According to the character of the soil, its grains or particles will vary from very large in coarse gravel to very fine in fine sand and clay.
Variable size of pores.
Surface attraction.
The size of the pores will vary as the size of its grains from large to small, but frequently a certain kind of soil will contain a mixture of large and small pores. The finer the pores the more energetic will, as a rule, be the surface attraction of the grains composing the soil.
Pore-volume.
With particles of equal size pore-volume amounts to about 38 per cent. of the total space, and sinks down to 10 or 15 per cent. with particles of unequal size.
With equally sized particles the pore-volume is the same whether the particles are small or large.
The collective capacity of the pores or the pore-volume mainly depends on the equal or unequal sizes of the particles. When the same are of equal size the pore-volume amounts to about 38 per cent. of the total space occupied by the soil, but when this is not the case it may sink to as low as from 10 to 15 per cent. of this space. With equally sized particles the pore-volume is the same whether the individual particles are large or small. In nature it will be the exception to find all the particles of equal size, such a condition of things prevails only when careful sorting by sifting or riddling has taken place, and in the majority of cases the larger pores will be partly filled up by the smaller particles of the soil.
Permeability depends first on the size of the pores, and secondly on the pore-volume.
The permeability of a soil for the passage of air and water depends, in the first instance, on the size of the pores, and is further to some extent influenced by the pore-volume.
Effect of large and small pores.