SOIL CLASSIFICATION 37 



gravity and capillarity. The water from rain passes downward by gravity. 

 The rate of downward movement depends on the size of the little passage- 

 ways through the soil. In fine-textured, compact soils it is often very 

 slow. The depth to which it penetrates depends upon the character of 

 the subsoil or underlying strata. It is frequently intercepted by impervi- 

 ous layers, and consequently in times of excessive rainfall the soil becomes 

 saturated and water accumulates on the surface. It then seeks an escape 

 by passage over the surface and often carries with it portions of the soil, 

 thus becoming a destructive agent in soil formation. In dry periods the 

 surface of the soil loses its water through direct evaporation and through 

 the consumption of water by the plants growing in the soil. This should 

 be replaced by the water in the subsoil which returns to the surface by 

 capillarity. The distance through which capillary water will rise is 

 measured by a few feet. The height of rise is greatest in case of fine- 

 textured soils, but in this type of soil the rate of movement is slowest. 

 The rate of movement in sandy soils is much more rapid, but the height 

 of rise is much less. 



Gases of the Soil. — The soil atmosphere consists of air and the gases 

 resulting from decomposition of the organic solids in the soil. The domi- 

 nant gas is carbon dioxide, which, dissolved in water, increases the solvent 

 action of the water and helps to increase the available plant food. The 

 movement of the gases in the soil is affected by changes in temperature 

 which cause an expansion and contraction of their volume. It is also 

 affected by the movements of soil water. As the water table in the soil 

 is lowered air enters and fills up all spaces not occupied by water. The 

 movement is also facilitated by changes in barometric pressure and by 

 the movement of the air over the surface of the soil. Just as a strong wind 

 blowing over the top of a chimney causes a strong draft in the chimney, 

 so does such a wind cause a ventilation of the soil and increases the cir- 

 culation of the air within the soil. 



The roots of most economic plants require oxygen and this is secured 

 in properly drained and well aerated soils from the soil atmosphere. When 

 soils are filled with water the plant roots have difficulty in getting the 

 required supply of oxygen and the growth of the plant is retarded. A 

 proper aeration of the soil is necessary to the development of microscopic 

 organisms that live in great numbers in the soil and play an important 

 part in making available the mineral constituents necessary for the higher 

 forms of plants. It is essential that farmers understand the movement 

 of water and air in the soil in order that they may do their part in bringing 

 about that degree of movement that is essential to the highest productivity 

 of the soil. Drainage, cultivation and the judicious selection of the crops 

 grown are some of the means of influencing the movement of water and 

 air in the soil. 



Soil Classification. — Science is classified knowledge. In order that 

 there may be a science of the soil it becomes necessary to classify soils. 



