CHAPTER, X 
GRASS-CROP AREAS 
In previous chapters, reference has been made under 
each grass crop to the area in the United States in which 
it thrives. In this chapter a résumé will be given of the 
conditions and limitations of these areas. The crop areas 
depend entirely upon climatic conditions. Soil conditions 
modify or limit the distribution of crops within each area. 
So far as crops are concerned, the climatic conditions are 
moisture and temperature. 
108. Moisture——The moisture, so far as it concerns 
crop areas, depends on the annual rainfall and its seasonal 
distribution. Locally crops may receive water by seepage 
from rivers and springs but such sources have no effect 
on the general distribution of crops. The seasonal dis- 
tribution of the rainfall is of as much importance as the 
annual rainfall, for the crops require water during the 
growing season. The amount of water required by a crop 
varies with the evaporation, which depends in part 
upon the humidity. Without going into the physical and 
meteorological details, it will readily be understood that 
latitude and altitude modify greatly the relation between 
a crop and its water-requirement. There is also an inti- 
mate connection between temperature and rainfall. Soil 
conditions modify the water requirement in various ways, 
but not sufficiently to have any material effect on the 
large crop areas. 
109. The temperature affects evaporation and hence 
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