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In the beginning, soil is primarily rock. This together with animal and vegetable decay over a long stretch or period of time crumbles or breaks up these rock masses into soil by heat, water action, and friction. Friction means the rubbing and grinding of rock mass against rock mass. Think of the huge rocks, in a perfect chaos, each bumping, scraping and settling against one another. What would be the result? Well, I am sure you could work that out. Here's what happened: bits of rock were worn off, a great deal of heat was produced, pieces of rock were pressed together to form new rock masses, some portions dissolved in water. You can almost feel the stress and strain of it all.
Next, there were great changes in temperature. First everything was heated to a high temperature and then gradually cooled down. Just think of all the cracking, crumbling, upheavals, caused by such changes in temperatures! You have seen some of the effects of winter — sudden freezes and thaws. But these little examples of water pipes bursting and jugs cracking are as nothing compared to what happens on the surface of the earth. The water and the gases in the atmosphere accelerates this crumbling.
This friction from rubbing, that is mechanical, so it is easy to understand how sand was formed. This is one of the great divisions of soil — sandy soil. The beach is a great mass of pure sand. If soil is just broken rock fragments then indeed it would be very poor and unproductive. But as the early forms of animal and vegetable life die, decay and became a part of this rock mass, a better soil results. So the soils we speak of as sandy soils is mixed with other matter, sometimes clay, sometimes vegetable matter or humus, and often animal waste.
Clay brings us right to another class of soils — clayey soils. When certain portions of rock masses dissolves as water trickled over them and together with heat clayey soils are formed. This dissolution took place largely because there is in the air a certain gas called carbon dioxide that dissolves in water, from rain, to form carbonic acid. This acid attacks and changes certain substances in rocks. Sometimes you see great rocks with portions sticking up looking as if they had been eaten away. Carbonic acid did this. It changed this eaten part into something else which we call clay. This change is not mechanical but chemical. This is the difference between these two kinds of change — in the first case, sand, a mechanical change, you end up with just what you started with, save that the size of the mass is smaller. You started with a big rock, and ended with little particles of sand. But you had the same kind of rock after. Mechanical action might be illustrated with a piece of lump sugar. Let the sugar represent a big mass of rock. Break up the sugar, and even the smallest bit is sugar. It is just so with the rock mass; but in chemical change you start with one thing and end with another. You started with a big mass of rock which had a portion changed by the acid acting on it. It ended up being an entirely different thing that we call clay. So in the case of chemical change the original substance is transformed into an entirely different thing. The clay soils are often called mud soils because of the amount of water used in their formation.
The third type of soil that farmers deal with is lime soil. Remember we are thinking of soils from the farmers point of view. This soil of course ordinarily was formed from limestone. Just as soon as one thing is mentioned about which we know nothing, another comes up of which we are just as ignorant. And so a whole chain of questions follows. Now you are probably saying within yourselves, how was limestone first formed?
At one time ages ago the lower animal and plant forms picked from the water particles of lime, a compound of calcium. With the lime they formed skeletons or shells to protect from larger animals. Coral is an example of this skeleton-forming animal.
When the animal dies the skeleton remained. Over a very long period of time, great masses of these skeleton are pressed together into rock and formed limestone. In some limestone formation, the shelly silhouette is still visible. Marble, another limestone, is somewhat crystalline in character. Another well-known limestone is chalk. Perhaps you'd like to know a way of always being able to tell limestone. Drop a little acid on some lime. See how it bubbles and fizzles. Then drop some on this chalk and on the marble, too. The same bubbling takes place. So lime must be in these three structures. One does not have to buy a special acid for this work, for even the household acids like vinegar will cause the same result.
These are the three types of soil that every farmer has to deal with. As for us, we will be a better gardener when we understand our soil better.