Environmental Problems, Their Causes, and Sustainability 1

Low-Tech Sustainable Agriculture in AfricaINDIVIDUALSMATTERIn 2003, Cubanbornsoil scientistPedro Sanchezwas awarded theWorld Food Prize,the “Nobel Prize”of agriculture. He received it for developinga low-tech and sustainableform of agriculture that has increasedcrop yields fourfold, restoreddepleted soils, and helpedmore than 150,000 Africans (most insub-Saharan Africa) escape fromhunger and poverty.Since 1992 Sanchez has beenworking with scientists in Kenya,Africa, to develop an effective cultivationsystem that grows crops andtrees together.A basic problem is that manyAfrican soils are thin and often depletedof nitrogen and phosphorusafter several years of intense cropgrowing. Adding commercial inorganicfertilizers would help, butmost African farmers cannot affordthem.Sanchez and his colleagues developedthe following soil replenishmentand crop-growth system todeal with these problems. First, atthe beginning of the rainy seasonfarmers plant corn in rows betweenlocal varieties of fast-growing trees(Figure 14-14c, p. 285). The corn isharvested and the trees are allowedto grow for a year.Second, just before the secondcorn-planting season the trees arecut down and their leaves are duginto the soil to add nitrogen. In addition,the trees can be used for firewood,which also helps preventdeforestation.Third, phosphorus is added tothe soil by crushing small depositsof phosphate rock found throughoutmuch of Africa. Africa’s mildlyacidic soils help dissolve the phosphatefertilizer into the soluble formof phosphate needed by corn plants.Fourth, farmers chop up theleaves and stems of a weedy shrubcalled the Mexican sunflower that isfound along many roadside andfarm boundaries. They place theplant pieces in planting holes withcorn seed to provide micronutrientsneeded for healthy crop growth.This four-part technologicallysimple system can then be used toprovide food and restore depletedsoil on a more sustainable basis. Thesystem also helps empower the womenwho raise most of the crops inAfrica by bringing in extra income.farmer innovation called tassas has tripled yields on atleast 100,000 hectares (250,000) acres of unproductiveland. Tassas are small pits dug in the soil, filled withmanure, and then planted with crops once they fillwith infrequent rain.14-3 SOIL EROSION ANDDEGRADATIONWhat Causes Soil Erosion? The Big ThreeWater, wind, and people cause soil erosion.Most people in developed countries get their foodfrom grocery stores, fast-food chains, and restaurants.But we need to remind ourselves that all food comesfrom the earth or soil—the base of life. This explains whypreserving the world’s topsoil (Figure 4-25, p. 73) isthe key to producing enough food to feed the world’sgrowing population.Land degradation occurs when natural or humaninducedprocesses decrease the future ability of land tosupport crops, livestock, or wild species. One type ofland degradation is soil erosion: the movement of soilcomponents, especially surface litter and topsoil fromone place to another. The two main agents of erosionare flowing water and wind, with water causing mostsoil erosion (see photo on p. ix, right).Some soil erosion is natural and some is caused byhuman activities. In undisturbed vegetated ecosystems,the roots of plants help anchor the soil, and usuallysoil is not lost faster than it forms. Soil becomesmore vulnerable to erosion through human activitiesthat destroy plant cover, including farming, logging,construction, overgrazing by livestock, off-road vehicleuse, and deliberate burning of vegetation.Soil erosion has two major harmful effects. One isloss of soil fertility through depletion of plant nutrientsin topsoil. The other harmful effect occurs wheneroded soil ends up as sediment in nearby surface waters,where it can pollute water, kill fish and shellfish,and clog irrigation ditches, boat channels, reservoirs,and lakes.Soil, especially topsoil, is classified as a renewableresource because natural processes regenerate it. However,if topsoil erodes faster than it forms on a piece ofland, it eventually becomes nonrenewable.How Serious Is Global Soil Erosion? MostlyBad NewsSoil is eroding faster than it is forming on more than athird of the world’s cropland, and much of this landalso suffers from salt buildup and waterlogging.A 1992 joint survey by the United Nations (UN)Environment Programme and the World ResourcesInstitute estimated that topsoil is eroding faster than itforms on about 38% of the world’s cropland (Figure14-7, p. 280). According to a 2000 study by theConsultative Group on International AgriculturalResearch, soil erosion and degradation has reducedfood production on about 16% of the world’s cropland.http://biology.brookscole.com/miller14279