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These changes are crucial to the development process itself. But they may also generate environmental side effects of varying degrees of importance. Some of the more common of these side effects are described here.
2.10. Environmental side effects may manifest themselves even within the framework of traditional systems of agriculture under the pressure of rapid population growth. These systems have often persisted for centuries, sometimes successfully cultivating the same lands without irreversible damage. But a new situation may be created by the rapid growth of population that is now taking place. This may impose pressures that were perhaps not experienced before and
which could give rise to environmental problems.
2.11. Traditional agriculture in many tropical regions is characterized, particularly under stress of expansion, by a range of environmental hazards.
These include leaching - notably the rapid leaching of nutrients and degradation of planted farmland following the removal of a forest;
rapid soil depletion resulting from permanent cultivation which the relative infertility of the soil cannot support without the addition of nutrients;
soil erosion through variable and heavy rainfalls and prolonged droughts or flash floods;
and indiscriminate loss of forest resources through slash-and-burn techniques.
Although much of this kind of environmental deterioration can be corrected if unlimited funds are available, some is so costly to correct as to be effectively irreversible. The fragility of tropical ecosystems may cause environmental deterioration to proceed rapidly and their recovery to be slow. In one instance, the establishment of an agricultural colony failed when deforestation resulted in the hardening of lateritic fields within five years; restoration on the other hand will take decades. In another case, previously ungrazed savanna was destroyed by over-grazing in two to three years, and will probably be lost to production for a very long period. There are opportunities for preventing some of these environmental hazards through proper planning and anticipatory action.
For instance, under-employed labour that frequently abounds in rural areas may be mobilized in terracing mountain sides and in reforestation programmes. Many of Africa's current marginal lands, for example, have all the necessary elements for successful reclamation through new management techniques.
2.12. The environmental hazards in the case of modern agriculture arise mainly from the chemical control of weeds and pests and from irrigation works. Fertilizers, on the other hand, would not appear at present to pose a threat even at prospective level of their use in the developing countries. The side effects of insecticides and pesticides need to be watched fairly carefully. Their toxicity to fish and birds, as well as their persistence and mobility, make them a hazard beyond their target area. Irrigation projects, unless matched by drainage facilities, can result in salinization and water-logging. In one country modern canal irrigation serviced forty million acres in 1949, of which five million acres suffered from salinization and water-logging by 1959. However, much of this land has since been reclaimed through appropriate management.
Even the welcome emergence of the high yielding varieties of wheat, rice, maize and other cereals can sometimes give rise to certain negative side effects, both because these varieties require larger quantities of chemicals such as pesticides and also since they replace hardy native species which, by natural selection, are often better suited to the adversities of local conditions and are valuable for interbreeding. Again, constant tillage which is facilitated by mechanisation can also damage the soil structure. Let us reiterate that modem agriculture would be impossible without the use of chemical fertilizers and pesticides, high yielding varieties of seeds and irrigation works, and a degree of mechanisation, but it is important that their side effects should also be taken into account in planning the use of these inputs to expand agricultural production.