Controlling Soil Fertility

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Controlling Soil Fertility Approximately 2 billion hectares of land, 17% of the total vegetated area of the earth has been degraded for agricultural purposes since 1945 (Oldeman et al., 1990). Oldeman et al. (1990) classify about half of this degraded area as still permitting agricultural use, but with greatly reduced productivity, and in the rest no agriculture is deemed possible. The causes of this degradation are overgrazing (35%), agricultural activities (28%), deforestation (30%), over exploitation (7%) and industrialization (1%). Wind and water erosion are the principal mechanisms of this soil degradation, accounting for about 56% and 28% respectively of the total; 12% is attributable to chemical degradation (due to insufficient manure and fertilizer returned to cropped areas, salinization by unwise irrigation or drainage, soil acidification not corrected by liming, and pollution caused by industrial and other urban uses) and the remaining 4% is physical degradation from compaction or flooding. (Oldeman et al., 1990) Moreover, it has become increasingly evident that although crop yields seem to be increasing, they are increasing at a decreasing rate. Krauss and Allmaras (1982) argue that this is largely due to the fact that in many areas, poor soil management is leading to soil degradation which is ignored as long as crop yields continue to rise. Technological advances such as improved cultivation systems, irrigation and pest control mask the negative effects of poor soil management on crop yields (Krauss and Allmaras, 1982, p85). What are the factors that contribute to the suitability of a soil for agricultural purposes? To what extent can people control these factors to halt soil degradation and maintain, enha... ... middle of paper ... ..., L.R., V.W.P van Engelen, and J.H.M. Pulles. 1990. The extent of human induced soil degradation. Annex 5 of World Man of the Status of Human-Induced Soil Degradation: An Explanatory Note, rev. 2nd edn., International Soil Reference and Information Center, Wageningen, Netherlands as cited by J.Schnoor and V. Thomas, 1994, Soil as a Vulnerable Environmental System pp233-44 in R. Socolow, C. Andrews, F. Berkhour and V. Thomas (eds.), Industrial Ecology and Global Change: Cambridge University Press. Robinson, C.A., R.M. Cruse, and K.A. Kohler, 1994, Soil Management, pp 109-134 in J.L. Hatfield and D.L. Karlen (eds.) Sustainable Agricultural Systems: Boca Raton, Florida, Lewis Publishers, 316pp. USGS, 1988, Essential Elements and Soil Amendments for Plants: Sources and use for agriculture: U.S. Geological Survey Circular 1017, U.S. Government Printing Office, 48pp
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