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. 
