28 



Yearhook of the Department of AgHcvIture, 192t. 



Figs. 22 and 23.— The northern boundary of the Cotton Belt is approximately the 

 line af 200 davs average frost-free season (see Fig. 5) and 77° mean summer temperature, 

 the southern "boundary that of 11 inches autumn rainfall, l>ecause wet weather inter- 

 feres with picking and damages the lint. This southern boundary is now moving north- 

 ward, as the milder winter temperatures near the Gulf and longer seasoii permit in- 

 creased injury bv the boll weevil. The western boundary of cotton production witnouu 

 irrigation is approximately the line of 23 inches average annual rainfall (see Ing. 4) 

 The densest areas on the map are districts of richer soils, notably the Black Frairie or 

 Texas and the Yazoo Delta (see Fig. 6), or heavily fertilized soils, especially those of the 

 Piedmont and Uwoer Coastal Plain (see Fis. 109 >. 



