Composition Of Soils
With respect to composition, all soils are made of four ingredients — sand, silt, clay and
humus. No one of these ingredients alone makes a valuable soil, nor is it possible to find
any soil composed entirely of a single grade. The most valuable soils — the loams — are a
mixture of the four ingredients. The basis upon which the four ingredients of soils are
separated is the size of the grains, and here an arbitrary division is made. This is
called a "mechanical" analysis" of the soil as distinguished from a chemical analysis,
described in Chapter XI. The coarser materials are screened from the soil by passing it
through several sieves, with meshes of different sizes. Fine sand, silt, and clay are
separated by allowing them to settle in water, the fine sand settling first, then the
silt and finally the clay. The approximate size of the different ingredients is:
Coarse sand 1/25 to 1/50 of an inch in diameter
Medium sand 1/50 to 1/100 " "
Fine sand 1/100 to 1/250 " "
Very fine sand 1/250 to 1/500 " "
Silt 1/300 to
1/2000 " "
Fine silt 1/2000 to 1/5000 " "
Clay 1/5000 to 1/250 000 " "
Sand is made chiefly of particles of quartz, and all its grains are large enough to be
readily separated and distinguished without a microscope. The grains of sand are large
because quartz is very hard, almost as hard as diamond; hence the grains weather very
slowly. Sand contains very little plant food, since the spaces between the large grains
allow water to pass through very readily. The chief value of sand in a soil is in making
it mellow, porous and warm. Mix a handful of sand with a handful of stiff clay and note
that the latter is made much more workable, but less retentive of moisture. Clay is made
entirely of very fine particles, so small that a single grain cannot be seen without a
microscope. It would take 5,000 large grains of clay laid side by side to measure an
inch. Clay may be made from any kind of rock, as silica, limestone, mica and feldspar.
Clay is exactly opposite to sand in its physical properties.
Being very small, clay grains sink but slowly in water, so they are often carried long
distances by streams and lodge only when the current becomes sluggish. The sediment that
settles to the bottom of a glass of muddy water is mostly clay. Because it contains so
many very small spaces between the minute grains, clay absorbs water slowly, but holds it
tenaciously. Hence it is adhesive and unmanageable when wet. Pure clay is a powerful
cement. Clay in a soil gives it body and richness and increases its ability to hold
water, but if a soil has too much clay it is wet, cold and hard to handle. Silt is a name
given to the grains of a soil that are intermediate in size and in character between sand
and clay. It holds water well and is especially rich in plant food. For these reasons a
soil that contains a large proportion of silt is apt to be mellow and productive.
Most of the soils of the western prairies, and in fact a large part of the grain soils of
the United States, are composed mainly of silt. A high proportion of silt in a soil has
about the same effect upon it as a large amount of clay, making it tenacious of water and
of plant food. Many soils said to be clayey have more fine silt in them than clay. Humus
is mostly decayed vegetation. All the vegetable matter in a soil, however, is not humus;
the carpet of rotting leaves beneath a forest tree is not humus. Not until this is
entirely decayed and has become a loose, black mould, in which neither leaf nor stem may
be discerned, is it humus. There are all stages between this and the vegetation that is
just beginning to decay, and all have value. The value of humus in a soil for increasing
its capacity to hold water, for making it mellow, and for furnishing plant food has been
stated in preceding Chapters, and is considered yet more fully in Chapter XII.
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