DIFFERENT KINDS OF SOIL 27 



position is a brown substance called humus or vege- 

 table mold. The heart of old hollow willows is con- 

 verted into humus; it is the same with leaves that 

 fall from the trees and rot on the ground. Humus 

 from the remains of earlier generations of plant-life 

 nourishes the plant-life of to-day, and this in turn 

 will become mold from which future plants will 

 spring. It is in this way that vegetation is main- 

 tained in places not cultivated by man. Humus, 

 then, is nature's manure. Where it is allowed to 

 form freely, vegetation never loses its vigor, using 

 over and over again the same material, which takes 

 alternately the two forms of plant and humus. But 

 hay from the field is stored in the hay-loft, and the 

 annual harvest of wheat is taken to the granary. 

 Thus the land is robbed of the mold that would be 

 formed naturally by the rotting of this hay and 

 wheat; therefore we must give back to it, under 

 some form or other, this mold that has been taken 

 away, since otherwise the soil will become less and 

 less productive until finally it is quite sterile. This 

 restitution is made in the form of animal manure, 

 which is a sort of humus produced by digestive proc- 

 esses instead of by natural decay. 



"Humus plays a twofold part in the soil. First, 

 it mellows the land, or in other words makes it more 

 easily permeable by air and water. Secondly, by the 

 slow combustion taking place in the humus there is 

 constantly being liberated a small quantity of car- 

 bonic acid gas, which is taken up by the adjacent 

 roots. Agriculture can succeed only in so far as the 



