GENESIS OF THE SOIL AND ITS POSSIBILITIES 187 



iron, converting them into peroxids, and thus tends to 

 produce decay. 



" There are other constituents of rocks which oxygen 

 attacks and thus helps to their final "breaking up. It is 

 true that, as a rule, the constituents of rocks are already 

 oxidized to nearly as high a degree as possible, and on 

 these constituents of course the air would have no ef- 

 fect. But on others, especially when helped by water 

 with the other substances it carries in solution, the air 

 may greatly aid in the work of destruction. 



" In a general view, the action of the air in soil forma- 

 tion may be regarded as of secondary importance, and 

 to depend chiefly on the oxidation of the lower to the 

 higher basic forms. These processes, while they seem 

 of little value, have, nevertheless, been of considerable 

 importance in the production of that residue of rock 

 disintegration which constitutes the soil. 



THE SOIL MORE THAN" MINERAL. 



" But for agricultural purposes the soil consists of 

 more than decayed mineral matter. By the decay of 

 organic matter there is introduced into the soil the ele- 

 ment, humus, which is one of its principal character- 

 istics from an agricultural point of view. The soil is 

 filled with millions of organisms of a lower form, with- 

 out whose activity the growing of crops would be im- 

 possible. The soil, therefore, not only contains the 

 mineral matters which are necessary to sustain the life 

 of plants, but also those organic elements without which 

 these mineral matters would not be available for plant 

 growth. The three principal mineral foods of plants 

 are potash, phosphoric acid, and nitrogen. Lime, mag- 

 nesia, iron, and many other mineral substances are also 

 found in plants, but these are not absolutely essential 



