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.