THE WATER-COST OF DRY MATTER 155 



To the irrigation farmer, a large yield, however, is of 

 interest only if it is produced with little water. In the 

 second and third columns of the preceding table, therefore, 

 are shown the pounds of water required for one pound of 

 dry matter, or the evapo-transpiration ratio, when 10 

 and when 25 inches were used. The uniform variation 

 is remarkable. There was a steady diminution in the 

 evapo-transpiration ratio, from corn to the lowest yielder. 

 In the case of the two exceptions, oats and wheat, and 

 corn and carrots, the yields were almost identical. Only 

 under the 25-inch heading was there a notable exception 

 that of oats. The variation, however, is so regular, over 

 so large a range of crops, that it may be suggested, as a 

 law, that the water-cost of dry matter varies inversely 

 as the inherent power of the plant to produce dry matter 

 per acre. That is, the crop that yields most largely pro- 

 duces the yield at the lowest water-cost. It may be 

 observed in this connection that the crops that yield most 

 heavily with the least expenditure of water are those of 

 the longest growing period. 



Summary. The factors that determine the water- 

 cost of dry matter fall into two classes: First, those 

 like the season, nature of the soil, mineral food, tillage, 

 vigor of plant and nature of plant, that favor the produc- 

 tion of dry matter, and at the same time diminish the 

 rate of transpiration and reduce the water-cost of dry 

 matter; and, second, those, like the varying quantity of 

 water, that favor the production of dry matter, but at 

 the same time accelerate transpiration and increase the 

 water-cost of dry matter. All these factors are of great 

 importance in the establishment of practices for the 

 economical production of dry matter, but, the last, the 

 varying quantity of water, because it is under the easy 



