FIG. 4. From rock into soil. The lower stratum of the soil has a composition approaching that of crumbled and decayed rock. Lower down the rock characteristics appear more plainly, and still farther down occurs the solid rock itself.

SOIL OF MANY KINDS

9

into a firm, stony sphere, and made more complicated because of the eternal forces which wrought such tremendous changes since the earth began-which forces brought into existence continents and oceans, lakes and rivers, mountains, hills and valleys, deserts and prairies.

Weathering Continues Indefinitely.-Weathering does not cease when it has reduced rocks to soil, but continues to act indefinitely upon the soil itself. If it were not so the best prairie soils would be incapable of supporting a single blade of grass. In this sense crops, vegetation and life are possible upon this earth largely because of decay.

Soil from Rock May be Observed. Soil wherever found is underlaid by some kind of rock-it may be the rock from which the soil was formed or some foreign rock. Where soil is found underlaid by its parent rock there is no definite dividing line between the solid rock and the soil above it-the one grades into the other. The lower stratum of the soil has a composition approaching that of crumbled and decayed rock. Lower down the rock characteristics appear more plainly, and still farther down occurs the solid rock itself.

Soils are Carried Away after They Form.-All the soil we see was not formed from the rock that may be found under it. This is due to the fact that many soils are carried away after they are formed and deposited on other rocks. Water, ice and wind are and have always been the main soil transporting agents.

Soil of Many Kinds.-Because of the source of soil building materials, the nature of soil formation, the forces to which the earth has been and is being subjected, and because of the transporting and mixing action of water, wind and ice, all soil can not be the same, but must necessarily vary in composition, both physically and chemically. These variations give rise to many kinds and types of soil, and necessitate convenient classification. A knowledge of these soil variations, classes and types becomes of primary importance.

Illustration Material for Lessons. Students should bring to class: 1. Specimens of rocks undergoing weathering.

2. Samples showing gradations of rock into soil.

Field Studies. 1. Observe if possible the weathering of native rock into soil, and note the action of the various weathering agents. 2. Observe the transportation and movement of soil. 3. Observe various kinds of soil-note location, etc.

QUESTIONS

1. What is soil and when was it formed?

2. Name two classes of materials composing soil.

3. Distinguish between them.

4. What is meant by rock weathering?

5. Name some of the forces which break up and cause rocks to decay. 6. In what ways do plants and animals aid in the weathering of rocks? 7. Is soil formation a simple process? Explain.

8. When does weathering cease?

9. What is the importance of "soil weathering"?

10. Is it possible to observe the stages in the changing of rock into soil?

11. What are the facts to be observed?

12. In every case is soil formed from the rock found underlying it? Explain. 13. Why should there be many different kinds and types of soil?

14. What kinds have you seen?

15. For an outline summary of this chapter see table of contents.

CHAPTER II

SOIL COMPOSITION, CLASSES AND TYPES

Common Meanings of the Term "Soil."-In the previous chapter the term "soil" included the total residue resulting from rock weathering-being hundreds of feet deep in some places. In common usage the term soil has restricted meanings; viz., it may mean that portion of the ground which is tilled, or that portion which is black or dark in color.

Subsoil Defined. That portion of the ground below the tilled or dark colored portion is called "subsoil." (Prefix "sub" means "under.") By some it is considered as that portion below 62% inches deep and 20 inches deep.

Soil to Mean Tilled Portion.-Soil as it is used in the following chapters is taken to mean that portion of the ground which is tilled. This is the more common meaning since it is that portion which is most important in supplying the needs of crops, and which is most. affected by farming operations.

Subsoil May Differ Widely from Soil.-With this definition of soil in mind, it is easy to see that subsoil may have extreme variations-it may have the same color and composition as the soil or it may show very little or no similarity either in color or composition.

THE COMPOSITION OF SOIL

The physical or mechanical make-up of dry soil may be briefly expressed in outline form as follows:

Soil components

1. Mineral particles of various sizes (derived ((a) Sand from rocks)

2. Organic matter-mostly plant remains.

(b) Silt
(c) Clay

Sand particles are the coarser and heavier soil grains which do not cohere when wet (Fig. 5). Between the fingers they feel rough and gritty. They are the first to settle out of running water carrying sediment-material washed from the upland.

Clay particles are the finest of individual soil grains. When moistened they become sticky. They settle out of quiet water very slowly the finest clay particles are so small that they are known to remain in suspension for months, or to be scarcely discernible under a powerful microscope.

Silt particles are soil grains medium in size between sand and clay. When moistened, silt has a velvety "feel," but not sticky like clay. Sand, silt and clay as used here refer simply to size of individual soil grains and nothing more.

Organic matter in most soils occurs largely as a well-decomposed residue, black or dark in color and coating the soil grains. Frequently plant fiber of recent growth can be distinguished.

Humus may be defined as a black, waxy, complex substance

Coarse sand

Medium Sand

Fine sand
Silt

Fine silt

Coarse clay схеру Very fine clay

FIG. 5.-Diagram illustrating the sizes of soil grains. The coarsest particles grade

into the finest.

of coating the soil grains, and which is derived from partial decay organic matter. All humus is organic matter, but all organic matter is not humus.

The term humus is sometimes used as a general term meaning organic matter. In these chapters we shall use the term "organic matter" since that is more readily understood.

Soil is a Complex Medium.-What we call soil is something more than a mere mixture of sand, silt, clay and organic matter. It is a composite, the framework of which is mineral matter. Aside from organic matter it contains:

(a) Water (moisture) which in reality is a dilute solution containing weak acids and small amounts of any soluble substance found in the soil.

(b) Soil organisms-bacteria, fungi and worms. These have