THE SOIL 45 



be planted; and the object of surface cultivation, aside 

 from controlling weeds, is to provide a satisfactory seed 

 bed containing the right amounts of moisture, heat and 

 air for germination and growth. Surface cultivation 

 also provides a mulch which lessens to some extent the 

 cracking of the soil and thereby checks evaporation. 



SOIL FERTILITY OR CHEMISTRY. 



33. The Food of Plants. Food is as important a con- 

 sideration to plants as it is to animals. Animals require 

 for food vegetable matter, such as starch, sugar, fat, 

 protein, etc., the highly organized, complex materials 

 built up by plant life. While animals frequently eat 

 animal flesh, plant life is the primary source of food. 

 The nutrition of plants, however, is different. They re- 

 quire as food the simple, inorganic materials derived 

 from the solution of rock materials or from the decay 

 of plant or animal remains. This food is taken up in 

 water solution, and by the plant is built up into tissue, 

 i.e. sugar, starch, cellulose, protein, the things for 

 which we prize plants as food for human or stock con- 

 sumption. 



34. The Essential Elements. The earth's crust is com- 

 posed of about 80 elements in varying amounts and 

 combinations or compounds. Of the 80 elements, 40 are 

 more or less common, and 15, because of their wide 

 occurrence in plants, soils, water and in the atmosphere, 

 are important in soil fertility. Of these 15 elements, 10 

 are essential for the growth of plants; deprived of any 

 of them the plant would succumb, or would, at least, be 

 unable to reproduce its kind. These elements which are 



