76 BACTERIA AND SOIL FERTILITY 



blocks of hardened lava and small fragments. The ash con- 

 sists of volcanic sand, cinders, and still finer volcanic dust. Vol- 

 canic dust may be carried great distances before being deposited. 

 Widespread formations in Nebraska and adjacent states contain 

 beds of volcanic dust, the origin of which is unknown for there 

 are no probable sources within hundreds of miles. It is stated 

 that millions of acres have an appreciable quantity of volcanic 

 dust in their soils. It is probable that nearly every square mile 

 of the earth's surface contains some volcanic dust. 



CHEMICAL AGENCIES 



We have seen that in the process of soil formation the rocks 

 are pulverized, but the process of making soils consists in some- 

 thing more than the grinding of rock. At the same time that the 

 particles are becoming smaller certain other transformations are 

 going on which change the chemical nature of the soil. The 

 agents at work we can conveniently consider under the heading 

 of atmosphere and water. 



Atmosphere. — The atmosphere consists of a mechanical mix- 

 ture of about four volumes of nitrogen and one volume of oxy- 

 gen. In addition to these it contains very small but important 

 quantities of carbon dioxid, nitric, sulfuric, and other acids. 

 It also contains small but appreciable quantities of ammonia. 



Iron exposed to the air rusts. In so doing it changes in color, 

 volume, and solubility. Many rocks contain appreciable quan- 

 tities of iron, and on exposure to the atmosphere oxidation takes 

 place. The resulting mineral being more bulky crowds the 

 grains of rock apart. Water slowly dissolves out the new com- 

 pound, and the rock soon falls prey to other agents. 



Many rocks contain sulfides. These are oxidized with the 

 formation of sulfates which are soluble and hence are readily 

 washed from the parent rock. One may at times see clear-cut 

 crevices which have been thus formed in rocks. 



Complex silicates, such as feldspar, mica, and hornblend, are 

 often attacked by oxygen and carbon dioxid and reduced to 

 simpler and more stable compounds. 



