PHYSICAL STRUCTURE OF THE SOIL 291 



While rock particles are the basis of most soils, these 

 particles alone would furnish little support for animal and 

 plant life. As already shown, dead plant bodies may accumu- 

 late 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. To be fertile all soils must have some humus in 

 them ; 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. 



The influence of earthworms in soil formation is often 

 very large. They eat their way through the soil, thus mak- 

 ing burrows. In their bodies digestive fluids are added to the 

 soil that has been eaten, and when this is voided from their 

 bodies the soil is changed so that its fertility is increased. 

 The burrows of the earthworm provide added opportunity for 

 air to enter the soil a matter of much importance. A gar- 

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



292. Raw materials for plant food. There are numerous 

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

 portant of these are carbon, oxygen, hydrogen, nitrogen, iron, 

 phosphorus, and potassium. The carbon is obtained from the 

 air ; for from a three-thousandth to a ten-thousandth of ordi- 

 nary air is carbon dioxide, a compound of carbon and oxygen. 

 All the other elements are secured from the soil, where they 

 appear in various combinations. Obviously a soil is fertile 

 that has an abundance of the needed substances and that 

 does not hold other injurious substances, for instance, plant 

 poisons, which sometimes do exist in the soil. Very little is 

 known about plant poisons in the soil, but recent investiga- 

 tions show that some plants excrete substances which may 

 be injurious to them and possibly to other plants. Much 

 attention is now being given to problems of soil fertility. 



