THE IRRIGATION AGE. 



49 



THE FORMATION OF THE SOIL* 



By Alfred Vivian 



One could scarcely imagine any subject for discussion 

 more commonplace than that of the soil. Nor could one 

 think of anything which would be less likely to prove 

 interesting to the careless observer. We are accustomed 

 to think of the soil as "dirt," a thing to be shunned as 

 far as possible, and kept hidden from sight. Perhaps you 

 will not think the soil worthy of interest and study, but 

 did you ever stop to think that without the soil we could 



Underneath all soils are found solid rocks, 



not be living in this world today? The food which you 

 eat could not be produced if there was no soil, for the 

 plants which make the food for animals, in their turn, 

 derive all their nourishment from the soil. So, you see, 

 the soil is after all very important to mankind. 



We are so familiar with the soil as it now exists that 

 most of us do not stop to consider that it was ever any- 

 thing different, but it has really taken a long 

 time for Nature to form what we call the soil, 

 and in doing so she has employed the wonder- 

 ful agencies about which something will be 

 said in this article. Some one has denned the 

 soil as "that portion of the earth at or near 

 the surface, which consists largely of fine parti- 

 cles." And again it has been described as that 

 part of the earth into which the plants send 

 their roots and from which they take much of 

 their food. Well, if the soil is the portion of 

 the earth at the surface, what is below the 

 soil? Most of you know that if you dig down 

 deep into the soil you will come to solid rock. 

 Sometimes rock is reached a few inches below 

 the surface, and again you must dig many feet 

 before you come to it, but sooner or later 

 you are sure to find a bed of stone. We learn 

 therefrom this first interesting fact that under- 

 neath all soils are found solid rocks. 



Now, if you were to examine a sample of soil 

 with a strong magnifying glass or a micro- 

 scope, you would find that it is largely made 

 up of very fine particles of rock. Mixed with 

 these particles is a much smaller quantity of 

 black material which is called organic matter. 



or sometimes humus. A little closer examination will 

 show you that the organic matter is simply the remains of 

 plants which have formerly grown upon the land, and 

 which have partially decayed or rotted in the soil. Take 

 a small quantity of black soil, heat it in the lid of a baking 

 powder can, and see if the odor that comes off is not 

 very much like that you notice on heating bits of leaves 

 in the same way. 



We find then that the soil is composed of small par- 

 ticles of rock mixed with the remains of former plants, 

 and that by far the larger part consists of these rock 

 particles. This suggests the thought that the soil has 

 been formed from the solid rocks such as are found be- 

 neath it, and this, indeed, is what the men who have 

 studied the subject have found to be true. 



Geology teaches us that at one time all the sur- 

 face of the earth was solid rock. At tht time there 

 was nothing like what we now know as the soil. 

 These rocks contained all the constituents neces- 

 sary to mage a soil, all the substnces which the 

 plants use as food with the exception of nitrogen. 

 This plant food, however, was not in a form 

 in which the plants could use it. Suppose you 

 had a sack of wheat. You know there is plenty 

 of food there to nourish you for some time, 

 but it is not in a very good form to eat so long 

 as it is in the whole wheat kernel. One of the 

 first things you would do would be to grind it 

 to a flour. And that is one of the first things 

 that Nature does in preparing the food for 

 plants; she grinds the rocks to flour. In other 

 words, the first process in the formation of a 

 soil is the pulverization of the rocks. 



.Nature uses several methods to bring about 

 the grinding or pulverization of the rocks. 

 The first of these is change of temperature, or 

 heat and cold. If you examine a piece of gran- 

 ite, you will find that instead of being a simple 

 rock, it is composed of different minerals ce- 

 mented together. Now, these minerals are 

 differently affected by heat and cold. You 

 know that most substances expand when 

 heated. The amount of expansion varies for 

 the different minerals in the granite and as a 

 result the effect of change in temperature is 

 to separate the minerals, thus breaking the rock into 

 smaller pieces. 



If you look carefully at any piece of stone you may 

 pick up, you will find numerous cracks and openings in 

 it. These cracks become filled with water and in the cold 

 weather the water freezes. You know that when water 

 turns into ice it expands with great force, and conse- 





Abstract from the Agricultural College Extension 

 Bulletin, Columbus, Ohio, October, 1910. 



Bits of Stone Are Broken Off the Surface of the Large Rocks by Weathering 



