Several classifications of soil types exist. The zonal classification treated here is considered most appropriate at this level. The classification relates soil to the major climatic and vegetation zones of the world. The world’s major climatic belts have their distinctive vegetation and distinctive soil types.
Zonal soils are mature soils. They result from the maximum effects of climate and vegetation upon parent rock in areas where the landscape and climate have been stable for a long time. Consequently, zonal soils have had time to develop distinctive profiles and clear horizons.
Some of these soil types will now be described as follows:
- Ferralitic Soil: These are tropical soils found in forest regions. They are very old soils that are weathered to great depths. They are red to yellow in colour and predominantly clayey in texture. The high clay levels are encountered right from the surface and increase only gradually with depth. Due to the heavy rainfall, the soils are leached of bases and silica, but the iron (Fe) is not leached. Rather the iron oxides remain in the profile, giving the soil it characteristic red colour. The climatic conditions are ideal for chemical weathering and the resultant rapid decomposition of the bed rock gives a deep soil. Leaching and mixing agents (living organisms) prevent the formation of well-defined horizons. The heavy continuous leaf fall gives a thick litter layer. Due to mineral recycling by the forest vegetation, the surface is generally rich in humus and plant nutrients while the subsoil is poor in nutrients. But when the natural vegetation is cleared that natural fertility deteriorates fast; the vegetation that supplies the humus through leaf-fall can no longer do so; thus the mineral recycling stops, whereas there are no reserves of weatherable minerals. The soils are therefore best suited for tree crops such as kolanut, orange, rubber and oil palm because these plants can recycle nutrients through leaf-fall.
- Ferruginous Soil: Ferruginous soils are found in tropical regions outside the rainforest i.e. in the savanna environment. They are not as deep as Ferralitic soils. Most profiles are less than 150cm deep below which occur partially weathered rocks of the parent materials. As a result, deep-rooted trees can extract nutrients released by the decomposition of minerals in the parent rock. A common feature of these soils is the downward movement of clay within the profile, a process that tends to produce a sandy surface soil rather low in organic matter. Savanna grasses die back during the dry season, but they can only produce a thin dark-brown humus layer after decomposition. During the wet season, the soils are leached and become acidic, but during the dry season, capillary action (movement of salts to the surface) neutralizes the effect of leaching. Hence, on the whole the soils are only slightly acidic and may even be neutral. This seasonal change produces a cemented layer or hard pan called duricrust just below the surface where the clay has been washed. Ferruginous soils contain few nutrients and so are not particularly suitable for agriculture. Indeed, tropical grasslands are more suitable for animal rearing than arable farming. However, the soil responds well to the application of fertilizers for the growth of grain crops. An extreme form of the red tropical soils is called Laterization is the process whereby soils are heavily leached of bases and silica leaving behind concentration of aluminium and iron minerals. Usually the soil forms a crust or hard pan at or near the surface. Laterite soils are good for road construction but poor for locations are so cold that heavy snowfall is experienced in winter. In spring when the snow melts, water percolates down through the soil, leaching salts downwards. In summer the warm and dry weather encourages the nutrients to move back up through capillary action. Thus minerals are moved up to the zone of plants’ roots at the time they needed maximum plant growth. The upper horizon is usually very thick in colour and tends to be loose and crumbly and this coupled with high soil fertility, due to the productive nutrient cycle and good drainage, contribute to the high agricultural value of czernozem which is used principally for cereal cultivation, especially wheat.
- Podzol: These soils are found in the coniferous forest region and the cool temperate region e.g. the British type of climate. They are developed under cool temperate conditions with moderate to low rainfall. They are generally low in nutrients and leached due to sudden release of water consequent upon snowmelt in springs. As a result of the long cold winter, the activity of bacteria is limited; therefore the rate of decomposition and humus formation is low. Podzol soils tend to be acidic and infertile; therefore they are of very little agricultural value. Desert soils occur both in temperate and hot deserts whenever there is enough water to allow loose, sand materials. They are often grey in colour in cool deserts but slightly red or yellow in tropical deserts. Owing to marked evaporation and very low rainfall, they are unleached and alkaline, and of course deficient in humus. They can support good crop yield under irrigation except in some places where there is a tendency for salts to crystallize and settle on the surface.