18 BULLETIN" 6*75, U. S. DEPARTMENT OF AGRICULTURE. 



ture with each heavy rain and tend to hold the water back. The 

 records taken on the selected areas in connection with the run-off 

 and erosion from melting snow showed clearly that the snow lies 

 longer under a vegetative cover than in the open, and more water 

 is therefore available for absorption by the soil in the spring of 

 the } T ear in the presence of a good plant cover. Aside from the 

 vegetation protecting the snow from the direct rays of the sun, the 

 roots create minute channels for the ready entrance of water into 

 the earth. To destroy this vegetative cover as shoAvn in Plate III, 

 then, is to decrease materially the x^ower of absorption of the soil. 

 The soil on fully vegetated lands contains a much larger amount 

 of organic matter than on denuded areas and this, greatly increases 

 both the water-holding capacity of the soil and its power of ab- 

 sorption. Accordingly, on fully vegetated lands there is practically 

 no erosion except during violent rainstorms of short duration or 

 after prolonged heavy rains, and even then the erosion is seldom 

 serious. On denuded or sparsely vegetated slopes, on the other hand, 

 run-off and erosion may occur after very small rainstorms. 



RELATION OF EROSION AND SOIL DEPLETION TO VEGETATIVE 



GROWTH. 



It has long been known that different plant species may exhibit a 

 great difference in the amount of water required in various soils to 

 produce a unit of dry matter, a function of profound economic im- 

 portance in the agricultural development of a region of limited rain- 

 fall. Carefully conducted experiments have also proved that when 

 certain fertilizers are added to a soil lacking in plant foods the 

 amount of water evaporated from a plant in the production of a unit 

 of dry matter is considerably reduced, and that the stand of vege- 

 tation may be dense or sparse according to the fertility of the soil. 



In view of these facts, it seemed probable that the sparseness of 

 the native vegetation generally observed on lands whose soils have 

 been subject to more or less serious washing and leaching for a num- 

 ber of years, the short stature of the plants, and the virtual lack of 

 seed production, might be accounted for by the low fertility of the 

 soil and lack of sufficient moisture coupled with a relatively high 

 water requirement of the vegetation in the production of growth. 



In order to determine the difference, if any, in the potential crop 

 production and water requirement of plants grown on eroded and 

 noneroded soils, samples of identical origin and type were selected 

 for comparative study. The soils in question were selected in the 

 spruce-fir type on typical summer sheep range at approximately 

 J 0.000 feet elevation. After being carefully sifted and thus freed 

 of the larger pebbles, etc., the soils were moistened moderately and 



