122 Plant Physiology 



Russia), famines may be expected any year when the rain- 

 fall is but slightly less than usual, and without their fairly 

 well-developed systems of irrigation much larger areas of 

 these countries would remain in doubt with respect to pro- 

 duction. Where irrigation is not practiced, it is frequently 

 necessary to introduce systems of dry-land farming 

 whereby the principles of soil-moisture conservation are 

 effectively applied, and sometimes a single crop is grown 

 in two years, water being allowed to accumulate every sec- 

 ond year. 



Precipitation has a maximum effect, of course, when all 

 conditions are favorable; that is, when the nature of the 

 soil and its depth, the type of sub-soil, the slope and ex- 

 posure of the ground, all combine to conserve moisture 

 and deliver it to the growing crop. 



68. Irrigation. - - Both in Europe and America (in 

 many sections where irrigation has not been considered 

 necessary) it has now been abundantly demonstrated that 

 the yield of most crops may be materially increased by a 

 rational use of water. In Wisconsin, for example, King 1 

 has found that during a six-year period the yield of pota- 

 toes was increased from 217.3 bushels to 301.7 bushels 

 per acre. Again, with twelve inches of rainfall during a 

 growing season for corn, the yield of grain was increased 

 by means of irrigation from 30.14 to 65.3 bushels. 



It has already been indicated that profitable crop pro- 

 duction is only possible in many regions when irrigation 

 practices supplement the effect of the normal rainfall. 

 In every drainage basin, large or small, there are oppor- 

 tunities for conservation. 



1 King, F. H., Wis. Agl. Exp. Sta. Report, 18 : 195, 1901. 



