THE WATER OF THE HABITAT 11 



be numbered with paint on both hd and side in such a way that 

 the number may be read at a glance. The rule is to weigh the 

 cans as quickly as possible after the sample is taken, though when 

 necessary they can be kept for several days without appreciable 

 error. For weighing, delicate balances are to be preferred, ])ut 

 when these are not available, coarser balances which weigh accu- 

 rately to one centigram give satisfactory results. The best method 

 is to w^eigh the soil sample in the can. Turning the soil out upon 

 the pan or upon paper saves one weighing, but there is always some 

 slight loss, and the chances of serious mishap are many. After 

 weighing, the sample is dried in a water-bath or oven. At a tem- 

 perature of 100 C. this is ordinarily done in twenty-four hours; 

 stiff clays require a longer time. High temperatures must be 

 avoided with soils that contain much leaf-mold or other organic 

 matter, in order that this may not be volatilized, and hence lead to 

 an error in the result. When a drying-oven is not accessible, soil 

 samples are dried in the air, preferably in sunshine. This usually 

 takes several days, and a test weighing is generally necessary to 

 determine that the drying-out is completed. The weighing of the 

 dried soil is made as before. The can is carefully brushed out and 

 weighed also. 



14. Computation of water content. To find the percent of 

 water content, the second weight, i.e., of the dried sample and can, 

 w', is subtracted from the first weight, w, of the original sample 

 and can. The weight of the can, uP', is taken from the dried weight, 

 w'. The first result is then divided by the second, and the result 

 is the percent of total water content, i.e., holard, figured upon the 



drv soil as a basis. The formula is 7 ?=TF, in which W is the 



holard in percent. Water content has generally been computed 

 upon the moist soil as a basis. ^ This method leads to inexactness 

 in the comparison of habitats, however, and should be abandoned. 

 Since most of the results so far obtained have been made in this 

 way, it is necessary at present that the basis be taken into account 

 in comparing the results of different workers. The most satisfac- 

 tory method for the present is to express the results in grams per 

 hundred grams of moist soil. For example, 20/100 indicates a 

 water content that is 20% of the moist weight or 25% of the dry 

 weight. 



' Research Methods, 28. 



