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 1, 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 heyond 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 when 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. 
HOLARD ECHARD CHRESARD 
Sands sce sccoesswumnansien as 14.3 12.6 3 25 14 12.3 
Clay’ sess rau eRe eee 47,4 32.5 9.3 6.3 38.1 26.2 
LO@SS!:..ct.2cncereaen seine 59.3 37.1 10.1 6.4 49.2 30.7 
LOAM isc ses, satiaeiacerstriosancieie 64.1 39.1 10.9 6.6 53.2 32.5 
FRUITS ovaia2psctanasouetanadepaveiorasess 65.3 39.6 11.9 7.2 53.4 32.4 
AITO ¥fecacass ssvevess savor snout apsesiavnlars 68.5 40.8 16.2 9.6 52.3 31.2 
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 
1Hepecock, G.G. The Relation of the Water-Content of the Soil to Certain 
Plants, Principally Mesophytes. Rep. Bot. Surv. Nebr., 6:48, 1902, 
