18 BULLETIN 675, 17. S. DEPARTMENT OF AGBICULTUBE. 
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 year 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 shown in Plate III, 
then, is to decrease materially the power 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 
10.000 feet elevation. After being carefully sifted and thus freed 
of the larger pebbles, etc., the soils were moistened moderately and 
