' THE SOIL 45 



>e planted; and the object of surface cultivation, aside 

 Tom controlling weeds, is to provide a satisfactory seed 

 )ed containing the right amounts of moisture, heat and 

 lir for germination and growth. Surface cultivation 

 ilso provides a mulch which lessens to some extent the 

 Tacking of the soil and thereby checks evaporation. 



- Soil Fertility or Chemistry. 



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

 ideration to plants as it is to animals. Animals require 

 'or food vegetable matter, such as starch, sugar, fat, 

 )rotein, etc., the highly organized, complex materials 

 luilt up by plant life. While animals frequently eat 

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

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

 [uire as food the simple, inorganic materials derived 

 rom the solution of rock materials or from the decay 

 •f plant or animal remains. This food is taken up in 

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

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

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

 umption, 



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

 )osed of about 80 elements in varying amounts and 

 ombinations or compounds. Of the 80 elements, 40 are 

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

 •ccurrence in plants, soils, water and in the atmosphere, 

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

 re essential for the growth of plants; deprived of any 

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

 mable to reproduce its kind. These elements which are 



