1. Desert environments in Namibia Soils

2. Key Question 1.1 What are the characteristics of a desert environment that make it
extreme?
* The climatic, biotic and soil characteristics of a desert environment.
* The links between climate, biotic and soil characteristics.
Key Question 1.2 How is human activity causing pressures on the desert environment?
* The threats that are posed by
(i) population growth;
(ii) mineral exploitation;
(iii) farming;
(iv) tourism.
* The positive and negative outcomes of human activity.
Key Question 1.3 What are the strategies that can be used to manage human activity in deserts?
* Strategies that attempt to
(i) conserve the desert environment;
(ii) alleviate the impacts of human activity;
(iii) control the use of the desert environment
(iv) monitor the impacts of human activity.
* The role of local, national and international groups in the management of the desert environment.

3. Soils are a complex mixture of organic material, inorganic material obtained from the parent rock, water and air. Their formation depends upon 5 main factors.

4. Processes of change within soils
Additions Addition of material to the developing soil profile from outside sources, such as organic matter from leaves, dust from the atmosphere, or soluble salts from groundwater.
Losses Loss of material from the soil profile by leaching to groundwater, erosion of surface material, or other forms of removal (often transformation and translocation result in the accumulation of material in a particular horizon).
Transformation   Transformation of soil constituents from one form to another, such as through mineral weathering and organic matter breakdown.
Translocation   Translocation (transportation) of inorganic and organic materials from one horizon to another, either up or down (material is primarily moved by water but may also be moved by soil organisms).
Processes of change within soils

5. Most soils develop horizons, i. e
Most soils develop horizons, i.e. distinct layers formed by the movement of materials and minerals by moving water. Minerals can be taen away (leached or eluviated) from one layer and added to (illuviated) to another.

6. P/Pet
Usually the movement of soil water (and therefore the main formation processes of the soil) depend upon the amount of water being added to and taken away from the soil. Therefore the rainfall (precipitation) influences the downward movement and the evapo-transpiration affects the upward movement. The ratio P/Pet (precipitation / potential evapotranspiration) can therefore decide what horizons develop.
If the P/Pet ratio is between 1 and 1.5 (similar to most of lowland Britain) moderate leaching develops, and a brown earth is the soil which develops. Here some minerals are eluviated from the upper horizons and transported to the subsoil.
If the P/Pet ratio is high (over 1.5) in temperate or colder areas podsolisation is the soil formation process. Intense leaching removes most minerals from the upper horizons leaving them bleached and grey in colour. Iron and aluminium sesquioxides are deposited in the subsoil. These are found in upland areas of Britain.

7. A high P/Pet ratio in tropical areas results in a ferralitised (sometimes called laterized) soil. Here the heat and moisture result in intense weathering of the parent rock, and although leaching occurs, high evapotranspitration brings the minerals back towards the surface. Soils are very red with the iron and aluminium sesquioxides being deposited throughout. Sand is transported to deeper horizons however.
Drier areas (P/Pet ratio of 0.5 – 1) result in calcification. Here calcium ions are the main ones redeposited, and so calcium carbonate nodules are found in subsoils. Temperate grasslands and semi arid areas often have this sort of soil.
If water is freely available in areas of high evapotranspiration (P/Pet of <0.5) then salinisation develops. In these soils minerals are brought to the surface and redeposited. A hard saline crust may develop rendering the soil useless for farming. In most deserts this soil cannot develop because not enough water is available to bring salts and minerals to the surface. Desert soils are characterised therefore by little humus and some cementation of calcium carbonate in subsoils.

8. Podzolization
Podzolization occurs in cool and moist climates under pine forests. They are typical of the colder portions of the humid continental and subarctic climates. The E horizon is heavily leached and basically composed a of light coloured layer of sand.
Typical ashy, gray layer left from leaching of sesquioxides is apparent in this podzolized soil.
(Image source: Agriculture Agri-Food Canada. Used with permission)
The upper portion of the B horizon is stained reddish colour from the accumulation of sesquioxides. The profile gets lighter in colour as depth increases.

9. Calcification
Soil enriched with calcium carbonate
(Image source: Agriculture Agri-Food Canada. Used with permission)
Calcification occurs in warm, semi-arid environments, usually under grassland vegetation. Soil tends to be rich in organic matter and high in soluble bases. The B horizon of the soil is enriched with calcium carbonate precipitated from water moving downward through the soil, or upward by capillary action of water from below.

10. Laterization / ferralitization
This profile displays the typical features of a soil having undergone laterization
(Image source: USDA NRCS. Used with permission)
The deep red to bright orange-red soils of the tropics are a product of laterization. Laterization occurs in the hot, rainy tropics where chemical weathering proceeds at a rapid rate. Soils subject to laterization tend toward the acidic and lack much organic matter as decomposition and leaching is extreme. Exposure of the soil to the hot tropic sun by deforestation bakes the soil dry, reducing infiltration, increasing runoff, and reducing fertility. Silica (sand grains) have been leached downwards leaving cemented red iron and aluminium sesquioxides.

11. Salinization
Accumulation of salts is easily seen in this salinized soil.
(Image source: Agriculture Agri-Food Canada. Used with permission)
Salinization occurs in warm and dry locations where soluble salts precipitate from water and accumulate in the soil. Saline soils are common in desert and steppe climates. Salt may also accumulate in soils from sea spray. The rapid evaporation of salt-rich irrigation water has devastated thousands of acres of land world-wide.

12. Aridisols: Aridisols are the dry soils of deserts ( the root arid or id comes from the Latin aridus for dry). They do not receive enough rain to permit normal agriculture. They may have a clay enriched subsoil and /or cemented to non cemented deposits of salts or carbonates.
Salinization, or salt buildup is the greatest danger in using Aridisols for irrigated agriculture. Most areas of true desert are retained as wilderness though the margins may be used for grazing.

13. Gleization
Cultivated gley soil in southern Canada
(Image source: Agriculture Agri-Food Canada. Used with permission)
Gleization occurs in regions of high rainfall and low-lying areas that may be naturally waterlogged. Bacterial activity is slowed in the constantly wet environment thus inhibiting the decomposition of dead vegetation allowing it to accumulate in thick layers. Peat is found in the upper portion of the soil. Decaying plant matter releases organic acids that react with iron in the soil. The iron is reduced rather than oxidized giving the soil a black to bluish - gray colour

16. Soils
Most soils in Namibia are aridisols with four main types depending on the parent material and relief.
Soils in Namibia are dominated by four main types of soil, with most not being suitable for agricultural use.
Lithosols are shallow and rocky
Calcareous soils are developed on limestone, and are also shallow and stony
Arenosols are sandy and therefore infertile and able to be blown away
Halomorphic soils are very salty, e.g. Etosha Pan

17. Soils Halomorphic soil Arenosol (Kalahari Sands Arenosol
Soils in Namibia are dominated by four main types of soil, with most not being suitable for agricultural use.
Lithosols are shallow and rocky
Calcareous soils are developed on limestone, and are also shallow and stony
Arenosols are sandy and therefore infertile and able to be blown away
Halomorphic soils are very salty, e.g. Etosha Pan
Calcareous soil
Lithosol