46 NATURE’S FOOD SUPPLY. THE CARBON CYCLE 
rocks that constitute the earth’s surface, soil being sometimes de- 
scribed as ground-up rock. The agents that cause the grinding of 
the rocks are physical, chemical, and biological. The physical 
agencies are chiefly those of freezing and thawing, together with 
the solvent action of waters. The chief chemical agent is direct 
oxidation by the oxygen of the atmosphere. The physical and 
chemical agents together produce what has been called the ‘“‘ weath- 
ering” of rocks, resulting in their crumbling into fine fragments. 
With these we are not particularly concerned. The dzological 
agencies are those of the soil microdrganisms. We do not yet 
know very definitely how great a part they play in this process, 
but that it is an important part is surely proved. One of the 
results of their growth is the liberation of carbonic dioxid from 
decomposing masses. This gas is readily dissolved in the soil 
water, and water containing carbonic dioxid in solution is able to 
dissolve a considerable quantity of carbonate of lime. These car- 
bonated waters, therefore, play a great part in the disintegration 
of limestone, which is one of the prominent factors concerned in 
the formation of soils. Again, the microdrganisms which decom- 
pose organic matters in the soil produce a variety of organic acids. 
Among these are the lactic, butyric, and acetic acids, as well as many 
others. These acids have a solvent action upon various rock 
formations, and, by dissolving out certain parts of the rocks, they 
slowly but surely cause them to crumble. Some of these matters 
will be considered on later pages in other connections, but we are 
interested in them here as showing that bacteria are prominently 
concerned in the disintegrations of rocks which result in the 
formation of soil. 
The Humus.—There is a vast difference in the fertility of a 
sand and a garden soil. Sandy soil may contain all the necessary 
mineral matters, but it lacks the something needed for plant 
growth which the garden soil contains. This something is called 
humus, an element rather difficult to define and still more difficult 
to describe in chemical terms. It is abundant in fertile soil, but 
