Fine Particles in Soils
Nature Bulletin No. 582 November 28, 1959
Forest Preserve District of Cook County
Daniel Ryan, President
Roberts Mann, Conservation Editor
David H. Thompson, Senior Naturalist
FINE PARTICLES IN SOILS
If a farmer, while plowing, is visited in the field by another farmer,
invariably the visitor will pick up a handful of turned over earth and
knead it with his fingers while they talk. The "feel" of it tells him a lot
about the texture and structure of that soil. He knows that both are
important factors in the growth of plants and determine the crops that
may be obtained from the land.
Soil is a combination of three different things About half of it is solid
matter; the other half consists of air and water The solid portion is
composed of organic and inorganic materials.
The organic matter consists of both living and dead plants and animals,
and their products. Plant roots, worms, insects, rodents, fungi, and
bacteria comprise most of the living things. The remains of plants and
animals, together with the products of their decay, make up the dead
material. When this has become more or less decomposed it is called
humus. It helps bind soil particles together and serves as food for plants
but especially as food for the bacteria and other organisms which are
essential to a healthy soil in good physical condition. Peat-like organic
matter, which has not yet decayed much, helps to keep soil loose,
porous, and able to contain needful amounts of air and water.
The inorganic or mineral matter was derived from the decomposition of
rocks by the chemical and mechanical processes of weathering, or from
rocks ground by glaciers into gravel, sand, and that powder-like
substance called clay. The mineral particles in soils are classified,
according to size, into three principal groups: sand, silt, and clay. The
particle sizes in each group vary between certain limits which have been
arbitrarily established: diameters of from 2 millimeters to 0.05 mm for
sand, 0.05 to 0.002 mm for silt, and less than 0.002 mm for clay. Sand
grains are plainly visible and feel gritty. Silt particles are barely visible
to the naked eye; it has the appearance and feel of flour. Clay particles
are too fine to be distinguished by eye and a large proportion of them
cannot be seen under an ordinary microscope. It is the clay content that
makes a soil sticky when wet.
There are three general classes of soil: sands, loams, and clays. Coarse
and fine sands comprise more than 70 percent of a sandy soil. From 25
to 35 percent of a clay soil is clay. Loams range in clay content from 10
to 25 percent, less than 50 percent sand, and from 25 to 50 percent silt.
That, regardless of their organic matter, makes loam soils most
desirable because they have the good features of both sands and clays,
plus the silt. As a result they are friable -- cohesive but easily crumbled
and pulverized -- the ideal structure and texture.
That introduces a beautiful, a most descriptive word that you should
know: tilth. It is derived from "till", an old Anglo-Saxon verb meaning
"to plow and cultivate the soil". When a plowed field is in good tilth the
soil is crumbly, mealy, porous, and ready to grow a crop. Loams are
ideal for good tilth.
Clay is both the angel and the villain in soils. Because its particles are
so fine and have such tremendous capacity for holding infinitesimal
films of water, it can cause trouble. If a clayey soil is plowed or
cultivated when too wet, if livestock trample it in early spring, the
microscopic particles are puddled into a solid mass. On the other hand,
the clay particles in soil are the pantry in which plant foods are kept,
and the storage place where chemicals are held, to be gradually released
to nourish plants. Paraphrasing I Corinthians 13-13:
And now abideth sand, silt, and clay, but the greatest of these is clay.
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Update: June 2012