30 THE HABITAT 
48. Depth of samples. The general rule is that the depth of soil sam- 
ples is determined by the layer to which the roots penetrate, The prac- 
tice is to remove the air-dried surface in which no roots are found, and to 
take a sample to the proper depth. This method is open to some objection, 
as the actively absorbing root surfaces are often localized. There is no 
practical way of taking account of this as yet, except in the'case of deep- | 
rooted xerophytes and woody plants. It is practicable to determine the 
location of the active root area of a particular plant and hence the water- 
content of the soil layer, but in most formations, roots penetrate to such 
different depths that a sample which includes the greater part of the distance 
concerned is satisfactory. Some knowledge of the soil of a formation is also 
necessary, since shallow soils do not require as deep samples as others. The 
same is true of shaded soils without reference to their depth, and, in large 
measure, of soils supplied with telluric water. In all cases, it is highly de- 
sirable to have numerous control-samples at different depths. The normal 
cores are 12 or 15 inches; control-samples are taken every 5 inches to the 
depth desired, and in some cases 3-inch sections are made. It has been 
found a great saving of time to combine these methods. A 5-inch sample is 
taken and placed in one can, then a second one, and a third in like manner. 
In this way the water-content of each 5-inch layer-is determined, and from 
the combined weight the total content is readily ascertained. 
49. Check and control instruments. A number of instruments throw 
much light upon the general relations of soil water. The rain-gauge, or 
ombrometer, measures the periodical replenishment of the water supply, 
and has a direct bearing upon seasonal variation. The atmometer affords 
a clue to the daily decrease of water by evaporation, and thus supplements 
the rain-gauge. The run-off gauge enables one to establish a direct connec- 
tion between water-content and the slope and character of the surface. The 
amount and rapidity of absorption are determined by means of a simple in- 
strument termed a rhoptometer. The gravitation water of a soil is ascer- 
tained by a hizometer, and some clue to the hygroscopic and capillary water 
may be obtained by an artificial osmotic cell. All of these are of importance 
because they serve to explain the water-content of a particular soil with 
especial reference to the other factors of the habitat. It is evident that none 
of them can actually be used in exact determinations of the amount of 
water, and they will be considered under the factors with which they are 
more immediately concerned. 
Physical and Physiological Water 
50. The availability of soil water. The amount of water present in a. 
soil is no real index to the influence of water-content as a factor of the 
