Further, in the presence of lime and
similar substances, these complex soil grains are grouped into yet
larger and more complex groups. The beneficial effect of lime is
usually due to this power of grouping untold numbers of soil
particles into larger groups. When by correct soil culture the
individual soil grains are thus grouped into large clusters, the
soil is said to be in good tilth. Anything that tends to destroy
these complex soil grains, as, for instance, plowing the soil when
it is too wet, weakens the crop-producing power of the soil. This
complexity of structure is one of the chief reasons for the
difficulty of understanding clearly the physical laws governing
soils.
Pore-space of soils
It follows from this description of soil structure that the soil
grains do not fill the whole of the soil space. The tendency is
rather to form clusters of soil grains which, though touching at
many points, leave comparatively large empty spaces. This pore space
in soils varies greatly, but with a maximum of about 55 per cent. In
soils formed under arid conditions the percentage of pore-space is
somewhere in the neighborhood of 50 per cent.
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