5 
Native Vegetation 
water and fertilizers. The amount and quality of air in the soil 
may be changed by cultivation and by the use of water and fertil¬ 
izers. The quantity of water in the soil and soil temperature may 
be controlled by irrigation, drainage, fallowing, windbreaks, and 
by the kind of crop planted. The nutrient material in the soil may 
be changed by the use of fertilizers, by irrigation, drainage,.the 
kind of crop planted and the cropping system. And even the life 
of the soil, composed as it is of innumerable microscopic plants 
and animals, may be regulated by irrigation, drainage, method of 
cultivation, cropping system, and by the use of fertilizers' 
Of the two groups of environmental factors;, climatic and 
local , the former group is less under the control of man than the 
latter, except when he is dealing with a very small group of plants. 
However, it is possible for man to modify and control, even in an 
extended way, the structure of the soil, the “plant food” of the 
soil, and the plant and animal life of the soil. It is quite beyond 
his powers to modify and control, in general farming practice, 
light, air temperature, moisture of the air, precipitation and move¬ 
ments of the air, except in a limited way by means of windbreaks 
in the last. 
Pig. 1.—The heavy black line indicates itinerary of automobile trip 
during summer of 1916. Dotted line indicates wagon and train trips. 
