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A comparison of the USDA Soil Taxonomy and the Zimbabwe soil classification system with reference to some red soils of Zimbabwe
Abstract and Figures
The Zimbabwe soil classification system was compared to the United States Department of Agriculture Soil Taxonomy in terms of logical structure, diagnostic criteria and applicability to some red soils in central Zimbabwe. Soil Taxonomy places little direct emphasis on parent materials and surface soil texture in contrast to the Zimbabwe system. At the higher levels, especially the soil order, the Zimbabwe System is less quantitatively defined than is Soil Taxonomy. The use of the latter system required a considerably larger number and more sophisticated type of laboratory analyses than does the Zimbabwe System. Four pedons, two on mafic and two on granitic rocks, were studied. The first two pedons were classified as 7E and 5E, while the third and fourth were classified as 7G and 5G, respectively, in the Zimbabwe System. Using Soil Taxonomy, the first two pedons were classified as clayey mixed isothermic Oxic Haplustalf and the fourth as sandy, mixed isothermic Haplustalf
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Soil fertility is largely a function of climate, parent material and management of the soils. Parent materials are derived
from various rock types. Most of the rocks in Africa are of Precambrian age, more than 544 million years old. The oldest rocks
in Africa, Archean granites and granite gneisses as well as old volcanic rocks, called greenstones, are more than 3.2 billion
years old. They form the stable granite–greenstone nuclei of the continent. Folded Precambrian metamorphic belts, the so-called
mobile belts, accreted around these nuclei. While the granite and granite gneiss-dominated areas are mineralogically and chemically
more homogenous, the areas underlain by deeply eroded Precambrian fold belts vary widely in composition. The African post-Precambrian
(<544 million years) history is marked by the development of sedimentary basins with varying sedimentary rocks at the perimeter
of the igneous and metamorphic stable continent. Extensive parts of the interior of Africa are covered by Mesozoic to Cenozoic
sandstone-dominated basins. Soil rejuvenating volcanic rocks extruded from lower parts of the earth crust and mantle mainly
during Mesozoic and Cenozoic times in linear zones. Massive volcanic outpourings took place since about 30 million years,
in Tertiary Rift environments. Soils derived from these rock types vary. Soils derived from silica oversaturated igneous and
metamorphic rocks as well as sandstone-dominated lithologies form sandy soils with limiting, inherently low nutrient concentrations
and low water-holding capacities. In contrast, soils derived from volcanic, silica-saturated or silica-undersaturated igneous
rocks, young or old, contain higher concentrations of total Ca, Mg, P and micronutrients. Under suitable climatic conditions
they weather into more clay-rich, fertile soils with high water-holding capacities. Examples of strong inherent soil differences
related to the underlying parent materials are given from Zimbabwe and Uganda. Inherent differences in soil properties can
spatially vary over short distances. The main nutrient constraints of African soils are N and P deficiencies. Most soils are
also low in organic matter. There are many soil fertility restoration management strategies, making use of local organic and
imported and local inorganic nutrient resources, such as phosphate rocks and fertilizers. Africa is endowed with large deposits
of naturally occurring agrominerals and fertilizer raw materials. Natural gas, the principal feedstock for industrial N fertilizer
production, is found in coastal zones of Africa, mainly at the northern and western sides of the continent. Extensive sedimentary
phosphate rock resources occur along the west coast of Africa and in North Africa. Igneous phosphate rocks are found mainly
in eastern and Central Africa and along linear zones in West and southwest Africa. Large gypsum deposits occur in Mesozoic
strata in coastal sedimentary basins. Calcium–magnesium carbonates, in various forms, occur in almost every country of the
continent.
KeywordsAfrica-Agrominerals-Fertilizer raw materials-Geology-Soil rejuvenation
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