158 TERRESTRIAL CONDITIONS 
with the addition of humus which takes up water by imbibition. The 
amount of water in the soil is usually expressed in terms of per cent of 
weight, but a soil with 8 per cent of moisture may not give up water to an 
organism as readily as another soil with only 2 per cent. It is necessary 
therefore to determine the capacity of a soil to retain or give up moisture. 
This has been determined for a number of soils (117, 118), in terms of 
what is called the moisture equivalent. The moisture equivalent of a 
soil is the percentage of water which it can retain in opposition to a cen- 
trifugal force 1,000 times that of gravity. The maintenance of turgor 
in plants is believed to be a purely physical matter. If the roots of a 
plant are in a mass of soil, the plant gradually reduces the water content 
until the permanent wilting occurs. The wilting coefficient of a soil is 
the moisture content (in percentage of dry weight) at the time when the 
leaves of the plant growing in the soils first undergo a permanent reduc- 
tion in moisture content, as a result of a deficiency of moisture supply. 
The moisture equivalent of a soil is 1.84 times the wilting coefficient for 
wheat, used as a standard plant. Fuller (119) states that the wilting 
coefficient of dune sand is about 0.75 per cent, while the usual moisture 
content of the cottonwood dune sand is two or three times this amount. 
For the clay soil of the oak-hickory forest, according to McNutt and 
Fuller (1 19a), the coefficient is about 8 per cent. These standards of soil 
moisture indicate the amount of water available to animals through 
direct contact with the soil or available for evaporation into the air of 
cavities which they construct for themselves beneath the surface of the 
soil. A soil gives water to or takes water from the body of a subter- 
ranean animal in proportion to the availability of water in the soil in 
question. The amount of available water increases with depth (119). 
3. TEMPERATURE 
Transeau found that the temperature of bog soil and bog water is 
below that of other soils and waters. This has, however, not been 
observed for different dry soils. The differences between soil on the 
beach at Sawyer, Mich., August 19, 1911, at 3:00 P.M. and in the beech 
woods near at hand was as follows: Air 20° C., upper half-inch of beach 
sand 38°-39° C., sandy soil of beech woods 19°-20° C., a difference of 
19° C. The upper half-inch of bare sand goes as high as 47° C. on the 
hottest days of summer, while the soil in the beech woods is probably 
always a little cooler than the air at the time of the air maximum. 
Dune sand temperature on the hottest summer days at about 3:00 P.M. 
has been found to be as follows: 
