2. THE ROLE OF VEGETATIVE COVER 



LET us assume for a moment an earth different from that 

 which we know. If the surface consisted entirely of hard, 

 granular materials such as gravel and sand, free from decom- 

 posed organic matter, then absorption, infiltration, and the 

 ground-water store would be at a maximum; and seepage, the 

 discharge of springs, and the flow of streams more regular. 

 Conditions would be suitable for the freest and most rapid 

 circulation of waters. But if to these materials there is 

 added matter in the form of dust, or that swells when wet, 

 precipitation will wash the fine particles into the spaces of the 

 granular structure or cause them to swell. In this way 

 absorption and infiltration are retarded by a clogging of the 

 channels of penetration. Therefore, assuming such an earth 

 surface, if may be said that vegetative cover, the principal 

 source of organic matter (humus) in the soil, retards absorp- 

 tion and infiltration and affects unfavorably the free circula- 

 tion of waters. 



However, that is not the kind of earth with which Man is 

 concerned. Whatever the condition of other planets, the 

 earth is a place of organic life vegetation, animals, Man. 

 From the point of view of his problems of use and conserva- 

 tion of natural resources, he must deal with surface soils of 

 which some have been pulverized to the consistency of dust 

 and of which most contain organic matter. While there are 

 many spots of sand or gravel, the soils which are most suit- 

 able for the production of food, clothing, and shelter are gen- 

 erally humified soils. 



Given this earth of humified surface soils necessary for the 

 existence of Man, and assuming that the introduction of the 

 humus to basic granular materials may have caused an initial 

 retardation of absorption and infiltration, beyond that point 

 relations between vegetative cover and humified surface 

 soil were arranged in such a way as to promote maximum 

 absorption and infiltration under the assumed conditions. 



Figure 9 shows in approximate detail the normal relations 

 between vegetative cover and absorption-infiltration over 



57288 36 - 2 



