24 BULLETIN- 615, U. s. DEPARTMENT- OF AGRICULTURE. 
and subsequent decay of the roots ; and the addition of humus to the 
soil by the decomposition of the portions of the plant developed both 
below and above gTound, finally accomplish wonders in improving 
the physical and chemical condition of the soil, provided, of course, 
that serious erosion in the meantime has been checked. 
As the soil is improved and absorbs and retains more water than 
in the beginning, the annual plants develop more- luxuriantly. At 
this point, however, the space occupied by the annual species is 
gradually encroached upon by slightly deeper-rooted, more robust 
annual plants, usually accompanied by a few shalloAv-rooted biennial 
and perennial plants. As the fertility of the soil is further improved, 
even the more robust annual species disappear and the more perma- 
nent perennial type of vegetation predominates as formerly. For 
many years after the latter type becomes conspicuous, however, less 
forage is produced, and of a poorer quality for stock generally, than 
before the soil became depleted. 
From the above facts-, then, it is evident that soil depletion, as re- 
lated to forage production and revegetation, does not imply merely 
a temporary change in the character of the vegetation and. nutritious- 
ness of the forage; on the contrary, the time element enters as a 
highly important consideration. To reestablish completely the more 
desirable and permanent species, such as occupy the soil before it 
becomes depleted, often requires years of time coupled with expert 
management. Too much care can not be exercised by the stockman 
and farmer in preserving the dark surface layer of soil, for that 
portion is the very life of any land. Preserving the surface soil in 
the first place is much cheaper than replacing it, and this is not a 
difficult matter if proper precautions are taken when incipient 
erosion becomes apparent. 
INFLUENCE OF GRAZING ON EROSION AND STREAM FLOW. 
While it is evident that the extent of run-off and erosion are 
roughly proportionate to the effectiveness of the ground cover in 
binding the soil, other factors being equal, the question as to whether 
run-off and erosion are augmented or retarded by grazing is one upon 
which opinions vary widely. Some stockmen contend that if a soil 
is cut up more or less by the trampling of stock, or the surface pretty 
thoroughly pulverized, more water will be held and subsequently 
absorbed by the soil than if the surface is undisturbed. Others are of 
the opposite opinion, contending that the packing of the soil, which 
unavoidably results from grazing, especially if the soil is fairly moist 
when stock travel over it, prevents the rain from being absorbed in 
maximum amounts. In carying out the details of the experiment on 
