The physical make-up of soil and its effect on plant growth

3 The physical make-up of soil and its effect on plant growth 

The farmer considers the soil from the point of view of its ability to grow crops. To produce good crops the soil must provide suitable conditions in which plant roots can grow. It should supply nutrients, water and air. The temperature must also be suitable for the growth of the crop. 

Approximately 50% of the soil volume is made up of the solid matter (mineral and organic) and 50% air and water. The relative amount of air and water and organic matter depends on soil type as well as cropping and cultivation systems. 

The description of a soil in relation to the amount of mineral material, clay, silt and sand, is called the soil texture. There are large variations in the texture of soils found on farms. Soil texture can be analysed accurately in a laboratory but can also be assessed in the field using hand texturing. 

If mechanically analysing a soil, the organic matter, stones and gravel are first removed. The rest of the mineral fraction is separated using sieving and sedimentation techniques. This gives an accurate measurement of the amount of elay, silt and sand particles present. The particle size for eaeh mineral fraetion is shown in Table 3.1. Because these mineral eomponents have very different properties it is important to know the relative amount of each, hence the importance of knowing the soil texture. Soil texture is classified into 11 textural elasses in the UK (see Appendix 1). 

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