50 



THE IRRIGATION AGE. 



quently when the water in the cracks freezes it tends to 

 break the stones to pieces. If you have ever known 

 water to freeze in a bottle or jug, you know what force 

 it exerts and from this you can see how easy it would 

 be for the ice to break bits of stone off the surface of 

 larger rocks. 



More important than either of these factors, however, 

 is the action of running water. You would hardly think 

 that such a soft substance as water would do much grind- 

 ing, but water running over stone grinds off the surface 



make the plant food in the rocks available, but it has been 

 found that the mineral matter alone cannot support plant life. 

 A soil to be fertile must contain nitrogen as well. All 

 the nitrogen in the soil came originally from the atmos- 

 phere. The air is four-fifths nitrogen, but it is in such 

 a form that most plants cannot use it. Before it can 

 serve as a plant food it must be combined with oxygen 

 to make nitrate nitrogen. A little of this is formed in 

 the air, and is carried into the soil by rain water. This 

 amount, though very small, is probably sufficient to enable 



Poor Soil 



Rich SoO 



Stream' 



-Showing movement of soils from higher to lower levels. 



Running Water Grinds Off the Surface of the Stones Slowly but Su'rely. 



The Effect of Freezing on Rock. 



slowly but surely. If the stream is swift enough to carry 

 along particles of sand or stone, the grinding takes place 

 more rapidly. A rapid mountain stream, for instance, tum- 

 bles the boulders along, causing them to rub against each 

 other until they are ground to powder, and at the same 

 time the bed of the stream itself is worn away. In this 

 way deep valleys are sometimes worn into the surface 

 of the earth and the fine material is carried away to 

 form a soil at some other place. 



Another agency which helps to grind the rocks is 

 moving ice in the form of glaciers. At one time all of 

 the northern part of our country was covered by a thick 

 sheet of ice. This immense glacier pushed its way slowly 

 down from Canada. As it moved south it carried with i* 

 large quantities of rocks, grinding them against each 

 other until they were reduced to particles of various 

 degrees of fineness. Later when the climate 

 became warmer, the ice melted, and this rock 

 material remained behind to become a part of 

 our soils. So you see there are a number 

 of ways in which the rocks are ground to smal- 

 ler and smaller fragments until they become as 

 fine as the particles in what is called soil. 



But a soil produced by mere grinding of 

 the rock alone is not suitable for the growth 

 of farm crops. If you grind your wheat into 

 flour you must still further prepare it before 

 it is fit for food. In the same way the food 

 in the rocks must be prepared for the plants. 

 We say that the food must be made "available" 

 to the plant, or in other words, it must be 

 made soluble so the plants can absorb it 

 through the roots. Water is important in 

 bringing about this change also. Pure water 

 \yill not dissolve much of the rock, but the 

 water which falls on the soil contains carbonic 

 acid gas taken from the atmosphere and wa- 

 ter containing this gas will dissolve much 

 larger quantities of the rock minerals. The 

 oxygen of the air also helps to make the plant 

 food available. You will see, then, that at the 

 same time the rock is being ground its nature 

 is being changed so that it is more readily dis- 

 solved. 



These processes so far described combine to 



plant growth to begin. 



The first plants were undoubtedly microscopic in size 

 and belonged to the class of bacteria or similar organisms. 

 Even the first visible plants were of very simple forms, 

 such as the lichens and mosses. These plants can evi- 

 dently live on very small quantities of plant food, and 

 simple as they are can take this food and build it up into 

 organic matter which, upon the death of the p'ant, be- 

 comes a part of the soil. Food which has once been used 

 by a plant is again readily made available to succeeding 

 crops through the process of decay. In this way there is 

 a gradual accumulation of food materials in the soil so 

 that year by year it is able to produce an ever-increasing 

 amount of vegetation. 



The decaying organic matter produces humus and 

 this substance in its turn becomes an important factor 



The Glaciers Were Important Factors in Soil Formatian. 



