there is just enough sand to make the soil absorptive and retentive of 

 moisture, and easily tillable. Near Forestburg, S. Dak., the amount 

 of sand becomes excessive and the result is sand dunes. The other 

 extreme is shown by the stiff gumbo soil of the Red River Valley. As 

 a rule, while clay soils permit tree growth, sandier ones are more 

 favorable to it. 



. CLIMATE. 



The rainfall of the prairies decreases gradually with progress 

 north. At Elk Point, S. Dak., the annual amount is 27.25 inches, 

 while at Grafton, N. Dak., it is but 13.54 inches. 



The average annual rainfall for the entire region is approximately 

 20 inches. In a hilly country this amount would not be enough for 

 tree growth. On the prairies, however, there is practically no run- 

 off, and a small rainfall is sufficient to produce crops. 



While a low normal in rainfall may affect the rate of growth and 

 general thriftiness of groves, the extremes of variation affect their 

 actual existence. Table 1 shows the rainfall at Huron, S. Dak., for 

 years which have departed widely from the normal. 



TABLE 1. Extremes of rainfall. 



WET. 



DRY. 



+ indicates more than normal, less than normal. 



It will be seen that, in most cases, by far the greatest amount of 

 rain fell during the growing season, June to September. In the three 

 dry years, however, the rainfall during this season was consider- 

 ably less, and in 1894 was even less than that of the months from 

 September to April. Just such years make it necessary for the 

 planter to select his species with care lest, through the inability of 

 the trees to resist drought, his plantation should fail. 



[Cir. 145] 



