Soils of the Limpopo River Basin

The dominant soil types in the Limpopo River basin are generally broken down into two main groups:

Older soils that have formed from deep weathering of parent material - examples of such deeply weathered, old sediments are the soils occurring on the highveld plateaus of South Africa and Zimbabwe; and
Considerably younger, shallower sediments from more recent erosional activities or deposited alluvium - the lowveld and coastal plains of Mozambique are good examples of younger, less weathered soils.

These two periods of soil formation are a result of erosional activities in different climates at different times. The older soils were created during a time of higher temperatures and higher rainfall, whereas the shallower younger soils have developed in drier climates, less conducive to soil development.

The map below illustrates the distribution of the main soil types, which are briefly described in the table below.

Dominant soil types of the Limpopo River basin.

Name	Description	Distribution
Arenosols	Sandy soils, developed from weathering of quartz-rich material or rock. Loamy sandy consistency up to approximately 100 cm depth. Less than 35 % rock fragment. Parent material unconsolidated calcareous or sandstone rocks. Code: AR	The Mozambique coast, in a zone adjacent to the Lebombo range, and in the southern half of the Botswana part of the basin
Solonetz	Soils with a densely structured, clay subsurface layer, with a high proportion of absorbed Sodium and Magnesium and are generally alkaline. Parent materials are generally unconsolidated, fine-textured sediments. These soils generally occur in flatter lands in warmer climates that experience hot, dry summers. Code: SN	The Mozambique coastal plain
Luvisols	Soils with a higher clay content in the lower horizons, that the upper horizons. Wide variety of different parent material types, including glacial, aeolian (wind-blown), alluvial (water-borne) or colluvial (gravity) deposits. Code: LV	Zimbabwean and northern Botswanan parts of the basin.
Leptosols	Shallow soils found over continuous stony/gravelly rock and soils. Parent materials are various, with fine earth volumes of less than 20 %. Code: LP	Zimbabwean and northern Botswanan parts of the basin.
Regosols	A group of Weakly developed mineral soils originating from unconsolidated material that do not conform to other classes. Code: RG	Dominate the Lowveld between the eastern escarpment and the Lebombo range in the South African part of the basin
Leptosols	Very shallow soils developed over hard rock or calcareous material. Common in mountainous regions. Less than 10 % fine earth content. Code: LT	Wherever the terrain is hilly in the South African part of the basin
Vertisols	Heavy, churning (internally moving) clay soils that include clay minerals that swell with water, resulting in large cracks when dry. Code: VR	Found overlying mafic rocks
Nitisols	Deep, well-drained red tropical soils with diffuse horizons and >30 % clay content in the subsurface. Parent materials include intermediate basic parent rock products. Code: NT	Found overlying mafic rocks
Acrisols	Soils with higher clay content in the subsurface than upper portions. Parent materials include strongly weathered acid rocks and weathered clays that are degrading. Found mostly in older landscapes of undulating topography in sub-tropical or former sub-tropical environments. Code: AC	Dominate the Highveld of the southern basin

Adapted from FAO 2004; IUSS 2006

In 1990, UNEP conducted a Global Assessment of Soil Degradation (GLASOD), which produced as one of its outputs, a global dataset of human induced soil degradation. Although the application of this data at river basin scale provides a relatively coarse and somewhat out of date result; in the absence of any other consistent basin-wide assessment, it provides a preliminary insight into the state of the soils of the Limpopo River basin. FAO 2004 drew the following conclusions from this map:

Human Induced Soil Degradation in the Limpopo River Basin

No degradation, or stable terrain, along the lower northeast part of the Limpopo River Basin in Zimbabwe and Mozambique and in a north-south zone roughly following the Escarpment and associated mountains.
Slight degradation along the upper Limpopo River Valley, in most of the adjacent southwest catchment in South Africa, and in southeast Zimbabwe. Most of these areas coincide with private farms. Most of the remainder of Mozambique also falls into this class.
Moderate degradation in northeast Botswana and adjacent Zimbabwe, a north-south zone covering northeast South Africa (including Kruger National Park) and the southern tip of the catchment.
High degradation in the southwest upper catchment in Botswana and in an area southwest from Pretoria.
Extreme degradation in three areas in Limpopo Province in South Africa, corresponding with densely populated communal areas (former homelands of Venda and Lebowa).

The Formation of Soils

Home to over one quarter of all living species, soil is the living skin (ISRIC 2010) that covers the Earth, supporting life and providing nutrients for plant growth, anchorage for their roots, and hold water. Soil is a highly influential factor in the biophysical environment. It provides plants with a medium for growth and therefore is a fundamental source of nutritional requirements for terrestrial food-webs. Therefore, the fertility of an ecosystem's soil is a determining factor in plant growth and can either limit or promote productivity/success of consumer organisms further down the food chain (Pidwirny 2008). A comprehensive understanding of the nature and distribution of soils is very important for sustainable development of a region, as the chemical and drainage properties of soils has a direct impact on agricultural potential.

Soil itself is created by the physical and chemical weathering of bed rock, deposition of other sediments and soils and the breakdown of organic matter. Soil formation is affected by a series of factors including the organisms that live on and in it, the climate of the region, the topography (aspect, slope, etc), bedrock below and time (Pidwirny 2008). The study of soils is known as Pedology and the evolution of soils is often referred to as Pedogenesis.

Noorallah (2009) and Mattson (1938) suggest that the development of the Pedosphere (soil layer) can be considered as the intersection of two, three and four of the four spheres - lithosphere (L), hydrosphere (H), atmosphere (A), and biosphere (B). See the diagram below for a graphical representation of this concept.

Classification of soils

Soils are commonly classified according to the size and proportion of the mineral particles found in the soil substrate. The mineral particles that are considered in soil classification are sand, silt and loam.

Table 1: Particle sizes of sand silt and clay in soils.

Mineral Type	Size Range of Particle
Sand	2.0 - 0.06 mm
Silt	0.06 - 0.002 mm
Loam	< 0.002 mm

Source: Pidwirny 2008

Due to gravitational effects on sediment and drainage, soils developing on slopes are often thinner.

Soil pH

Depending on the parent material, the minerals within, the amount of organic matter and the nature of the drainage of a particular soil profile, the pH can vary considerably. As a rule, the pH of soil is determined by the concentration of free hydrogen ions (Pidwirny 2008). Lower pH (acidic) soils have higher concentrations of hydrogen ions and higher pH (alkaline) soils have a lower concentration of hydrogen ions.

Soil Profiles

Soils are commonly classified using a principle called soil profiles, which sub-divides the cross-sectional profile of soil at a location into a series of Horizons.

There are 5 types of horizon, which are listed in the table below.

Table 2: Explanation of soil horizons.

Horizon	Descriptions
O	The upper most layer of the soil, composed primarily of litter, fallen from plants growing in the soil or close by.
A	The layer below the O Horizon, is usually the darkest layer in terms of colour, and the part of the profile from which the finer particles and soluble substances are removed - a process known as eluviation. This layer is where the organic matter derived from plant litter accumulates and is mixed with mineral particles. Due to its proximity to the O Horizon, the upper portion of the A Horizon usually has a higher organic content than the lower portion.
B	While the A Horizon is the location of the eluviation, the B Horizon is the layer that receives the minerals and particles from above - illuviation. The eluviated material from upper horizons may impact the colour of the soil, such as iron, which may turn the soil orange as it oxidises. This layer generally has a higher density than layers above, as it is often more compacted and less porous. This often results in poorer drainage.
C	The C Horizon is composed of weathered parent material from the bedrock below. The size and distribution of particles depends on the weathering processes, drainage of the soil and the movement of minerals in upper horizons of the soil profile.
R	Un-weathered parent material/bedrock

Adapted from Pidwirny 2008; Gilluly 1968

The depth, composition, definition and formation of these horizons are important aspects of a soils development, as they all play a significant role in the drainage and fertility of the soil.

Soil Colour

Soil colour is usually determined by the mineral content, organic content (from plant and animal litter) and the drainage of the soil. Darker colours and tones are usually the result of increased organic content. Other colours or tones, such as orange or white are the result of oxidation of minerals in the soil - orange is caused by oxidation of iron minerals.

Soil Contamination

Soils are naturally occurring substrates, forming through the combination of inputs from the Lithospheric, Hydrospheric, Biospheric and Atmospheric conditions described above. They comprise different amounts of minerals and organic material; however, their general composition is entirely natural. Soil contamination occurs when anthropogenic (human) inputs to the soil profile occur, altering the chemical and biological composition of the soil.

Soil contamination can be caused by a number of different inputs including agricultural run-off, effluent and waste, dumped refuse from domestic and municipal sources, and chemical and hydro-carbon run-off, spillage and waste from mining and industrial sites (including Persistent Organic Pollutants or POPs), to name but a few. The primary concerns related to soil contamination are related to the following issues:

People and animals coming directly into contact with poisonous substances in the soil;
Releases of poisonous vapours and/or gases from the soil; and
Pollution of water flowing through the soil.

Depending on the nature and intensity of the contamination, soil can be remediated (restored to former state), but sometimes only with the application of complex and expensive measures such as thermal treatment, or complete removal and replacement of the contaminated soil.

To access an example of a soil assessment completed for the soils of the Botswana Mining town of Francistown undertake by the BGR (Vogel and Kasper 2002), please refer to the Document Library. In addition to providing a detailed assessment of the soil contamination and the characteristics of the main contaminants, the report also proposes a series of measure for remediating sites.