HOME GARDEN TECHNOLOGY LEAFLET 7

EROSION CONTROL AND SOIL CONSERVATION

The most beneficial part of the soil is the dark layer of topsoil, which takes many years to develop. Topsoil is rich in plant nutrients and beneficial soil organisms such as worms. Humus is the layer of rotting plant debris that feeds the topsoil. Topsoil and humus are lost easily through erosion by rain, wind, cultivation, foot traffic and by the ground being swept clean every day. The yellow or light brown subsoil below the topsoil may be acidic and is harder to grow plants in.

The loss of topsoil from the home garden can be prevented by putting up windbreaks, planting cover crops and using cultivation practices that minimize soil erosion.

WINDBREAKS

Windbreaks are rows of trees, or shelterbelts, planted across the direction in which the wind blows to reduce the wind's speed. Their capacity to block the flow of wind depends upon their height and density or thickness. A home garden windbreak also can serve as a fence. Apart from preventing the loss of fertile soil, windbreaks also:

• reduce wind damage to crops;
• reduce the loss of soil moisture;
• improve the microclimate.

To provide proper shelter, a windbreak must have nearly uniform vegetation density or thickness from top to bottom. The higher the windbreak, the more effective it is, although it should not impede the growth of crops by depriving them of sunlight. To ensure good thickness in a windbreak, at least two rows of trees should be planted because wind will pass through any gaps, reducing effectiveness.

The most suitable tree species for a shelterbelt are those most adapted to the soils and climate of a given area. They should have the following properties:

• rapid early growth;
• adequate height;
• duration (i.e. they should last a long time before replacement);
• wind resistance (i.e. they are not easily bent, broken or uprooted by wind).

Table 1 lists suitable windbreak tree types for different agro-ecological and topographic regions. The local agricultural extension worker can advise communities on alternatives that are suitable for a given area.

TABLE 1
Some suitable windbreak plants

COVER CROPS

The soil in a home garden must not remain bare for a long time. Cover crops should be planted to help reduce wind and water erosion. Details on cover crops are provided in Home Garden Technology Leaflet 9, "Cover cropping".

PLANTING ALONG THE CONTOUR

Contour lines trace horizontally across a slope, joining points of the same elevation. These lines are important for certain soil conservation measures, such as contour planting, terracing or making contour ridges. Before planting on the contour line or making a contour ridge, it is important to ascertain and mark the contour lines. This can be done simply, by using an A-frame.

Making an A-frame

Constructing an A-frame requires two poles of at least 3 m, one pole of 2.5 m, a piece of string 2 m long and a small stone to serve as a weight.

1. Place the two poles (>3 m) in an inverted V shape and tie the top ends together firmly. The poles must be exactly the same length.

2. Tie the ends of the 2.5-m pole to the legs of the inverted V-frame (about 1 m from the ends of the >3-m poles) to constitute the crossbar of the A.

3. Find the mid-point of the horizontal pole by measuring the pole with a piece of string and then folding the string in half.

4. Use the folded string to mark the mid-point of the A-frame crossbar.

5. Tie the stone to the end of the 2-m piece of string. Tie the string to the tip of the
A-frame (i.e. where the two long poles are joined). The string should dangle a few centimetres below the horizontal crossbar of the A-frame. It will be kept straight by the weight of the stone.

FIGURE 1
Making an A-frame

When the weighted string touches the mid-point mark of the A-frame crossbar, this indicates that the two legs of the A-frame are standing on the same level ground. If the string is not touching the mid-point mark, the A-frame legs are not on the same level ground. They can be brought to the same level ground by moving one leg up or down until the weighted string is at the mid-point mark of the A-frame crossbar.

Establish a level contour line using an A-frame

After placing the two legs of the A-frame on the same level ground, mark the spot where one of the legs is standing with a peg or stone. Then lift that leg, while the other leg remains in place, and rotate the A-frame (in a semicircle) until the lifted leg is on the opposite side. Now place it on the ground and adjust it until the weighted string touches the mid-point mark of the crossbar. Mark the ground (with a peg or stone) where the second leg of the A-frame has been standing, then lift the second leg and rotate it until it is on the opposite side of the frame. Repeat this process, marking the ground where each leg stands before moving it to the opposite side. When these points are marked across the slope, the line that joins the pegs (or stones) that have been placed on the ground is the contour line.

Crops, including trees, then can be planted along the contour lines. This will slow water runoff after rains and enable water to seep into the soil.

FIGURE 2
Using an A-frame

TIED RIDGING

Tied ridging involves building ridges along the contour line. These ridges will reduce soil erosion and allow water to seep into the ground. They also trap silt. Furrows can be blocked at regular intervals with smaller ridges of soil. This system is suitable only for areas with marginal rainfall.

GRASSED CHANNELS

Grassed channels are rows of crops that channel excess water from the fields, preventing erosion from taking place along the contour. The channels should be planted with grass or other suitable crops that slow down the movement of runoff rainwater and hold soil together. Vetiver grass and Crotalaria and Cynodium species are commonly used.

CONTOUR RIDGES

Making contour ridges involves building a series of earthen or stone barriers (bunds) along the contour line. These retain water and allow it to seep into the ground. Excess water is drained off in vertical channels.

Where possible, keep home garden soil moist and maintain a vegetative cover. This will reduce the chances of losing topsoil.