28 BULLETIN 355, U. S. DEPARTMENT OF AGEICULTUBE. 
to reduce the surface tension of soil water and so lessen to a slight 
extent the rise of water in the soil by capillarity. 
Rainfall in relation to water requirements of crops. — According to a 
report from the United States Weather Bureau the normal annual pre- 
cipitation from rain and snow in different parts of this country between 
1870 and 1901 varied from 1 inch to 100 inches. The great agricul- 
tural area included in the central basin of the Mississippi River had a 
mean annual precipitation varying between 30 inches and 50 inches; 
the North Atlantic and Middle Atlantic States had from 40 to 50 
inches; the South Atlantic and Gulf States from 50 to 60 inches; the 
Great Plains States from 15 to 30 inches; the Rocky Mountain States 
from 1 to 20 inches; while the annual precipitation of the Pacific 
States ranged from 10 inches in the extreme southwest to 100 inches 
in the extreme northwest. It has been stated under "Variation in 
water requirements," that the growth of 90 bushels of corn per acre 
requires approximately 15 inches of water; 75 bushels of oats, 12 
inches; 300 bushels of potatoes, 6^ inches; and 2 tons of clover hay, 
9 inches. Comparing these figures with the normal annual precipi- 
tations of the principal agricultural areas, it will be seen that the 
moisture falling as rain or snow would in nearly every instance be 
sufficient to produce large yields of staple crops if it could all be 
held in the soil and utilized for plant growth. It will be recalled, 
however, that the different classes of soil can hold in the upper 4 feet 
only from 3 inches to 7 \ inches of water available for plant growth 
at any one time. On the other hand, over all the agricultural areas 
of highest precipitation the fall of moisture is very unevenly dis- 
tributed throughout the year, and the larger quantities do not fall dur- 
ing the growing season. Because of this, large quantities of water 
drain away from the land, making it necessary in nearly every farm 
area to adopt means to prevent the escape of moisture from the soil. 
Prevention of evaporation (Ref. No. 6, pp. 108-119; or No. 10, pp. 
147-164). — The most effective preventive of loss of capillary water 
from soil is a dry surface which retards the movement of moisture 
through it. Probably everyone has tried the old experiment of 
making a path for a little stream of water by wetting a finger and 
drawing it along a gently inclined board and has been astonished to 
see how irregular a path the water can be made to follow by this 
means. This is because the film of water supplied by the wet finger 
offers less resistance to the movement of the remainder of the water 
than does the surface of the dry wood. In the same way moisture 
in the subsoil can pass upward by capillary action much more readily 
when the soil is moist than after it has been dried. A surface layer 
a few inches in depth of thoroughly dry soil practically prohibits the 
further capillary rise of water to the surface. Water does, of course, 
