Transpiration and Water Movement 101 



are considered. If, however, we should compare evapora- 

 tion with the loss of water in such a plant as prickly pear 

 (Opuntia), the former might be one hundred tunes as great. 

 On the other hand, it should be remembered that a crop 

 on a given plot may develop in leaf surface an area many 

 times that of the soil upon which it is growing Practi- 

 cally all direct measurements of the relative water-content 

 of bare soil as contrasted with areas producing crops 

 indicate that the percentage of water-loss is greater where 

 a crop is grown. In other words, it is possible to conserve 

 water in the soil by fallowing. The use of a fallow, to- 

 gether with sufficient cultivation to keep a constant surface 

 mulch, is one of the first principles in dry-land farming. 



King 1 has cited a case showing the effects of fallowing 

 versus cropping, which is striking. Two plots which had 

 been almost identical in water-content were used in the 

 experiment. After the summer fallowing there was the 

 next spring in the upper surface foot 9.35 pounds per 

 square foot (or 203 tons per acre) more water than in the 

 soil cropped the previous season. A considerable differ- 

 ence was still manifest after both plots had been cropped 

 alike the succeeding season. 



Practically, therefore, plants deprive the soil of moisture. 

 It is well known that willows or birches in a moist spot in 

 a yard or meadow keep the soil fresh and mellow. In 

 some cases trees or other vegetation may seem to increase 

 the soil moisture, but a closer examination will generally 

 reveal the fact that in such instances the vegetation pre- 

 vents rapid run-off and, therefore, appears to use the 

 smaller quantity. 



1 King, F. H., "The Soil," pp. 291-292. 



