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 dried as rapidly as possible in a 

 water bath or oven. At a temperature of ioo 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, ziP-, is subtracted from the weight of the original sample, w, and 

 the weight of the can, zv^, is likewise subtracted from w. The first result 

 is then divided by the second, giving the per cent of water, or the physical 



w w^ 

 water-content. The formula is: r,= W. 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 (i) a series 

 of readings in the same place, (2) a series at practically the same time in . 

 a number of diflFerent 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 



