94 A YEAR IN AGRICULTURE 



change. Water which falls on the soil conducts carbonic 

 acid gas from the air, and this helps dissolve quantities 

 of the rock minerals. The oxygen of the air also helps 

 to make plant-food available in the soil. Mineral mat- 

 ter alone, as we shall learn later, can not support 

 plant life. A soil to be fertile must contain nitrogen. All 

 the nitrogen in the soil came originally from the atmosphere. 

 The air is four-fifths nitrogen, but it is in a form which 

 most plants can not use. Before it can serve as a plant-food 

 in the soil, it must be combined with oxygen and certain min- 

 eral elements in the soil in the form of a nitrate. A little 

 of this is formed during electrical storms and is carried into 

 the soil by the rain. 



The growth of vegetation is a factor in soil formation. The 

 vegetation began with the smallest forms, such as lichens and 

 mosses. These die and become a part of the soil. The soil 

 is soon able to produce larger plants and to add the residue 

 of these plant growths to the soil in its formation. The plants 

 upon decaying give rise to the organic matter, and this in- 

 creases the fertility of the land, by being a source of plant- 

 food and soil aeration, and by increasing the water-holding 

 power. During the decomposition of the plants, acid sub- 

 stances are formed which act upon the rocks in such a way 

 as to make more plant-food available. One of the products 

 of this decay is carbonic acid gas. This gas is dissolved by 

 the water and is an important factor in disintegrating the 

 rocks. The roots of plants often penetrate the soil to great 

 depths and exert tremendous force in breaking apart rocks 

 and stones, if they once obtain a foothold in the crevices; 



