Wadleigh and Richards 421 



soil moisture tension increases, there is a very rapid decrease in the 

 readiness with which the soil transmits water. Two or three days after 

 a heavy rain or irrigation, tensiometers in field soils usually read in the 

 range from 50-150 cm. of water. It thus appears that field capacity 

 corresponds to the moisture content at which the moisture films are so 

 thin that the unsaturated permeability becomes very small and, com- 

 pared with plant root extraction, further downward drainage can be 

 neglected. 



It is of interest to consider some soil moisture systems that are related 

 to plant nutrition and experimental plant growth setups. Take for 

 example the small pots in which plants are frequently grown. When 

 the soil in a pot is allowed to come to equilibrium with a water table 

 at the bottom of the pot, we have the condition that the soil moisture 

 tension is zero at the water table. When equilibrium is established the 

 soil moisture tension at each level in the pot is just equal to the vertical 

 distance to the water table. Near the water table the tension is low and 

 the moisture content of the soil is correspondingly high. All the soil 

 pores, except the very largest, will thus be filled with water. As the 

 distance from the water table increases, the larger pores are increasingly 

 filled with air, the soil moisture tension is higher and the moisture films 

 on the soil particles are thinner. This soil, pore-space, moisture system 

 has been nicely pictured by Gardner and Chatelain (47). For a soil 

 water system that is at equilibrium with the water table and which is 

 protected from evaporation, the hydraulic head of the water in the 

 system is uniform throughout; the hydraulic gradient and hence the 

 water-moving force in the system are balanced because the downward 

 force of gravity is just counterbalanced by the increase in tension with 

 height. If evaporation is allowed to take place from the soil surface, 

 this increases the tension gradient over and above that required to 

 overcome gravity and an upward flow takes place. When plants are 

 grown in pots, it is interesting to note that the soil moisture conditions 

 at the bottom of the pot are very similar to those just described. No 

 outflow of water will take place until the soil moisture tension at the 

 bottom of the pot becomes zero, thus downward drainage of water 

 from a pot takes place just as if there were a water table at the bottom 

 of the pot. Consequently, the soil moisture that exists in a shallow pot 



