[Sidenote: Size of grain and pores.]
Here is of interest the size of the grains or particles composing the
soil, the size of the pores and their collective capacity.
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.
[Sidenote: Variable size of pores.]
[Sidenote: 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.
[Sidenote: Pore-volume.]
[Sidenote: 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.]
[Sidenote: 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.