6 Using cropping systems to control erosion Rotation and fallow Many tropical farming systems have originated from shifting cultivation; a system of food production based on a rotation of cultivation followed by a long fallow period. The fallow period ensures a natural recovery of soil fertility. | 6 Using cropping systems to control erosion Rotation and fallow Many tropical farming systems have originated from shifting cultivation a system of food production based on a rotation of cultivation followed by a long fallow period. The fallow period ensures a natural recovery of soil fertility. After a recovery period of 15 - 20 years the vegetation is again cut back and burned and the land can again be used for farming. This farming system is still very common it accounts for 8 of the world s food supply but unfortunately a high population pressure in many places undermines the system. The land is then used too intensively and the fallow period is shortened. Too few nutrients are built up so that the soil fertility declines. The reasons for this system being more susceptible to erosion are In the cropping period and to a larger extent after a year erosion is much greater than in the fallow period because the soil is less covered. The longer the cropping period the more the erosion. If the cropping period is longer more crops are taken from the field and thus more nutrients are removed from the soil. This results in a decline in the organic material production the structure of the soil deteriorates and with it the stability too. The condition of the crop and the cover crop also declines with an increasing chance of diseases and pests. In time erosion itself again results in lower yields. This illustrates that a sufficiently long fallow period in the cropping system is very important for the soil. If the fallow period has to be 48 Erosion control in the tropics shortened out of necessity precautions should be taken in the cropping as well as in the fallow period to minimize the risks. The most important measure to keep the production up to level is to fertilize with organic manure or artificial fertilizers. Extra precautions will have to be taken to avoid the chance of diseases and pests. The cropping techniques discussed in Chapter 5 are especially directed .
