SOIL 



5431 



SOIL 



water freezes the ice occupies more space than 

 the water ; we have seen pitchers broken by the 

 freezing of the water in them. When water 

 freezes in the crevice of a rock it works in the 

 same way and makes the crevice larger, so that 

 in the course of a number of years the piece 

 of rock is broken off. If we examine the base 

 of any rocky cliff we shall find an accumula- 

 tion of pieces of various sizes, ranging from 

 those weighing hundreds of pounds to tiny 

 particles of soil. This mass of loose rock at the 

 foot of a cliff is called talus. In addition to 

 this, repeated freezing and thawing of water 

 on the surface of rocks constantly wears them 

 away, forming soil. This process is known as 

 weathering. Water also acts chemically, since 

 the oxygen in it attacks substances that have 

 been dissolved from the rock. 



In past geological ages great glaciers covered 

 most of the northern part of North America, 

 and by their movement crushed and ground the 

 rocks into soil, which they deposited where 

 they melted. Much of the soil in Canada and 

 in the United States north of the Ohio River 

 and east of the Mississippi was formed by 

 glaciers (see GLACIER). 



Work of Plants. The roots of plants, the 

 leaves of trees and the stems of all plants that 

 are allowed to stand and die down in the fall, 

 decay and become a part of the soil. More- 

 over, the roots, by extracting certain plant food 

 from rocks, help to break them up. Sometimes 

 large rocks are split apart by the roots of trees. 

 Certain minute plants, such as molds, grow on 

 decaying plants and hasten the process of 

 changing these plants to soil. 



Work of Animals. Animal waste is con- 

 stantly added to the soil, and sooner or later 

 the animal dies and its body decays and be- 

 comes a part of the earth. Animals not only 

 add to the soil, but they are important agents 

 in mixing the soil. Gophers burrow in the al- 

 falfa field and damage the crop. Woodchucks 

 burrow in the clover field and make mounds 

 that are a hindrance to the farmer, and he at- 

 tempts to destroy these and all other burrow- 

 ing animals; yet in all ages these animals have 

 performed an important part in making soil. 

 But the small members of the animal kingdom, 

 such animals as the earthworm and the ant, 

 perform a work of far greater value. An au- 

 thority on agriculture says, "Each ant hill is a 

 real soil-mixing mill." 



Earthworms burrow in damp, rich soil and 

 derive their nourishment from the animal and 

 vegetable matter it may contain. In order to 



obtain their food these worms eat the earth 

 but reject the portion containing no nourish- 

 ment. In passing the soil through their diges- 

 tive system the worms pulverize it and cast it 

 forth in a highly fertile state. Darwin esti- 

 mated that in England these worms brought to 

 the surface more than ten tons of dry mold to 

 the acre. Air enters the soil through the holes 

 made by the worms and aids in the decomposi- 

 tion of substances beneath the surface. 



Transportation of Soil. We often find the 

 soil on one side of a hill or valley to be very 

 different from that on the other. This is be- 

 cause the soil in one place has been brought 

 there from some other locality, probably by 



COMPOSITION OF LOAM 

 In twenty grams (about seven ounces) of loam 

 the constituent parts are as follows : 



1. Organic matter 5. Fine sand 



humus 6. Coarse sand 



2. Very fine sand 7. Medium sand 



3. Clay 8. Gravel 



4. Silt 9. Fine silt 



water or ice, and that in the other place is still 

 lying where it was formed. We have already 

 seen that rivers carry soil to the lower part of 

 their course. Rain washes the soil down the 

 slopes of a hill and deposits it on the lowlands. 

 Here the soil is deeper on these lands than on 

 the hillsides. As water carries the soil it also 

 sifts it. In the upper course of the stream, 

 where the current is usually swift, all grades of 

 soil, including coarse gravel, are swept along, 

 but as we go downstream the soil becomes 

 finer and finer, until in the lower course only 

 the finest silt is carried. 



Kinds of Soil. Soil is classified according to 

 its composition and according to its structure. 

 Soil contains mineral (inorganic) and animal 

 and vegetable (organic) matter. The minerals 

 in soil are chiefly clay, sand and lime, the last 

 of which is in the form of pulverized limestone. 

 The organic matter, consisting of decaying ani- 

 mal and vegetable substances, such as leaves, 

 twigs and the dead bodies of animals, is called 

 humus. A clay soil is more than one-half clay; 

 a sandy soil is at least one-half sand. A loam 

 is a soil in which clay, sand and humus are 

 mixed in about equal proportions. Since hu- 

 mus is necessp-V to fertility, loams are the most 



