GRAIN FARMING 19 



Temperature in Soil Formation.— Freezing and thawing 

 produce slighit motion among different particles of roclc and 

 soil. This results in friction similar to that produced by 

 water and wind. But water in freezing expands. Therefore, 

 when water settles in a crevice and freezes, it splits the pieces 

 •apart. When a concrete sidewalk, in a climate where the tem- 

 perature goes below freezing in winter, becomes cracked, the 

 crack constantly becomes wider. Thus, when freezing and 

 thawing occur at short intervals, they are forces of consider- 

 able importance in the making of soil. If a hollow iron ball 

 be filled with water and allowed to freeze it will break. This 

 will occur although the ball is of such strength that it is prac- 

 tically impossible to break it with a sledge hammer. 



A sandy soil is composed of comparatively large particles, 

 coming from rock of one kind; while clay is made up of very 

 fine particles coming from another kind of rock. 



Humus in Soil. — Humus mixed with the soil contributes to 

 fertility. Humus is decayed and decaying vegetable matter. 

 In nature considerable humus lies on the surface of the soil in 

 the form of grass, leaves, and weeds. Where there is vege- 

 tation the roots of plants die and decay and add to the supply 

 of humus. 



Certain plants grow on soils which contain no humus. Such 

 plants supply humus in the earlier processes of soil formation. 

 Their roots, and acid and alkali by-products which they give 

 off, help, also, to reduce the rock and soil particles to fine 

 grains. Moreover, the parts of soil that aie put into solution 

 act directly as plant food. As the soil becomes adapted to 

 vegetation of a different kind, nature seems to provide the 

 seed for such growth. 



Humus also loosens the soil, or holds the particles apart, 

 so that the air can get in. Air is needed to furnish oxygen, 

 nitrogen, carbon dioxide, etc. These elements are usually 

 held in solution by the water. This is taken up by the roots 

 and forced into the plant, taking along the food materials. 

 A heavy clay soil is often materially benefited by plowing 

 under coarse rye straw. This puts a lot of tubes into the soil 

 making passages for the air to circulate. When the roots of 

 deep-rooting plants decay they leave openings or pores which 



