70 DRY-FARMING 
ditions are favorable, it encourages bacterial life, 
which, as is now a well-known fact, is an important 
factor in developing and maintaining soil fertility. 
(3) By somewhat subtle chemical changes it makes 
the relatively small percentages of other plant-foods, 
notably phosphoric acid and potash, more available 
for plant growth. (4) It aids to convert rapidly 
organic matter into humus which represents the main 
portion of the nitrogen content of the soil. 
Of course, an excess of lime in the soil may be 
hurtful, though less so in arid than in humid re- 
gions. Some authors state that from 8 to 20 per 
cent of calcium carbonate makes a soil unfitted for 
plant growth. There are, however, a great many 
agricultural soils covering large areas and yielding 
very abundant crops which contain very much larger 
quantities of calcium carbonate. For instance, in 
the Sanpete Valley of Utah, one of the most fertile 
sections of the Great Basin, agricultural soils often 
contain as high as 40 per cent of calcium carbonate, 
without injury to their crop-producing power. 
In the table are two columns headed ‘‘Soluble 
Silica” and “Alumina,” in both of which it is evident 
that a very much larger per cent is found in the arid 
than in the humid soils. These soil constituents 
indicate the condition of the soil with reference to 
the availability of its fertility for plant use. The 
higher the percentage of soluble silica and alumina, 
the more thoroughly decomposed, in all probability, 
