26 BULLETIN 355, U. S. DEPARTMENT OF AGEICULTUEE. 



matter stored in ordinary crops an average of about 350 pounds of 

 water is taken from the soil. This amount varies widely under differ- 

 ent conditions and with different crops. Vivian says, ''There is no 

 doubt that the proper condition of moisture is the most important 

 single factor in determining the fertility of the land, and that more 

 soils fail to produce good crops for lack of it than for any other cause." 



Variations in water requirements. — There is no necessary relation 

 between the rate of growth and the quantity of water transpired by 

 the plant. When all conditions are favorable to rapid growth the 

 quantity of water transpired for each pound of dry matter produced 

 seems to be distinctly less than when an essential element of plant 

 food is lacking, or when disease attacks the plant, or any other cause 

 exists which lessens the rate of growth. Moreover, there is a very 

 marked influence of climatic conditions, especially temperature and 

 humidity of the atmosphere, on the quantity of water which plants 

 require. Most staple crops growing in the dry, clear atmosphere of 

 Utah, for example, require from 50 to 100 per cent more water than 

 in Wisconsin. But there also seems to be a marked difference among 

 crops in respect to the relative quantity of water they require. Ex- 

 pressed by rainfall m inches, it has been found that in the eastern 

 part of the United States and in Europe a crop of corn yielding 90 

 bushels per acre requires on the average 15 inches of water, one of 

 oats yielding 75 bushels per acre requires 12 inches, 300 bushels of 

 potatoes per acre, 6i inches, and 2 tons of clover hay, 9 inches. These 

 figures include the water lost by evaporation from the surface imme- 

 diately under the plant when careful tillage and mulching to prevent 

 evaporation are practiced, as well as that transpired by the plant. 



DeptJi to which roots extend for water (Ref. No. 10, pp. 86-93). — In 

 climates which have frequent showers during the summer period, 

 crops get most of their water comparatively near the surface and do 

 not usually extend their roots for moisture more than 3 or 4 feet in 

 depth. On the other hand, in regions in which there is a heavy 

 winter rainfall and a long, dry summer, crops sown in the spring must 

 go deeper and deeper for their moisture as summer advances and the 

 rains cease. Some crops, especially ahalfa, are able to send their 

 roots to great depths, often 20 feet or more. Under such conditions 

 the water-holding capacity of the soil to great depths must be con- 

 sidered. In the Mississippi VaUey, with considerable rainfall during 

 the summer, one may be satisfied with a soil having a good water- 

 holding capacity to a depth of 6 or 8 feet. On the Pacific coast and 

 other parts of the country, where the rainfall comes all during one 

 season, it is important that a fine-textured soil continue to a depth 

 of 15 to 20 feet. This is particularly true for fruit trees. 



Capillary rise of water (Ref. No. 4, pp. 30, 31). — Fortunately, crops 

 are not entirely dependent on the moisture held in the layer of soil to 



