34 



THE HABITAT 



The plate for the bottom should be somewhat wider than the block, and is 

 slipped into place by raising the block if the soil is not too loose; in the 

 latter event, it is carefully driven in. The side plates are then pushed down 

 to meet the former. The size of the plates depends upon the block; in 

 general, plates of i, 2, and 3 feet square, with the bottom plates a trifle 

 larger, are the most serviceable. Access of rain and dew is prevented by an 

 awning of heavy canvas which projects far enough beyond each side of the 

 block to prevent wetting. The height will depend of course upon the size of 

 the plants. The awning must be used only w^hen rain or heavy dew is threat- 

 ened, as the shade which it produces changes the power of the plant to draw 

 water from the soil. 



The time necessary to cause wilting varies with the habitat and the 

 weather. When the block is large and in position, two or three weeks are 

 required. This period of drying incidentally furnishes an excellent oppor- 

 tunity for determining the rate at which the particular soil loses water. 

 The holard sample is taken daily for several days before the block is cut 

 out, in order to obtain an average, care being taken of course to avoid a 

 period of extreme weather. The echard samples are taken as soon as the 

 wilting is sufficient to indicate that the limit of available water is reached. 

 The air-dry soil above the roots is first removed. The treatment of the 

 samples and the computation of the chresard are as previously indicated. 



54. Chresard values of different soils. The following table gives the 

 water-content values of six representative soils. The per cents of holard 

 (at saturation) and of echard are those determined by Hedgcock^ with six 

 mesophytes as test plants for each soil. The chresard has been computed 

 directly from these. 



The first column indicates the per cent based upon the dry weight, the second upon 

 the weight of the moist soil. 



While these can not be considered absolute for a particular soil other than 

 the ones investigated, they are found to correspond somewhat closely to 

 the results obtained for other soils of the respective groups. For accurate 



^Hedgcock, G. G. The Relation of the Water-Content of the Soil to Certain 

 Plants, Principally Mesophytes. Rep. Bot. Surv. Nebr., 6:48. 1902. 



