THE SOIL STRUCTURE 101 
it may be thousands, of the smaller silt grains; or 
a regiment of smaller soil grains may themselves 
be clustered into one large grain by cementing 
power of the clay. 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. Any- 
thing 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 physi- 
cal 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 some- 
where in the neighborhood of 50 per cent. There 
