276 REPORTS ON THE STATE OF SCIENCE.—1914. 
types of clay.18 Obviously, then, the percentage of water in the soil 
that is available for the growth of plants, or the ‘ growth-water’ as 
Fuller 3° hag termed it, cannot be determined until this unavailable 
residue is known. 
Alway ?° has used the hygroscopic coefficient, i.e., the percentage 
amount of water that a dry soil absorbs on exposure to a saturated 
atmosphere, to represent the unavailable portion. Briggs and Shantz ** 
have measured the moisture-content at which plants undergo permanent 
wilting when growing in a limited soil mass, protected from surface 
evaporation. By permanent wilting is meant a condition from which 
the plants cannot recover when exposed to a saturated atmosphere.*? 
The percentage of moisture remaining in the soil under such conditions 
has been termed the ‘ wilting coefficient’ of that particular soil, and 
has been found to vary slightly with the kind of plant used as an indt- 
cator. The ‘ wilting coefficient ’ in connection with a total moisture 
determination provides a means for calculating the ‘ growth-water,’ 
the latter being the surplus above the wilting coefficient. By the aid 
of such determinations it is possible to calculate the amount of stored 
erowth-water—the bank-balance, so to speak, in the water account, 
against which the crop may draw. 
It is not necessary always to measure the wilting coefficient directly, 
since it can be calculated from other physical properties of soils that 
can be more readily measured. ‘Thus the moisture equivalent, hygro- 
scopic coefficient, and mechanical composition have all been shown 
to bear a linear relationship to the wilting coefficient.2* Of these 
indirect methods, that based on the moisture equivalent 74 is the most 
rapid and satisfactory. The latter represents the percentage of moisture 
remaining in the soil when brought into equilibrium with a centrifugal 
force 1,000 times that of gravity. The wilting coefficient is approxi- 
mately one-half the moisture equivalent. 
Where a small grain-crop has extended its root-system to a depth 
of 4 feet or more, the moisture-content of the second and third feet 
is sometimes reduced below the wilting coefficient. This is practically 
sure to occur if the crop is suffering for water, for plants are able to 
reduce the moisture-content far below the wilting coefficient while in a 
wilted condition, or during the ripening process. But it appears also 
to take place while the crop is still growing, provided the root-system 
is in contact with growth-water in some other part of the soil mass.?° 
18 Briggs, L. J., and Shantz, H. L., The Wilting Coefficient yor Different Plants and 
its Indirect Determination, U.S. Department of Agriculture, Bureau of Plant Industry, 
Bulletin 230, 1912, pp. 56-59. 
19 Botanical Gazette, 58, p. 513, 1912. 
20 Journal of Agricultural Science, 2, 1908, p. 334. 21 Op. cit. 
22 As the plant approaches a wilted condition its transpiration is reduced. Further- 
more, aS soon as wilting occurs it is necessary to transfer the plant to a saturated 
atmosphere, in order to determine whether the observed wilting is temporary or per- 
manent. Consequently during the final stages of a wilting coefficient determination 
the transpiration rate is greatly reduced. 
°3 Briggs and Shantz, op. cit. 
4 Briggs and McLane, Jour. Am. Soc. Agron. 2, 1910, p. 138. 
» Briggs, L. J., and Shantz, H. L., ‘Application of Wilting Coefficient Determi- 
nations to Agronomic Investigations,’ Jour. Am. Soc. Agron. 8, 1911, p. 250. 
