of harvesting water from roads or other sources of runoff. They consist of a ditch, 0.7-1.5 m deep, dug along the contour, upslope from a crop field. Water is diverted from the roadside into the ditch, which is blocked at the other end. Water trapped in the ditch seeps into the soil. On soils with an impervious layer (such as a hardpan) below the surface, the water does not sink straight down into the soil. Instead, it moves downslope just below the surface, towards the crops in the field below.

Infiltration ditch spreading water from a road into a field. Make sure that the bank blocking the ditch at the far end is higher than the road so that in a big storm, excess water spills down the road and does not burst the bank.

Water-retaining pits

Water-retaining pits trap runoff and allow it to seep into the soil. A series of pits are dug into the ground where runoff normally occurs. The soil from the pits is used to make banks around the pits (keep the topsoil and put it on top of the banks). Furrows carry excess water from one pit to the next. The size of the pits depends on the amount of runoff: a typical size is 2 m square and 1 m deep. Plant bananas and other tree crops around the pits. Keep children and livestock away from the pits.

Broadbeds and furrows

In a broadbed-and-furrow system, runoff water is diverted into field furrows (30 cm wide and 30 cm deep). The field furrows are blocked at the lower end. When one furrow is full, the water backs up into the head furrow and flows into the next field furrow. Between the field furrows are broad beds about 170 cm wide, where crops are grown.

Broadbed-and-furrow system. Ensure that it is constructed so that excess water spills down the road.

Fanya juu Fanya juu terraces are made by digging a trench along the contour and throwing the soil uphill to form an embankment. The embankments are stabilized with fodder grasses. The space between the embankments is cultivated. Over time, the fanya juu develop into bench terraces (see below). They are useful in semi-arid areas to harvest and conserve water.
New fanya juu terrace	Same terrace after 5 years

Fanya chini

A fanya chini is like a fanya juu, except that the soil is put on the lower side of the contour trench, not on the upslope side of it (as in a fanya juu). Fanya chini are used to conserve soil and divert water. The resulting embankment can be used to grow fodder. Fanya chini are easier to make than fanya juu, but they do not lead the formation of a bench terrace over time. They can be used on slopes up to 35%.

Bench terraces

Bench terraces are level (or nearly level) steps constructed on the contour, and separated by embankments (risers). They can be formed by excavation or may develop over time from a grass strip or fanya juu.

Harvesting rainwater from trees This technology is useful for people cannot afford a metal roof and are not near other sources of “free” water like springs, wells or boreholes, but who have trees in their compounds. Rain falling on the foliage runs down the tree trunk, and is funnelled by a banana leaf or metal sheet attached to the trunk into a bucket, pot or storage tank. Rainwater can be harvested from trees anywhere: it is a question of finding out which trees are appropriate. The best ones have short, smooth trunks, thick crowns and heavy foliage. Jackfruit (Artocarpus heterophyllus) and wild fig (Ficus natalensis) are very suitable. This method is cheap and easy to set up. While the water collected is not as clean as that from a metal roof, it is cleaner than that collected from thatched roofs. It should be filtered and boiled if it is used for drinking. Mr Mukasa of Mpigi district in Uganda has two grass-thatched houses and a few trees on his compound. He harvests rainwater from one of the trees, a jackfruit. He uses banana leaves tied to the trunk with banana fibres, and collects the water in a clay pot. While the amount of water collected is not large, he says it helps his family a lot because the nearest well is about 1 km away.

Stones terraces

Stone terraces are useful in areas with steep slopes but a high population density and scarce land. The terrace risers are made of stones collected from the land. The terraces themselves can be sloping or level. In the Konso area in southern Ethiopia, farmers form the terrace steps into a series of shallow pits, in which they plant several different crops.