WATER-CONTENT 33 
from the holard to give the chresard in grams for each roo grams of dry 
weight. The chresard may also be expressed with respect to Ioo grams of 
moist soil. As a final precaution in basal work, it is advisable to determine 
the ‘chresard -for six individuals of the same species under .as- nearly the 
same conditions as possible. When it is desired, however, to find the 
average chresard for a particular soil, it is necessary to employ various 
species representing diverse phyads and ecads. Such an investigation is 
necessarily very complicated, and must be made: the subject of special 
inquiry. 
53. Chresard readings in the field. The especial difficulties which must 
be overcome in the field are the exclusion of rain and dew and the cutting 
off of the capillary water. It is evident, of course, that experiments of this 
sort must also be entirely free from outside disturbance. The choice of an 
area depends upon the scope of the study. If the chresard is sought for a 
particular consocies, the block of soil to be studied should show several 
species which aré fairly representative. In case the chresard of a certain 
species is to be obtained, this species alone need be present, but it should 
be represented by several individuals. Check plots are desirable in either 
event, and at Jeast two or three which are as nearly uniform as possible 
should be chosen. The size and depth of the soil block depends upon the 
plants concerned. It must be large enough that the roots of the particular 
individuals under investigation are not disturbed. There is a limit to the 
size of the mass that can be handled readily, and in consequence the test 
plants must not be too large or too deeply rooted. The task of cutting out 
the soil block requires a spade with a long sharp blade. After ascertaining 
the spread and depth of the roots, the block is cut so that a margin of several 
inches free from the roots concerned is left on the sides and bottom. If 
the block is to be lifted out of place, so that the sides are exposed to evapor- 
ation, this allowance should he greater. In some cases, it may be found 
more convenient to dig the plant up, place it in a large pot, and put the latter 
back in the hole. As a general practice, however, this is much less satis- 
factory. 
After the block has been cut, it may be moved if the soil is sufficiently com- 
pact, and then allowed to dry out in its own formation or elsewhere. The 
results are most valuable in the first case, though it is often an advantage 
to remove blocks cut from shade or wet formations to dry, sunny stations 
where they will dry more rapidly. The most satisfactory and natural 
method, however, is to leave the block in place, and to prevent the reestab-. 
lishment of capillary action by enclosing it within plates. This is accom- 
plished by slipping thin sheet-iron plates into position along the cut surfaces. 
‘ Pa 
