40 NATURE'S FOOD-SUPPLY. THE CARBON CYCLE. 



The Mineral Ingredients. These come primarily from the 

 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 oxida- 

 tion by the oxygen of the atmosphere. The physical and chemical 

 agents together produce what has been called the "weathering" 

 of rocks, resulting in their crumbling into fine fragments With these 

 we are not particularly concerned. The biological agencies are those 

 of the soil microorganisms. 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 carbonated 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 microorganisms 

 which decompose 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 scarce or 

 wanting in barren soil. Though its chemical value is too complex 

 to be stated or even known, its origin is easy to understand. 



