THE MAKING OF THE SOIL 243 



Fertile soils contain a considerable proportion of the 

 partially decayed remains of plant life. This material 

 is commonly called humus. In general, humus is a 

 convenient word to use when we wish to speak of the 

 organic material in a soil as distinguished from the in- 

 organic material. The organic material is that which 

 has come from living things, — plants or animals, — 

 though we should remember that all animal materials 

 are derived from plants. The inorganic material is the 

 part of the soil that is derived from rocks. 



If you visit a swamp, especially a peat bog, you are 

 likely to find a soil composed very largely of humus. It 

 is made up chiefly of the partially decayed remains of 

 plants that have lived and died in the swamp. 



From the point of view of successful crop production, 

 the presence of humus in the soil is of greatest impor- 

 tance. This dead vegetation helps more than any other 

 one thing to render the soil fit for the root growth of crop 

 plants. It furnishes the best material for improving the 

 physical condition of both clay and sandy soils. It 

 helps to fill the pores between the soil particles and serves 

 as an absorbent to hold water, thus storing it up for the 

 use of plants. It forms a breeding place for the mil- 

 lions of bacteria of many kinds, which help so largely in 

 rendering the soil fertile for plant growth. It furnishes 

 many tiny particles for the support of root hairs and 

 readily gives up to these the supply of food and mois- 

 ture which they require. 



While it is true that soils originally were made up of 

 particles of rock mixed with the remains of plants or 

 animals, soils as they now exist are much more complex 

 than is usually supposed. Every fertile soil is filled 



