WATER-CONTENT 33 



from the holard to give the chresard in grams for each lOO grams of dry 

 weight. The chresard may also be expressed with respect to lOO grams of 

 nioist 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 are 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 least 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 be 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. 



