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 resume 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 



(79) 



