624 
Journal of Agricultural Research voi. xxvn, no. 9 
CONDITIONS OF SOIL WATER 
• a 1188 iS 31 customar y to say that water may be contained in the soil 
m three different conditions which are designated as follows: 
thi I air HySr ° SCOpiC WatCr ’ OT Water that ‘ S held by the S0il when in equilibrium with 
(2) Capillary water, or water that is held as liquid films around the soil particles in 
such a way as to exert no hydrostatic pressure; in other words, in equilibrium with the 
force of gravity, but not m equilibrium with the air. 4 
in f Water ’ f ° r W ? ter tf ; at exists in the soil, but subject to movement 
in response to the force of gravity unless such movement is hindered by some barrier 
While this classification is useful in connection with the study of the 
physical properties of soils, it does not serve so well in dealing with the 
re ations between the soil and the crop plant. The soil may be regarded 
as . a reservoir which is replenished with water from time to time by. 
rainfall or by irrigation, from which there may be losses by percolation 
°rr ev ^P ora ^ on an< 3 from which the plant draws its daily supply. The 
effective reservoir capacity of the soil is limited on the one hand by 
0AfOUA/T OT W/9T£R /A/ SO/L 
BETOBE /B0/&9T/ON. 
0MOUA/T OT HWTEB 000EP 
TO SO/L BT /BB/G0T/OA/. 
Fig. 2. Quantity of water held before and after irrigation by two soils. 
the fact that most crop plants do not thrive when the soil is saturated with 
water so that air is excluded and on the other hand by the fact that 
plants can not absorb all of the water held in the soil. 
From the standpoint of its use by crop plants, the soil water may be 
classified as to condition as follows : 
(1) Subavailable water, or that portion of the soil water which is held by the soil 
when plants can no longer obtain water for the normal processes of growth. The 
moisture content of the soil which represents the upper limit of the subavailable supply 
is often referred to as the wilting point of the soil. 
(2) Available water, or that portion of the soil water that is in excess of the sub¬ 
available supply and has an upper limit somewhat below the duration point. 
(3) Superavailable water, or that portion of the soil water between the saturation 
limit and the limit above which plant roots do not function normally, presumably 
because of lack of air. 
The soils which one finds in irrigated fields differ so greatly from place 
to place that it is impossible to indicate in any precise way how much 
water any soil may hold, even were it possible to establish definitely 
