26 BULLETIN 355, U. S. DEPARTMENT OE AGRICULTURE. 
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 in 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, 6J 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. 
Depth 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 alfalfa, 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 Valley, 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 
