168 THE STUDY OF PLANT COMMUNITIES • Chapter VII 



Maximum "water holding capacity is the water held by a sat- 

 urated soil. It may be determined by weighing a unit volume of 

 soil before and after it has been immersed in water for twenty-four 

 to forty-eight hours. 



Field capacity is the amount of water a soil retains after all 

 gravitational water is drained away. Soils in the field attain this 

 condition within one to five days after a rain except when the 

 water table is near the surface or saturation extends to a depth of 

 many feet. After prolonged rain, soil may be assumed to be at field 

 capacity if samples taken at eight- to twelve-hour intervals have 

 essentially the same moisture content. 



It is now common practice to express most soil moisture values 

 on a volume basis. In addition, it is desirable that most of these 

 values should apply to the soil as it lies in the field. It is, therefore, 

 advisable to obtain undisturbed samples of a certain volume and to 

 make all determinations without modifying the structure of the 

 samples. Such samples may be obtained with metal cylinders, 63 

 which, when forced into the soil, cut a sample of exact volume, 

 which is then enclosed with airtight lids. Rocky soils may make it 

 impossible to obtain undisturbed samples. It then becomes neces- 

 sary to use special techniques, which, although they give much 

 the same results, require more time and pains than are ordinarily 

 necessary. 163 Some investigators obtain all their samples only when 

 the soil is at field capacity. This system has several advantages, 

 such as eliminating the problems related to swelling on wetting, 

 simplifying sampling, and giving a value for field capacity that is 

 strictly determined by field conditions. When soils are dry, it is 

 often possible to soak them, in place, and permit them to come to 

 field capacity before sampling. 



Capillary capacity (water holding capacity) is the water re- 

 tained against the pull of gravity. Although this appears to be 

 essentially what is meant by field capacity, it is a value determined 

 under laboratory conditions and may run slightly higher than field 

 capacity. The saturated samples of undisturbed soil used for de- 

 termining maximum water holding capacity are permitted to drain 

 over sand for a fixed time, usually two hours, and the weight of 

 water retained, expressed as a percentage of the volume of the 

 sample, is termed the capillary capacity. 



