172 THE STUDY OF PLANT COMMUNITIES • Chapter VII 



Measurement of Soil Moisture.— For ecological purposes, it is 

 of prime importance to know how much soil water is available for 

 plant use and often to be able to follow its variations from day to 

 day throughout a growing season. Because of soil variation, it is 

 usually desirable to have determinations from numerous places in 

 a stand and usually from more than one stratum in the soil. It is 

 undesirable to use sampling methods that disturb any considerable 

 amount of the soil or injure roots in the experimental area, and, 

 again, any expression of soil moisture should preferably refer to a 

 unit volume of sample obtained in an undisturbed condition. The 

 last qualification is advisable because interest is in the volume of 

 water available to roots occupying a given volume of soil, rather 

 than weight of water in a given weight of soil. 



It should be clear from our previous discussion that, to inter- 

 pret soil moisture conditions, several soil moisture constants are 

 necessary and that some physical analyses of the soil may be de- 

 sirable. A single collection of samples from each local area of study 

 may suffice for these purposes. Thereafter, some method must be 

 fixed upon, which, within the time available to the worker, will 

 give as adequate a notion as possible of the variations in soil mois- 

 ture content of the experimental areas. Finally, it must be possible 

 to express the soil moisture data in terms of what is available to 

 plants. 



Methods currently in general use are of two types : ( 1 ) deter- 

 mining the actual content of water, (2) measuring the forces with 

 which water is held or the rate at which it is supplied to an ab- 

 sorbing surface. 



The actual content of water is determined by weighing samples 

 before and after drying to constant weight in an oven at 105° C. 

 The loss in weight, representing the water content, is expressed as 

 a percentage of the dry weight or, if the samples are undisturbed, 

 on a volume basis. The disadvantages of the method are numerous. 

 Sampling takes time and disturbs the soil, the samples must be 

 transported, weighing and drying are time-consuming, and a con- 

 tinuous record is impossible. However, the method has its uses, 

 and, where only a few determinations are wanted, it is undoubt- 

 edly the procedure to use. Note, too, that it requires no equip- 

 ment that is not ordinarily available. 



