WATER-CONTENT 29 



base already exists, or, better, while it is being established, scattered readings 

 may be used somewhat profitably. As a practical working rule, however, 

 it is most convenient and satisfactory to make all determinations consecu- 

 tively, i. e., in a series of stations or of successive days. Under ordinary 

 conditions, the time of day at which a particular sample is taken is of lit- 

 tle importance, as the variation during a day is usually slight. This does 

 not hold for exposed wet soils, and especially for soils which have just been 

 wetted by rains. In all comparative series, however, the samples should be 

 taken at the same hour whenever possible. This is particularly necessary 

 when it is desired to ascertain the daily decrease of water-content in the 

 same spot. In the case of a series of stations, these should be read always 

 in the same order, at the same time of day, and as rapidly as possible. When 

 a daily station series is being run, i. e., a series by days and stations both, 

 the daily reading for each place should fall at the same time. While there 

 are certain advantages in making readings either early or late in the day, 

 they may be made at any time if the above precautions are followed. 



47. Location of readings. Samples should invariably be taken in spots 

 which are both typical and normal, especially when they are to be used as 

 representative of a particular area or habitat. A slight change in slope, soil- 

 composition, in the amount of dead or living cover, etc., will produce con- 

 siderable change in the amount of water present. Where habitat and 

 formation are uniform, fewer precautions are necessary. This is a rare cir- 

 cumstance, and as a rule determinations must be made wherever appreciable 

 differences are in evidence. The problem is simpler when readings are taken 

 with reference to the structure or modifications of a particular species, but 

 even here, check-readings in several places are of great value. The varia- 

 tion of water in -a spot apparently uniform has been found to be slight in the 

 prairies and the mountains. In taking three samples in spots a few inches to 

 several feet apart, the difference in the amount of water has rarely ex- 

 ceeded I per cent, which is practically negligible. Gardner^ found that 16 

 samples taken to a depth of 3 inches, in as many different portions of a 

 carefully prepared, denuded soil plot, showed a variation of jYz per cent. 

 This is partially explained by the shallowness of the samples, but even then 

 the results of the two investigations are in serious conflict and indicate that 

 the question needs especial study. It should be further pointed out that all 

 readings should be made well within a particular area, and not near its 

 edge, and that, in the case of large diversified habitats, it is the consocies 

 and the society which indicate the obvious variations in the structure of 

 the habitat. 



1 Gardner, F. D. The Electrical Methods of Moisture Determination in Soils. 

 Bull. Div. Soils, 12:12. 1898. 



