An e-publication by the World Agroforestry Centre
IMPERATA GRASSLAND REHABILITATION USING AGROFORESTRY
4.1 Soil erosion control with contour planting
The removal of Imperata to convert grassland to other uses may expose the soil to erosion and do more harm than good. Vegetative barriers planted on the contour control erosion and provide organic material and nitrogen for the subsequent crops. They are very useful in Imperata grassland rehabilitation.
It is important to control the flow of water from above the field. If another farmer owns land upslope, encourage him or her to implement soil conservation measures. Check dams, soil traps, or diversion canals may be needed to control water coming from the plot above. If the farmer owns enough land, it is useful to plant trees at the top of the field.
Contours are level lines across a slope at a constant elevation. Contours may curve from side to side to stay level, but they never upslope or downslope.
Vegetative barriers (such as grassy strips) are be located on the contour to control soil erosion. Water flowing down the slope picks up soil. When it reaches a contour barrier it slows down, the soil particles settle out, and more water enters into the soil.
Farmers must be encouraged to lay out contour lines fairly accurately. If lines are laid out by eye, they may go up and down on irregular slopes.
If water gathers and starts running along a sloping barrier, it may cause more erosion than if the barrier were not there.
Contour barriers can include piled crop wastes, grassy strips, and hedges. Crops are planted between the contour barriers. Plowing should also follow the contour. Even with contour barriers, land steeper than a 60% slope should not be cultivated.
Gradually, a terrace builds up between permanent contour barriers. The terrace is flatter than th( original slope, so soil erosion decreases.
Making the A-franic. Use 3 wooden or bamboo poles about 4 cm in diameter. Join the poles securely with notches and string so that the joints do not slip. Tie a string to the top of the A-frame. Tie a rock or other weight to the other end of the string. The weight must be heavy enough that it will not be blown by the wind. The weight should hang about 20 cm below the crossbar.
Calibrating the A-frame. Place the A-frame on nearly level ground. Mark the spots where the legs (A and B) touch the ground. Mark the crossbar where the weighted string passes it ("mark 1").
Turn the A-frame so that leg A is exactly where leg B was, and leg B is exactly where leg A was. Mark the crossbar where the string falls now ("mark 2"). If the two marks are the same, they are the midpoint. If they are different, the midpoint is halfway between them.
Check the midpoint by moving one leg until the string hangs at the midpoint. Mark the positions of legs A and B with stakes in the ground. Reverse legs A and B. If the string hangs at the midpoint again, the A-frame is level and the midpoint is accurate. Mark the midpoint clearly on the A-frame.
Marking a contour line. It is easiest for two people to work together: one to handle the A-frame, and one to mark the contour line.
A new way to find contour lines: "The cow's back method"
Another method to find contours has been developed using the plow and a cow or water buffalo (carabao). This method is faster than using an A-frame, especially if the line will be plowed subsequently.
With a little practice this method can increase the speed of laying out contour lines quite strikingly compared to the A-frame.
Placing your vegetative barriers every 2-3 m vertical distance apart will control soil erosion very well. Contour barriers at a 2 m vertical distance will be a little over 10 m apart on a 20% slope. Farmers prefer alleyways at least 8 m wide for ease of field operations.
Even vegetative barriers spaced at 4, 8, or more vertical meters can greatly reduce soil erosion. If a farmer is doubtful about contour barriers, encourage him or her to put in just one at the center of the field, or just 3 to divide the field into quarters. The farmer can always add more later, if he finds that they work satisfactorily.
When Imperata grasslands are cultivated for annual crops, the easiest way to control soil erosion is to leave unplowed strips of land along the contour that are 0.5 to 1.0 m wide. These natural vegetative strips (NVS) re-vegetate rapidly with native grasses and weeds. They soon form stable hedgerows with natural front-facing terraces. Installing NVS is quite simple. Once the contour lines are laid out there is no further investment in planting materials or labor. They exhibit excellent soil erosion control and require little maintenance. Establishing NVS requires only a fraction of the labor needed for conventional contour hedgerows with tree legumes (see section 4.1.4). The fodder production of NVS is less than that of other hedgerow options, but that also means they compete less with associated crops for nutrients and water. They have proven to be popular in northern Mindanao in the Philippines, and have been adopted by hundreds of fanners in recent years.
NVS and other contour barriers occupy some field area and reduce the amount of land available for crops. Initially the yield of the total area may decrease at first because of the land taken out of production. However, the yield per area of cropped land usually increases later on due to erosion control, improved water retention, and improved soil fertility.
Gradually, farmers may plant grasses, cover crops, shrubs, and fruit or timber trees above the strips. These will replace Imperata. They will filter water, trap soil, and produce fodder and mulch. Grasses commonly planted are Pennisetum purpureiim (napier grass), Pannicum maximum (guinea grass), and Vetivera zizanoides (vetiver grass).
Some grasses, like napier, grow tall and fast in the wet season and might shade crops or compete for water and nutrients. Therefore plant only as much as will be cut and carried for forage.
As the field is plowed on the contour, the soil that is turned downslope will accumulate above the strip and begin to form a terrace. Plant trees along the strip so their roots will anchor the edge of the terrace as it develops. Maintain grasses on the steep edge of the terrace.
Hedgerows of nitrogen-fixing trees or shrubs arc a special kind of vegetative strip. The trees or shrubs are trimmed to keep them from competing with nearby crops for sunlight, nutrients and water. The trimmed leaves and twigs provide nitrogen-rich mulch or fodder. The hedgerows also serve as fuelbreaks within the field.
Intensive contour hedgerows require a great deal of labor (up to 80 days/crop/hectare). If nitrogen fertilizer is cheap and labor is expensive, hedgerows may be more costly than fertilizer as a source of nitrogen. However, the mulch from the hedgerows also may help control weeds and provides organic material. Over time, the farmer might save labor for weeding, land clearing, and cultivation.
Contour hedgerows can be implemented less intensively by carefully selecting the species, hedgerow height, and frequency of trimming. These choices will depend upon the fanning system. This manual offers many choices rather than a single method.
Try several different species in short hedgerows before making recommendations. Observe growth, effects on the crops, and labor requirements. Combine species that complement each other: for example, plant a row of Flemingia for leaves that decompose slowly and provide mulch, and a row of Leucaena for leaves that decompose rapidly and provide nitrogen. Mixtures of species can reduce the risk of pest attacks. Be careful not to introduce a new species that might become a weed! Try local native and naturalized species first, and avoid new species that bear many seeds.
The spacing of the trees and shrubs within contour lines depends on how much planting material is available, and how much labor is available for trimming. If trees are not closely spaced, be sure to pile crop wastes in the hedgerow to control erosion.
Farmers may find it simplest to cultivate the entire field and then plant hedgerows along with their crops. The disadvantage is that the field will have no erosion protection and the hedgerow seeds might wash out. Farmers looking for ways to reduce the labor of establishing hedgerows could be encouraged to start with natural vegetative strips (Section 4.1.2) and these plant trees in the hedgerows later.
The hedgerows should be pruned for the first time only after they are one year old or 1.5-2 m tall. This allows their root systems to become well established.
Subsequent cuttings should take place when the crop needs more light or when the hedgerows are starting to compete with the crop for water and nutrients. Trimming the branches will cause the roots to die back, so trimming affects both shade and root competition. Trim the hedgerow before the crop starts to suffer. For example, with maize: trim the hedgerows when maize is planted, and trim a second time after the hedgerows reach 1 m height (30-45 days). A third trimming might not be necessary, once the maize is tall and the ears are maturing.
Fields with contour hedgerows may be allowed to fallow to rebuild soil fertility. After cropping is discontinued the contour hedgerows are allowed to grow to their full height and serve as a medium-term improved fallow (Section 4.2). After two or three years, they can be cut back to allow the field to be cropped again.
Compared to ordinary contour hedgerows, this system:
Species choice for rotational alleycropping
Fallowed hedgerows in Claveria, Mindanao, Philippines
Contour hedgerows are recommended in Mindanao as a method to prolong annual cropping. Even so, many farmers eventually stop pruning them and let them grow up as fallows. Farmers explain that:
When the fields are again opened, fertility is much increased. It also requires less labor to open these fallows for cultivation than to prepare new fields infested with Imperata.