Soil type

Soil is not a standard commodity.  The relative proportions of soil particles of various sizes has a profound effect on many essential processes.  The coarsest soil particles are termed ‘sand’; the intermediate – ‘silt’, and the finest – clay.  However, this still leaves a huge particle size range within each of these categories.  For example, the volume of a coarse grain of sand is approximately 37,000 times greater than that of a fine grain of sand!  Indeed, if one were to represent the thickness of a clay particle with that of a typical human’s hand then, at this scale, an equivalent coarse grain of sand would be 250 metres high.


Figure 2 The relative size of sand, silt and clay particles.

This has far-reaching implications for the way in which soil mixtures of different proportions of particle size behave due to the huge range in the surface area and the associated pore sizes between adjacent soil particles.  For example, if a large soil particle that occupies the volume of a cube is sliced into eight pieces, the resulting surface area increases six-fold.  If each of the resulting particles are sliced into eight pieces, then the resulting surface area increases to 48 times the original and this can be repeated many times to represent the huge difference between soil types.  For this reason, a teaspoon (5 ml) of clay can have a surface area of between 160 and 640 m2 and a population of microbes that exceeds the human population of China.
 

Figure 3 The increase in surface area with decrease in particle size

This has a dominant effect on how readily water moves through, and is held in, soil and also affects the retention and availability of nutrients.