16 
Research Bulletin Xo. 5 
water a point is reached where the attraction of gravity over- 
comes the force that holds the water to the soil particles, and the 
water is drawn downward into the lower soil. If no more water 
is added, a point is reached where the pull of gravity is equaled 
by the force that holds the water to the soil, and there is no 
further movement except by the slow action of capillarity. The 
amount of water in the soil at this time is termed "maximum 
water-holding capacity'' or "saturation point." 
The amount of water a soil will hold against gravity depends 
upon the type of soil. A clay soil composed of fine particles with 
very small spaces will retain more water than a coarser, sandier 
soil composed of larger particles and larger spaces. Also, the 
amount of humus, or decomposed organic matter, in the soil in- 
fluences the water-holding capacity. The more humus in the soil, 
the more water it will hold, providing the soil particles are of 
similar size. 
MINIMUM POINT OF EXHAUSTION BY THE PLANT. 
The minimum point of exhaustion of water from the soil by 
the plant is the point at which the force exerted by the plant in 
obtaining water is equaled by the attraction of the soil for the 
water. At this point the plant can obtain no more water from 
the soil and will suffer until water is supplied. The minimum 
point of exhaustion is in direct correlation with the maximum 
water-holding capacity of the soil. A fine soil that has a high 
water-holding capacity will exert a strong resistance to the use 
of water by the plant, while a sandy soil, with a lower maximum, 
has a correspondingly lower minimum of water which is not 
available for the use of the plant. The water held in the soil 
below the minimum point of exhaustion by the plant is called 
"nonavailable water.*' Some water may be taken from the soil 
below the minimum point, but not in sufficient amount or rapid 
enough to promote plant growth or even to sustain life for any 
considerable period of time. 
From an agricultural standpoint, only the available water is 
of value. The available water in the soil is the amount that the 
farmer has to use, in addition to the rainfall, to produce crops. 
Tn order to interpret results from moisture data, the maximum 
water-holding capacity and the minimum point of use should be 
known. These points rise or fall, depending upon the composi- 
tion of the soil. In the following table we give the data cover- 
ing these two points : 
