AGRICULTURAL PRACTICE AND CLIMATIC CYCLES. 



263 



for the most part from 15 to 30 inches. Above 30 inches the compensating 

 effect of accumulated water-content tends to minimize the consequences of 

 drought, while below 15 inches the margin of safety is so small that it is easily 

 destroyed by local or incidental causes. 



The evidence of definite cycles in crop production is difficult to obtain for 

 the further reason that accurate records in a particular place for a long period 

 are extremely rare. Few of these extend through a sun-spot cycle of 10 to 12 

 years, and practically none through the more significant double cycle of 21 to 

 23 years. However, the drought periods of 1870-72, 1893-95, and 1916-18 

 were so intense that a corresponding production cycle is shown in the crop 

 averages for the regions concerned. The sun-spot maximum of 1907 marked 

 a minor drought period which in most regions reached its culmination two or 

 three years later. This discrepancy seems to be explained, in part at least, 

 by the interference of a shorter cycle, probably the pleion or quarter cycle of 

 2.5 years, and by the action of the excess-deficit balance. Arctowski (1912: 

 745) has shown the relation of the corn crop by States and regions to the 

 interaction of these two cycles. He has found not only that areas of excess 

 and deficit in production bear a definite relation to each other, but also that 

 this relation is preserved as they shift about from year to year. Douglass 

 (1919: 106) has found that the 2.5-year cycle is regularly present in the 

 growth of trees. Hence, it seems probable that the major cycle of crop 

 production is the double sun-spot cycle of 21 to 23 years, and that this is 

 made up of smaller cycles resulting from the interaction of the sun-spot 

 cycle of 11 years and the quarter cycle of 2.5 years. Intensive research only 

 can determine how distinct and universal these may be. At present, it must 

 be admitted that they are often much disturbed by the compensating action 

 which follows an excess or deficit of rainfall. This is termed the excess- 

 deficit balance, and is itself a short-period cycle, based apparently upon 

 the fundamental physical correlation of action ajid reaction. Since it is 

 usually 2 to 3 years in length, it is not improbable that it may be the 2.5- 

 year cycle heightened by spatial variations in rainfall (fig. 15). 



7S0 *C Ja 20 /a 700 SO go 70 



)tar-a.c. 



Fio. 15. 2-year cycle in a Sequoia. After Douglass. 



60 



An analysis of the production of grain-sorghums at Amarillo from 1907 to 

 1918 has been made to illustrate the possible relation to the various cycles. 

 This has been drawn from the results of Ball and Rothgeb (1918), but is 

 limited to the production in bushels of grain, as representing the more com- 

 plete response of the plant to growing conditions. The seasonal rainfall has 

 been reckoned for the five months beginning with April and ending with 

 August. 



