WATEE OF SOIL IN ITS EELATION TO PLANTS 187 



As a rule, this depression of the ratio of grain to straw is 

 not due to an actual decrease in the grain, but to a corre- 

 spondingly greater production of dry matter in the vege- 

 tative parts. As available water is augmented, the dry mat- 

 ter of plants increases until a maximum is reached. The gen- 

 eral relationships are well exemplified by data from Widtsoe ^ 

 (Fig. 34), although other equally valuable figures may be 

 obtained from von Seelhorst - and Atterberg/ who have done 

 much work on the subject. 



99. The water requirements of plants. — ^As might be 

 expected, the pounds of water transpired for every pound of 

 dry matter produced in the crop is very large. This figure, 

 called the transpiration ratio, or water requirement, ranges 

 from 200 to 500 for crops in humid regions, and almost twice 

 as much for crops in arid climates. An accurate determina- 

 tion of the transpiration ratio of a crop is somewhat difficult, 

 due to the methods of procedure necessary and also to the 

 difficulty of controlling the numerous factors that influence 

 the transpiration. For really reliable figures the plants must 

 be grown in cans or pots in order that the water lost may 

 be determined accurately by weighing. If there is no percola- 

 tion the water ordinarily lost from a cropped soil includes 

 both that evaporated from the soil surface and that tran- 

 spired from the leaves. The former loss may be controlled 

 largely in one of two ways: (1) by covering the soil so that 

 evaporation is absolutely checked and the only loss is by 

 transpiration; or (2) by determining the evaporation from a 

 bare pot and, by substracting this from the total water loss 



^Widtsoe, J. A., TTie Froduction of Dry Matter with Different Quan- 

 tities of Irrigation Water; Utah Agr. Exp. Sta., Bui. 116, pp. 19-25, 

 1912. 



^Seelhorst, C, von, und Krzjmowski, E., Versuoh uher den Mnfluss, 

 welehen das Wasser in dem VerscMedeneu Vegetationsstadien des JSafers 

 auf sein WaeJistum ausubt; Jour. f. Landw., Band 53^ Seite 357-370, 

 1905. 



® Atterberg, A., Bie Variationem der Ndhrstoffgehalte "bei dem Mafer; 

 Jour. 1 Landw., Band 49, Seite 97-113, 1901. 



