Contour barriers (live, dead and mixed barriers)

Contour barriers are contour strips which intercept downslope flowing water and soil particles. These barriers slow down the water movement and reduce its erosive force. They also filter out and trap many of the suspended soil particles, keeping them from being washed out of the field. A long term advantage of barriers is that soil tends to build up behind them, creating a terrace effect. Barriers can be classified as live (strips of living plants), dead (rocks, crop residues), or mixed (a combination of the previous two).

Live barriers are strips of vegetation planted along the contour which serve to anchor the soil in place with their roots and to slow down the movement of water downslope with their stems and leaves (Fig. 8). They are planted above hillside ditches to prevent them from filling with soil and also by themselves to prevent hillside erosion. me most common types of live barriers are plants of the grass family because of the dense foliage and root nets produced. As soil builds up behind them, a bench terrace with a grass protected riser (sloping bank) is formed (see section on bench terraces). In addition the grasses are valuable as forage for animals, or for human consumption in the case of sugar cane or lemmon grass. Many species of plants have great potential as useful live barriers, some possibilities are listed in Table 1. The table is by no means all inclusive, the possibilities are almost limitless, especially when considering agroforestry systems where contour strips of nitrogen-fixing and/or wood or fruit producing trees may be used as barriers. An effort should be made to discuss the available barrier plants in the area with farmers so that each one can select the ones most suited to their needs.

As a management practice it is advisable to cut forage barrier plants and carry them to livestock rather than letting livestock graze them. If the barriers are not well established, the animals may uproot or overgraze them, thus wiping out the barrier. If farmers use the practice of opening their fields to grazing after the harvest, then an unpalatable barrier or planting early in the year to ensure a well established barrier is necessary. Maintenance of the live barriers is extremely important. Open spaces should be replanted so that the barrier forms an effective soil filter. Barriers should be cut or pruned to avoid excessive shading or root competition with the crops.

TABLE 1 Some suggested live barrier plants

TABI.E 1 (Continued)

Fig. 8. Contour live barriers

In planting contour barriers, the spacing from one barrier to the next depends on the slope of the land, with barriers spaced closer together on steep slopes and farther apart on gentler slopes. (Table 2)

TABLE 2

SPACING OF CONTOUR BARRIERS ACCORDING TO THE SLOPE

From Suarez Castro 1980, modified to show distance along ground surface rather than horizontal distance.

Contour planting of a crop and contour strip cropping are also techniques which may be considered functionally as live barriers since they are contour plantings which serve to control hillside erosion. The planting of windbreaks is also a use of live barriers (trees or tall grasses) in this case to avoid erosion or crop damage due to wind rather than water.

Dead barriers function similar to live barriers, the difference being that they are composed of rocks, plant residues, or other non-living materials. (Fig. 9). If rocks are present in a field, it is useful to construct these and in the process make the soil easier to work. Another advantage is that they can be completed during the dry season, meaning that they are in place and functioning at the start of the rainy season. If enough rocks are present' the barriers can be constructed as rock walls of sufficient height so that bench terraces are famed as the soil fills in behind each wall. If sufficient rocks are not present, the barriers lose their effectiveness as the soil fills in behind them, and they should be supplemented with the planting of live barriers.

Fig. 9. Contour dead barriers

The construction of dead barriers is simple, but requires a lot of manual labor. Once contour lines are marked out according to the spacings giving in Table 2,a hoe is used to form a furrow which serves to anchor the barrier (~ 20 cm deep). men the materials are laid out to form walls along the contour lines. In some cases, mixed barriers are used, a combination of live and non-living materials. This can consist of strips of trees with the intervening spaces filled with rocks or crop residues. (Fig. 8), or combinations of grass and rock barriers. If using plant residues, be aware that as they decompose they lose their effectiveness as barriers and will erode away, needing to be supplemented.

Contour furrows or raised planting beds may also be considered functionally as contour barriers. (Fig. 10). These techniques are useful in the cultivation of vegetables, basic grains, or any raw crops in which the soil is to be intensively cultivated. The contour furrows or beds serve various purposes: reducing water movement and therefore soil erosion down the slope, permitting drainage of excess soil moisture from the planting bed, providing for a more even distribution of irrigation or rainwater, and avoiding compaction of the planting bed surface by providing walkways The construction is simple. The furrows are plowed if possible, along measured contour lines, then cleaned out with a hoe or shovel, spreading the soil in the space between the furrows. The width between furrows is variable and depends on the crop and any irrigation requirements.

Fig. 10. Contour planting beds