The soil, as the farmer understands it, is the soft tillable covering or epidermis of the earth. It is derived primarily from disintegrated rock, but all productive soils contain organic remains, or materials derived directly from these remains. Some soils, as those in swamps, are very largely organic.

Classification of soils in respect to origin (Merrill)

Classification of soil constituents (TJ. S. Dept. Agric.)

Weight of soils.

Soils vary widely in weight according to their composition and the size of the particles. Humus soils are the lightest, and sandy soils are the heaviest. Clay soils weigh less per cubic foot than arable soils or sandy soils. The larger the amount of organic matter in a cubic foot of soil, the less it weighs. For this reason, surface soils are lighter, as a rule, than subsoils (Stevenson).

The weight of a cubic foot of dry soil is given by Shubler as follows: —

Warington gives the following data regarding the weight of soil per acre: —

1. Old pasture, Rothamsted, loam with clay subsoil

2. Arable land, Rothamsted, loam with clay subsoil

3. Arable land, Woburn, sandy soil

These tables show: (1) That each of these classes of soil is lighter at the surface; (2) that in each case the weight increases with an increase in depth. This increase in weight of the lower zones is due:

(1)  to the increase of pressure to which the lower zones are subjected;

(2)  to the fact that the surface soil is more loose and porous; (3) to coarser texture of subsoil. This condition is brought about by the removal of the finest soil particles from the surface into the sub-soil by the action of rain; by the accumulation of organic matter in the surface soil; and, in the case of arable soils, by tillage.

The specific gravity of a soil indicates its weight as compared with the weight of an equal volume of water. An English authority has published the following table, which gives the specific gravity of the more common soil constituents: —

Scheme gives the following for the specific gravity of soils: —

Clay soil......................    2.65

Sandy soil......................    2.67

Fine soil......................    2.71

Humus soil.....................    2.53

The true specific gravity of an arable soil varies from about 2.5 to 2.7.

Texture of the soil.

The size and shape of the particles of which the soil is composed determine its texture. The arrangement of the particles determines its structure, as " loose," " open," " mealy," " friable," " cloddy," " porous," " hard," " compact," " retentive," " leachy."

The texture determines the amount of soil-surface exposed to roots, and to a great extent the quantity of moisture that the soil may hold.

The size and form of the particles determine the number in a given volume of soil. It has been estimated by Whitney that a gram of soil contains 2,000,000,000 to 20,000,000,000 soil particles. The number of particles per gram of different soil types is approximately as follows: —

Early truck..................      1,955,000,000

Truck and small fruit..............      3,955,000,000

Tobacco...................      6,786,000,000

Wheat...................    10,228,000,000

Grass and wheat................    14,735,000,000

Limestone..................    19,638,000,000

Owing to the fact that a soil is made up of particles, there is between them a certain amount of space that is occupied by air or water; this is known as the " pore space." In ordinary soils the pore space varies from a little over 50 per cent in the finest clay soils to about 25 or 30 per cent in coarse sands of uniform texture.