28 THE HABITAT 
reliable results. The best procedure is to weigh the soil with the can. 
Turning the soil out upon the pan or upon paper obviates one weighing, 
but there is always some slight loss, and the chances of serious mishap are 
many. After weighing, the sample is dricd as rapidly as possible in a 
water bath or oven. At a temperatureof 100° C. this is accomplished 
ordinarily in twenty-four hours; the most tenacious clays require a longer 
time, or a higher temperature. High temperatures should be avoided, 
however, for soils that contain much leaf mould or other organic matter, 
in order that this may not be destroyed. When it is necessary on trips, 
soil samples can be dried in the sun or even in the air. This usually takes 
several days, however, and a test weighing is generally required before one 
can be certain that the moisture is entirely gone. The weighing of the 
dried soil is made as before, and the can is carefully brushed out and 
weighed. The weight of aluminum cans may be determined once for all, 
but with painted cans it has been the practice to weigh them each time. 
45. Computation. The most direct method of expressing the water- 
content is by per cents figured upon the moist soil as a basis. The ideal 
way would be to determine the actual amount of water per unit volume, 
but as this would necessitate weighing one unit volume at least in every 
habitat studied, as a preliminary step, it is not practicable. The actual 
process of computation is extremely simple. The weight of the dried 
sample, w*, is subtracted from the weight of the original sample, w, and 
the weight of the can, w?, is likewise subtracted from w. The first result 
is then divided by the second, giving the per cent of water, or the physical 
: _. we F 
water-content. The formula is: eo The result is expressed pref- 
erably in grams per hundred grams of moist soil; thus 20/100, from which 
the per cent of water-content may readily be figured on the basis of dry 
or moist soil. 
46. Time and location of readings. Owing to the daily change in the 
amount of soil water due to evaporation, gravity, and rainfall, an isolated 
determination of water-content has very little value. It is a primary re- 
quisite that a basis for comparison be established by making (1) a series 
of readings in the same place, (2) a series at practically the same time in 
a number of different places or habitats, or (3) by combining the two 
methods, and following the daily changes of a series of stations through- 
out an entire season, or at least for a period sufficient to determine the 
approximate maximum and minimum. The last procedure can hardly be 
carried out except at a base station, but here it is practically indispensable. 
It has been followed both at Lincoln and at Minnehaha, resulting in a 
basal series for each place that -is of the greatest importance. When such a 
