i] NATURAL HISTORY OF THE SOIL 9 



between a heap of mineral matter and a soil. There 

 is no soil without life, and no life is possible without 

 stored up energy. We are only beginning to know 

 what this soil life is, but already some hundreds of 

 different kinds of creature have been found. Some 

 few are large enough to be seen. Of these the most 

 important are the earthworms, which burrow in the 

 soil and effect a fine natural cultivation, letting in air 

 and drawing in leaves, stems, and other vegetable 

 debris from the surface to mingle with the mass of 

 soil below. Most of the soil organisms are micro- 

 scopic in size ; some lead an active life, others are 

 in the inert resting stage and are called spores or 

 cysts. The very incomplete census taken so far 

 shows that the numbers of micro-organisms living 

 in a single salt-spoon full of soil must be reckoned 

 in millions. 



Some of these organisms certain bacteria play 

 a controlling part in soil fertility because they bring 

 about the decay of the plant residues and consequent 

 liberation of plant food. Out of the old dead plants, 

 in fact, they make food for new ones. Thus the new 

 generation of plants is dependent on them, just as 

 they in turn are dependent on the past generations 

 of plants. As more and more knowledge is gained 

 the circle of soil life widens out and other varieties 

 of organisms are seen to come in, interacting one on 

 the other, not all making plant food, but all dependent 



