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SOIL: The farmer’s most important asset

PART 26: Soil classification (iii)

ProAgri Zambia acknowledges Grain SA for the use of this series which originally appeared in Afrikaans in SA Graan/Grain.

Martiens du Plessis, Soil Scientist, NWK Limited & Prof Cornie van Huyssteen, Lecturer: Soil Science, University of the Free State

In the previous two articles of this series, we discussed the first nine soil groups according to Fey (2010). In this issue, we discuss groups 10 to 14.

When a specific body of soil is classified, the properties thereof may be better communicated. The properties may then be interpreted and, in doing so, the soil may be evaluated for use.

Soil groups

In part 24, the current 73 soil types are grouped according to the soil formation processes into related soil groups, in a complete table.

Table 1 indicates those groups that are discussed in this article. Groups 1 to 4 are based upon the top soils. Groups 5 to 11 all have an orthic topsoil and are characterised by the enrichment of the subsoil. Groups 12 to 14 comprise those soils that developed in young sediments and thus have poor horizon differentiation. 

In the Hutton form, haematite (a red iron oxide) dominates and indicates a free draining soil with adequate oxygen (thus very little water saturation).

Soil group 10: Oxidic soils

  • The dominating process is the accumulation of iron oxides in the B horizon. The iron oxides are a strong colouring agent that cling to the soil particles and give the soil a red or yellow colour. Haematite is a red iron oxide, and when it dominates, it indicates a freely drained soil with adequate oxygen (thus very little water saturation). The Hutton form (Photo 1) is a good example of this. When a piece of iron lies upon the top of the soil for a long time, it also rusts to a reddish colour. Goethite is the yellow and more reduced form of iron oxide, and when it dominates, it indicates a well-drained soil with a bit less oxygen than in the case of red soils (thus a slight bit more water saturation). This happens when the soil remains wet for longer periods and does not drain as quickly as in the case of the red soils. The Clovelly form (Photo 2) is a good example of this. It is the same process as a piece of iron which lies under water and then rusts in a yellowish colour.
  • Oxidic soils are well drained and therefore occur in drained positions in the landscape, such as on top of the high-lying parts of the landscape. Oxidic soils are widely distributed throughout South Africa.
  • Due to their occurrence in the well drained positions in the landscape, they are frequently used for commercial grain production, provided the climate is favourable. The soil is hydrologically suited to root development, as they do not drown. These soils are not very suitable for the accumulation of water below overlying systems. Deep variants of these may be used for crops and trees that are sensitive to waterlogged conditions. Deep oxidic soils are normally very suitable for irrigation.
Photo 2: In the Clovelly form, goethite (the yellow and more reduced form of iron oxide) dominates and indicates a well-drained soil which remains wet for longer periods than in the case of red soils.
Table 1: Grouping of soil types based upon determining master horizons and materials (Fey, M. 2010).

Group 11: Gley (Russian: clay) soils

  • In contrast to oxidic soils which are well aerated and iron oxides accumulate, one finds gley (clay) soils. Characteristic of these soils is reduction (lack of oxygen) under anaerobic (waterlogged) conditions. Also characteristic is the dominant grey colouring in this horizon. (Photo 3). Gley soils are poorly drained with one or another poor or impenetrable underlying material. They are usually found in the lower lying areas of the landscape where water accumulates, or in level landscapes where most of the rainwater infiltrates the soil and stagnates the profile. They usually have a high clay percentage, but not necessarily.
  • The conditions under which gley soils form, lead to waterlogged conditions. The pH is usually higher than the higher lying soil types in the area. When the gleyed horizon occurs close to the surface (<600mm), waterlogged conditions may harm sensitive crops. As the water is then stored at a shallow level, it is also highly exposed to evaporation losses. When it occurs at deeper levels (>1 200mm), it could be beneficial to crop cultivation for the storage of water and, in so doing, help the crop to get through dry periods.
  • In general, soils where the gleyed horizon is deep (>1 200mm) are highly productive when utilised for crop cultivation, because a lot of water may be stored in the horizon. Shallow variants store too little water in the profile and crops easily wilt during dry periods. During wet periods, shallow soils (<600mm) are subject to waterlogging. Shallow variants should rather be used for grazing. Deep variants may be successfully irrigated, using very careful water scheduling. Irrigation of shallow variants is risky due to the inherent danger of waterlogging. Especially with poor quality water which could lead to a building up of salts in the root zone. They must therefore rather not be irrigated.
Photo 3: Characteristic of gleyed soils is reduction (lack of oxygen) under anaerobic (waterlogged) conditions with dominating grey colours in this horizon.

Group 12: New soils

  • The dominant property of soils in this group is poorly developed pedogenetic properties, which means that horizons are not well developed, as the soils in which the underground horizons develop, was recently laid down (in geological terms). Sediments are typical colluvial (material that has moved downwards against slopes, due to gravity), alluvial (material laid down by water) or aeolian (material deposited by wind). Colluvial deposits are typically found in the foothills of the landscape. Alluvial deposits are typically found in the low-lying parts of the landscape where water-borne material is deposited in flood areas, for example. The deposits are sometimes layered with, for example, a layer of clay, a layer of gravel, a layer of sand (Photo 4), etc. Otherwise, it is simply a mass of soil with poor horizon development. Aeolian deposits with poor horizon differentiation are typically young dunes. Another variant is incomplete accumulation of lime in the horizon (neocarbonate).
  • Due to the extensive nature of mother materials and lack of pedogenetics, the properties of new soils are not typical and should be evaluated in terms of a specific soil body.
  • New soils are usually deep and are potentially very productive. Root and water movement are problems due to the varying layers, particularly in the alluvial deposits. Good soil preparation can, however, solve this problem. Under irrigation, high returns may be achieved, provided the water problems can be managed. Aeolian soils are inherently transported by wind and are therefore very sensitive to wind erosion. Clayey alluvial deposits may also be cultivated with difficulty.
Photo 4: Alluvial deposits are sometimes striated with layers of material of various origins and textures.

Soil group 13: Stony soils

  • A characteristic of stony soils is that the soil formation and loss due to erosion take place at the same rate, with the result that the soil does not get the chance to form as a thick layer on top of the mother rock. In arid areas, weathering is also too slow to form a thick layer. In the wetter areas, erosion is quicker, but the soil is just as quickly removed by erosion as it forms. In both cases, the final result is shallow, stony ground. In some cases, the rock is semi-weathered and is known as saprolite. Cracks can also occur between individual blocks of rock and are then filled with soil.
  • The mother material has a very strong influence on the physical and chemical properties of the overlying layer of soil. It can therefore range from clay to sand, with all the variations in the chemical and physical properties thereof. The most important property is the shallow stone, which severely limits the water retention ability and workability of these soils.
  • Broken rock fragments often allow water and plant roots to penetrate the rock to a certain extent. In contrast, solid rock is absolutely impenetrable. Stony soils are normally not cultivated in the summer grain areas and are used as grazing. In the Western Cape, with its winter rains and accompanying low evaporation, the poor water retention may be overcome and winter crops may successfully be planted in these soils. The hydraulic properties of each body of soil must be studied carefully before irrigation is developed upon them.
Photo 5: Neocutaneous horizons are typical of new soils.

Soil group 14: Anthropogenetic soils

  • Anthropogenetic soils are soils that have, to a certain extent, been altered by human activity, so that the original properties thereof have been totally destroyed. Examples of these are mine dumps (or any pile of soil), excavations and refuse disposal sites.
  • The mother material determines the chemical and physical properties of the soil and range from totally unusable for agricultural purposes (gold mine slimes dams), to reasonably reusable (shallow excavations of lime mines).
  • Usually, anthropogenetic soils have to be adapted at high cost to recover them for agricultural purposes. They are usually then used for grazing and seldom for cash crop production.
Photo 6: In the case of shallow stony soils, soil formation and loss due to erosion takes place at the same rate, with the result that the soil does not get the chance to form a thick layer on top of the bedrock.

Soils with E horizons

In this series on soils, the soil types were dealt with according to Soils of South Africa (Fey, 2010). According to that, the soil types with E horizons are woven into the various soil types. However, these soils do deserve special mention, as the genetics of E horizons are special and these soils have special hydraulic and chemical properties.

  • E horizons mostly develop due to the lateral movement of water within the soil profile. The lateral flow of water then washes away the clay and sesquioxides, leaving only the white sandy quartz as a layer. In podzol soils, the loss of clay and sesquioxides is often not lateral but vertically down the profile.
  • E horizons with lateral water flow are hydromorphic and thus saturated with water during the rainy season. They are also impoverished in soil nutritional substances.
  • E horizons are often very unsuitable for the growth of plant roots because they are sometimes saturated with water (anaerobic) and also impoverished in soil nutritional material. Furthermore, they are usually difficult to till as agricultural vehicles often get stuck in the soil during the rainy season. When they dry out, these horizons occasionally become very hard. Grain crops must therefore be planted early in the season, so that the crop is reasonably high when the peak rain season starts and, in this way, the danger of drowning is reduced.
Photo 7: In E horizons, the lateral flow of water washes the clay and sesquioxides away, resulting in the white sandy quartz remaining as a layer.

Summary

Soil classification is a very useful way to group together soil types with certain properties. It is also the basis on which soil mapping is based. It makes communication easier, as the concept, which may be identified with a name, communicates a number of properties. With the properties known, they may be interpreted keeping in mind the specific desired use of the land, and the likely effect thereof may be predicted. The South African soil classification system is highly suited to this purpose and is widely utilised for this purpose.

For further information, please contact the authors on:
Martiens du Plessis: 072-285-5414 / martiens@nwk.co.za
Prof Cornie van Huyssteen: 051-401 9247 / vhuystc@ufs.ac.za

REFERENCES
Fey, M. 2010. Soils of South Africa. Cambridge University Press, Cape Town, South Africa.

Soil Classification Working Group. 1991. Soil Classification – A taxonomic system for South Africa, Department of Agricultural Development, Pretoria.
Van Huyssteen, CW. 2009. Unpublished class notes for GKD214, University of the Free State, Bloemfontein.

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