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The most common source of indoor radon is uranium in the soil or rock on which homes are built. As uranium naturally breaks down, it releases radon gas which is a colorless, odorless, radioactive gas. Radon gas enters homes through dirt floors, cracks in concrete walls and floors, floor drains and sumps. When radon becomes trapped in buildings and concentrations build up indoors, exposure to radon becomes a concern.
2.1.1 Soil compositionWhen dry soil is crushed in the hand, it can be seen that it is composed of all kinds of particles of different sizes.Most of these particles originate from the degradation of rocks; they are called mineral particles. Some originate from residues of plants or animals (rotting leaves, pieces of bone, etc.), these are called organic particles (or organic matter). The soil particles seem to touch each other, but in reality have spaces in between. These spaces are called pores. When the soil is "dry", the pores are mainly filled with air. After irrigation or rainfall, the pores are mainly filled with water. Living material is found in the soil. It can be live roots as well as beetles, worms, larvae etc. They help to aerate the soil and thus create favourable growing conditions for the plant roots (Fig. 26).Fig. 26. The composition of the soil2.1.2 Soil profileIf a pit is dug in the soil, at least 1 m deep, various layers, different in colour and composition can be seen. These layers are called horizons. This succession of horizons is called the profile of the soil (Fig. 27).Fig. 27. The soil profileA very general and simplified soil profile can be described as follows:a. The plough layer (20 to 30 cm thick): is rich in organic matter and contains many live roots. This layer is subject to land preparation (e.g. ploughing, harrowing etc.) and often has a dark colour (brown to black).b. The deep plough layer: contains much less organic matter and live roots. This layer is hardly affected by normal land preparation activities. The colour is lighter, often grey, and sometimes mottled with yellowish or reddish spots.c. The subsoil layer: hardly any organic matter or live roots are to be found. This layer is not very important for plant growth as only a few roots will reach it.d. The parent rock layer: consists of rock, from the degradation of which the soil was formed. This rock is sometimes called parent material.The depth of the different layers varies widely: some layers may be missing altogether.2.1.3 Soil textureThe mineral particles of the soil differ widely in size and can be classified as follows:Name of the particlesSize limits in mmDistinguisable with naked eyegravellarger than 1obviouslysand1 to 0.5easilysilt0.5 to 0.002barelyclayless than 0.002impossibleThe amount of sand, silt and clay present in the soil determines the soil texture.In coarse textured soils: sand is predominant (sandy soils).In medium textured soils: silt is predominant (loamy soils).In fine textured soils: clay is predominant (clayey soils).In the field, soil texture can be determined by rubbing the soil between the fingers (see Fig. 28).Farmers often talk of light soil and heavy soil. A coarse-textured soil is light because it is easy to work, while a fine-textured soil is heavy because it is hard to work.Expression used by the farmerExpression used in literaturelightsandycoarsemediumloamymediumheavyclayeyfineThe texture of a soil is permanent, the farmer is unable to modify or change it.Fig. 28a. Coarse textured soil is gritty. Individual particules are loose and fall apart in the hand, even when moist.Fig. 28b. Medium textured soil feels very soft (like flour) when dry. It can be easily be pressed when wet and then feels silky.Fig. 28c. Fine textured soil sticks to the fingers when wet and can form a ball when pressed.2.1.4 Soil structureSoil structure refers to the grouping of soil particles (sand, silt, clay, organic matter and fertilizers) into porous compounds. These are called aggregates. Soil structure also refers to the arrangement of these aggregates separated by pores and cracks (Fig. 29).The basic types of aggregate arrangements are shown in Fig. 30, granular, blocky, prismatic, and massive structure.Fig. 29. The soil structureWhen present in the topsoil, a massive structure blocks the entrance of water; seed germination is difficult due to poor aeration. On the other hand, if the topsoil is granular, the water enters easily and the seed germination is better.In a prismatic structure, movement of the water in the soil is predominantly vertical and therefore the supply of water to the plant roots is usually poor.Unlike texture, soil structure is not permanent. By means of cultivation practices (ploughing, ridging, etc.), the farmer tries to obtain a granular topsoil structure for his fields.Fig. 30. Some examples of soil structures GRANULAR BLOCKY PRISMATIC MASSIVE 2.2 Entry of water into the soil 2.2.1 The infiltration process 2.2.2 Infiltration rate 2.2.3 Factors influencing the infiltration rate
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