CHAPTER IV 



THE INCREASE IN YIELD DUE TO IRRIGATION IN 

 HUMID CLIMATES 



IN order to know how important the right amount 

 of soil moisture, applied at the right time, is, and in 

 order to know whether it will pay to irrigate in humid 

 climates, 'it is necessary to learn what yields are possi- 

 ble under the best conditions when the crop must 

 depend upon the natural rainfall, and, side by side 

 with these in time and place, to measure the possible 

 increase in yield due to irrigation, if any there be. 



When the study of the importance of soil moisture, 

 and the principles underlying the methods of saving 

 and utilizing it, were begun at the Wisconsin station 

 in 1888, it very early became evident that, in order to 

 learn just how important it is in plant culture to con- 

 serve the soil moisture, some method must be adopted 

 which would permit of giving to the plants under inves- 

 tigation all the water they can use to advantage. 

 This led to the series of experiments which have been 

 recorded in the introductory chapter, aiming to meas- 

 ure the amount of water which different cultivated 

 plants can use under the conditions of field life. But 

 when the results attained under the methods there used 

 showed that such large yields are possible, it became 



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