Soil formation is a slow process that requires 10,000 years or more to form one foot of depth. The host soils provide 10′ feet or more effective depth for the root zone.
Rocks are variable in composition but contain minerals such as feldspars, micas, apatite, hornblende, and others that are important in forming soils.
As the solid rock disintegrates, the minerals dissolve partially in the water. Taking the orthoclase feldspar mineral as an example, one of the results of the dissolving action of water is some soluble potassium.
Another result is the freeing of some silica which may become ordinary sand. Still another product of the reaction with water is the clay material of the soil, a new mineral derived from the feldspar-water reaction.
These reactions are typical of those that account for two important mechanical soil parts, sand and clay. The third mechanical constituent is the silt, composed of tiny pieces of minerals ranging in size from five-hundredths of a millimeter to five-thousandths of a millimeter in diameter.
Clay particles are smaller than this range, and sand particles are larger, up to one millimeter in diameter. The relative proportion of sand, silt, and clay determine soil texture.
Disintegration Of Rocks
The disintegration of rocks is partly physical and partly chemical. When a glacier grinds rocks to flour or when the alternate heating and cooling breaks big rocks into little ones, those are physical processes.
Hydration, solution, and oxidation are chemical processes.
After rocks are disintegrated, and the minerals undergo various changes by chemical reactions, the line materials are subject to some sorting and redistribution by wind and water.
Streams and lakes redistribute the sand, silt, and clay, sometimes leaving sand predominantly in one location and perhaps clay mostly in another. Or the wind picks up silt, carries it far from the source, and covers the earth, sometimes several feet deep, with fine material. This then becomes a fertile and highly productive loess soil.
Soil is not only a mass of minerals in various stages of weathering that gives sand, silt, and clay particles, but it contains 2% to 4% percent or more (by weight) of organic matter that is mostly humus, partially decomposed remains of plant roots, and various kinds of litter and organisms.
The soil’s humus, chemically, is largely lignin, hemicellulose, and protein materials. The lining, in part, at least in combination with the protein and the humus moss, is partly in combination with the clay.
This humus-clay combination takes on a sponge-like structure. It forms water-stable granules until, with an abundance of humus on the forest floor, the whole mass of soil becomes an accumulation of granules and crumbs, readily permeable to roots, water, and air — the ideal condition for supporting plant growth.
A System Of Organic Matter And Organisms
Soil is never an inert mixture of minerals and organic matter. It is a living system full of bacteria, fungi, and animal organisms such as earthworms, ants, and arthropods, all of which contribute something to the quality of the soil.
These organisms decompose the organic matter and help mix the minerals and humus of the soil to form the granules.
The larger organisms, such as ants and earthworms, make channels and burrows through the soil, effectively admitting air and water and providing openings for roots to penetrate.
The activity of soil organisms is responsible for much of the liberation of nutrients to plants. A certain kind of bacteria can produce nitrates from organic matter, supplying the plant with the growth-element nitrogen.
At the same time, many organisms make other nutrients ready for the plant, either as the direct result of decomposition or because solvents are produced to attack and free the nutrients from the more inert minerals.
A small amount of the nutrient will go directly into the water solution without soil organisms’ intervention.
Clay And Humus
The most active portion of the soil is the clay and humus, both capable of absorbing and holding soluble nutrients carrying such elements as potassium, magnesium, phosphorus, and others supplied by fertilizers.
These nutrients held by the clay and humus are prevented from loss by leaching and are gradually given up to the absorbing roots of a growing crop.
The plant roots contact the moist colloidal clay and humus, and through these surfaces, contacts absorb nutrients and moisture.
The spongy crumb structure of soils, rich in humus with only a moderate amount of clay (a loam, sandy loam, or silt loam texture), is most favorable to profuse root development and efficient functioning of the roots.
The undisturbed soil before a man arrives upon the scene to start cultivation is, therefore, a natural body formed by natural processes operating on natural materials over a long period to develop physical, chemical, and biological properties that enable the enriched soil to support plants.
The arrival of a man with tillage tools is not always good, so far as soil properties are concerned, as evidenced by much one-good soil now in varying stages of deterioration.
Soils must have good depth (10′ or more feet) without obstructions in the profile to allow crop roots not only to penetrate deeply but so that the roots can obtain air (oxygen), water and nutrients simultaneously from soil contact.
By volume, the ideal soil is about 40% percent rock (mineral) particles, 10% percent organic matter, and 50% percent open space.
The 50% percent open space should be approximately one-half occupied by water and one-half by air when the soil has all the crop water (capillary water) it can hold. Good soil will hold about 2″ inches of crop water for each foot of depth.
The soil is a storehouse of nutrients, nitrogen, phosphorus, potassium, sulfur, calcium, magnesium, iron, copper, zinc, manganese, and boron that plants require for normal nutrition.
A large portion of these nutrient elements in the topsoil is in organic combination (with the humus) and are liberated by the organic matter rots.
The chemical properties of the soil are most favorable to the plant when the reaction is about neutral due to the combination of clay and humus with lime and when the humus is regularly and liberally renewed.
Soil organisms (bacteria, fungi, and many animal organisms) are active in breaking down organic matter and attacking inert minerals, causing solutions and oxidations that prepare the nutrients for absorption by plant roots.
Some organisms can take nitrogen from the air where it is abundant and leave it in the soil in an organic form where gradual liberation feeds the plant. This was the source of all the nitrogen that the soil contained.
To the extent that man cooperates with nature to preserve the favorable properties of the soil developed through ages of activity, crop yields will be maintained at a satisfactory level.
Legumes will probably be needed to renew the supply of nitrogen and humus, lime will be needed to correct the soil’s acidity (where rainfall is greater than 10″ to 40″ inches annually), and fertilizers are necessary to supplement the supply of available essential elements in the soil.
Crops cannot be continually removed with anything put back, nor can the soil be left bare and subject to leaching and erosion that has destroyed the productivity of so much good land.
Nature covers her fields with grass or timber, and man must recognize that a plant cover is essential to preserving productive soil. I
f he observes these fundamentals, he should be able to produce profitable harvests over the years to come and maintain the soil in a high state of productivity for the generations to follow.
44659 by R. E. Stephenson