SOIL MOISTURE 51 



These differences in condition of the water present are important in 

 practical ways. They explain the increased moisture requirement of a 

 plant grown in moist surroundings; their application in cold resistance is 

 shown elsewhere ; it is possible that the greater adaptability of some plants 

 to varied environments is related to their capacity of forming water 

 retaining substances. These water retaining substances represent a 

 mechanism for the retention of moisture by living plant cells, a mechanism 

 which is entirely distinct from that represented by the anatomical modifi- 

 cations characteristic of xerophytic or semi-xerophytic plants. The 

 former has to do with the loss of water from the cells to the intercellular 

 spaces; the latter with the loss of water from the plant tissue to the 

 outside. The two means of protection against water loss may occur 

 together in which case the effectiveness of each would be increased, but 

 they may be quite independent of each other. 



MOVEMENT OF WATER IN THE SOIL 



After water once reaches the soil, following either natural precipitation 

 or irrigation, it becomes subject to the forces of gravity and surface 

 tension and in a general way these may be said to control its movement. 

 Percolation downward represents the result of the two forces working 

 together. Lateral movement and rise by capillarity represent what the 

 force of surface tension is able to do in opposition to the force of gravity. 



Percolation. — It has been pointed out that optimum growing condi- 

 tions for most crop plants are found when from 40 to 60 per cent, of the 

 total pore space of the soil is filled with capillary water. Immediately 

 after heavy rains the soil moisture occupies a larger percentage of pore 

 space in the soil. Therefore, the movement of water through the soil 

 must be considered. In humid regions its vertical movement is of chief 

 interest; in irrigated sections both its vertical and its lateral movement 

 are important. Few realize the rate at which water percolates through 

 the soil and the percentage of the total precipitation or of the total 

 amount applied by irrigation that is lost in this way. Table 29 averages 

 the percolation data obtained during a period of 34 years at the Rotham- 

 sted Experiment Station on a rather heavy loam, or clay loam soil. It 

 shows the amounts of water percolating through the soil columns 20, 

 40 and 60 inches deep. 



Attention is directed particularly to the great difference between the 

 proportions of rainfall removed by seepage in years of light and years of 

 heavy fainfall. The figures also show a much higher percentage of 

 percolation during winter months when there is comparatively little 

 evaporation than during the summer months when the evaporation 

 rate is high. As a rule, the hghter the soil, the larger is the percentage of 

 percolation water. Consequently in the irrigation of hght soils it is 



