I] NATURAL HISTORY OF THE SOIL 7 



of some sort of vegetation. As this dies its residues 

 mingle with the mineral particles, being carried in by 

 earthworms and various insects. The effect of this 

 addition is very great. In the first place it profoundly 

 influences the amount of plant food in the soil. The 

 first vegetation that sprang up must obviously have 

 got its food — its calcium and potassium salts, phos- 

 phates, etc. — from the mineral particles, but new 

 sources of food appear for the plants that come after. 

 The first crop slowly decayed under influences we 

 shall deal with later on, and in decaying it set free 

 those substances that its roots had taken as food and 

 returned them again to the soil. Hence subsequent 

 plants have food from two sources : the potassium 

 salts, etc. dissolved by the soil water from the soil 

 particles ; and in addition a supply of the same 

 substances drawn by previous generations fi'om the 

 soil during their lifetime, but afterwards set free on 

 the decay of the dead tissues. The plant food, in 

 fact, keeps circulating between the soil and the plant, 

 and the organic matter constitutes the medium by 

 which the circulation is completed. 



The second effect of the organic matter is even 

 more important. During its lifetime the plant has 

 been making a good deal of the substance of its 

 leaves and stems from the gases of the air and the 

 rain water, and the materials thus formed contain 

 stored up energy derived from the sunlight. When 



