MICROORGANISMS IN THE SOIL. 39 



Lime and magnesia should also be in the soil for reasons that 

 will be given later, but they are only slightly used by most plants. 



There are still other materials used by plants in very minute 

 quantities, but they hardly fall in the scope of our study. All 

 the foods above mentioned are commonly called inorganic foods, 

 since they come chiefly from the soil and the air. Organic foods 

 on the other hand refer to the more highly organized products, which 

 are the immediate remains of living things, like roots, starches, 

 fats, wood, cellulose and other similar bodies. .Our problem, then, 

 is to explain nature's methods of keeping the soil supplies of these 

 various inorganic ingredients from diminishing. 



MICROORGANISMS IN THE SOIL. 



The upper layers of the soil are exceedingly rich in bacteria, 

 the number varying according to conditions, from a few thousands 

 to many millions per gram. In sandy soil there may be very few, 

 while in soil polluted with organic matter, as in the vicinity of 

 manure heaps, there may be as many as 100,000,000 per gram or 

 even more, 1,600,000,000 per gram having been found in some soils. 

 They rapidly diminish in numbers, as we pass to the lower layers, 

 and at a depth of from four to six feet, they have almost disappeared. 

 Below this they are rarely found, except in places where drainage 

 currents carry them downward. The microorganisms thus found 

 in the soil include bacteria in the greatest abundance, and also 

 quantities of the higher fungi and yeasts. Each of these classes is 

 represented by many varieties, and each has an important share in 

 the complex activities going on in the soil. These functions and 

 the relation of the soil microorganisms to them may best be under- 

 stood by noticing in succession their relation to the various soil 

 ingredients that constitute plant foods. 



ORIGIN OF SOIL. 



The ingredients in the soil may be divided into two classes: i. 

 The purely mineral matters. 2. The organic ingredients consti- 

 tuting the humus. 



