ing soil moist and absorbent. The humus increases porosity 

 in a soil not only by a physical influence, but also by a chemi- 

 cal influence that promotes aggregation of soil particles 

 (tilth), and by harboring worms and other soil fauna and 

 bacteria. The roots of the trees and other vegetation open 

 channels for penetration into the deeper ground reservoir. 

 This function of vegetative cover to act as a mechanical 

 retarding agent, and to keep the surface open for absorption 

 and infiltration, is of major importance and should be kept 

 in mind, for we shall find that it is the basis for most of the 

 arrangements made by Man for conservation of soils and 

 water where he has removed the vegetative cover. 



As is the case with respect to all natural arrangements, 

 exceptional situations may be found. Measurements have 

 discovered, as should be expected, that in places where a 

 rock stratum is near the surface and the permeable soil is 

 shallow, a vegetative cover may transpire into the atmosphere 

 more water than it conserves in the manner noted above. 

 Under such circumstances there may be less run-off from 

 seepage during the dry season with vegetative cover than 

 without it; the water held in the root zone is transpired and 

 evaporated before it can seep into the streams. 9 



Generally therefore vegetative cover is an important factor 

 in Man's existence. Without this cover the greater part of 

 the precipitation on other than sandy soils would run off 

 into the rivers, lakes, and oceans; for after a few rains had 

 disturbed, compacted, and caused a swelling of colloidal 

 matter in the soil, there would be left inadequate capacity 

 for absorption and infiltration. There would therefore be 

 little ground-water supply to support vegetable life, and to 

 provide a supply for pumping and for maintenance of the 

 creeks, rivers, ponds, and lakes. 



Without this cover there would be another consequence to 

 which only passing reference has been made, although its 

 importance will appear in the next section. The light, fer- 

 tile soils, soluble or transportable in small particles, would be 



9 Consult W. G. Hoyt and H. C. Troxell, Forests and Stream Flow, Proceedings of 

 American Society of Civil Engineers, Vol. 58, No. 6, August 1932, p. 1037. 



