PHYSICAL STRUCTURE OF THE SOIL 203 



is so nearly pure plant material that when dry it makes a fuel 

 of importance in some parts of the world, and it is cut from 

 the earth in brick form, dried, and sold in the markets. Some- 

 times peaty soils get afire and may burn slowly for weeks or 

 even months. 



While rock particles are the basis of most soils, these par- 

 ticles alone would furnish little support for animal and plant 

 life. As already shown, dead plant bodies may accumulate so 

 as to produce almost pure plant material in the form of peat. 

 In deep woods we have another illustration of the deposition 

 of plant material in large quantities. Leaves, twigs, and stems 

 of plants fall and decay, thus forming a layer rich in humus, 

 in which luxuriant growths of plants are found. All soils, to 

 be fertile, must have some humus in them, and one of the 

 great problems of modern agriculture is to supply by natural 

 or artificial means the organic matter that may decay and 

 produce the needed humus in the soil. 



The influence of earthworms in soil formation is often very 

 large. They eat their way through the soil, thus making bur- 

 rows. In their bodies digestive fluids are added to the soil 

 that has been eaten, and when voided from their bodies, the 

 soil is changed so that its fertility is increased. Also, the bur- 

 rows of the earthworm provide added opportunity for air to 

 enter the soil a matter of much importance. A garden with 

 many earthworms in it is likely to be a rich garden. 



225. Raw materials for plant food. There are numerous 

 elements that plants need in order to grow well. The most 

 important of these are carbon, oxygen, hydrogen, nitrogen, 

 iron, phosphorus, and potassium. The carbon is obtained 

 from the air in combination with oxygen in the form of car- 

 bon dioxide, which is found in ordinary air in the proportion 

 of approximately 3 parts of carbon dioxide to 10,000 parts of 

 air. All the other elements are secured from the soil, where 

 they appear in various combinations. Obviously a soil is fer- 

 tile that has an abundance of the needed substances and that 



