80 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 



adapted to radically different habitats, there may be, at least under 

 certain conditions of wilting, radical differences in the coefficients. 

 Another important phase of the matter is that certain soils may 

 have a peculiar reaction on one species and not on others; as, for 

 example, a highly acid or strongly limey soil. It is therefore th< 

 part of wisdom to test the nonavailable moisture of any soil by the 

 use of at least the predominating or type species found on the soil, 

 and of as many other species as possible. 



DIRECT DETERMINATION OF THE WILTING COEFFICIENT. 



The writers cited above have given such thorough consideration 

 to this and the succeeding subjects that a complete discussion here 

 appears almost useless. The treatment of forest soils and forest 

 species, however, has brought out a number of new problems, so that 

 it is almost impossible . to overlook any phase of the question in 

 this discussion. Constant comparison will be made with the treat- 

 ment found desirable for field crops and related plants. 



It is well to bear in mind from the outset the point brought out 

 by Briggs and Shantz that the wilting coefficient represents merely 

 the moisture point at which wilting first occurs to such an extent 

 that the plant does not recover if placed in a saturated atmosphere. 

 The plant may actually draw considerable water from the soil after 

 this, and might be theoretically conceived to pass moisture to the 

 atmosphere until the soil and atmosphere were in vapor-pressure 

 equilibrium. The wilting coefficient is, however, the practical ex- 

 pression for nonavailable moisture. 



The fact must be also strongly emphasized that the point at which 

 wilting occurs must depend in a very large measure on the rate at 

 which the plant is transpiring; or, in other words, on atmospheric 

 conditions and sunlight. Therefore, as the soils approach dryness, 

 the conditions should be maintained at a fairly definite standard. 

 It will usually be feasible to prevent the occurrence of temperatures 

 in excess of 70° F., as well as sudden changes in temperature, and 

 to exclude direct sunlight. It would be also desirable to control 

 atmospheric moisture, though this is a very difficult thing to do in 

 ordinary rooms. 



In the tests with seedlings of coniferous trees it has been found 

 exceedingly difficult to determine when permanent wilting occurs. 

 There is no doubt that seedlings of this kind have developed a power 

 of resistance, or recovery, far in excess of that of most plants. This 

 probably consists in an extremely low rate of transpiration when 

 the moisture becomes deficient; but the difficulty of observation may 

 be mainly ascribed to the fact that the stems, and to a Lesser extent 

 the leaves, become stiff and woody at a very early age, so thai 

 shriveling rather than collapse is the phenomenon that evidences 



