Relation of NonavailaUe Water to Hygroscopic Coefficient 13 
Subsequently in their work in California they changed the 
temperature of drying from 200° C. to 110° C. to avoid expulsion 
of combined water and the partial carbonization of organic mat- 
ter at the former temperature; this method is still in use by them. 
In 1880 Hilgard 1 reported the hygroscopic coefficients of 435 
soils from eleven different states and territories, all the deter- 
minations having been made by himself or Loughridge in Mis- 
sissippi or by his assistants at the University of California. 
Hilgard, 2 using the results of his studies in the arid portions 
of California, replied to Mayers sweeping statements, his con- 
clusions being summarized in the following: 
1. "Soils of high hygroscopic power can withdraw from moist 
air enough moisture to be of material help in sustaining the life 
of vegetation in rainless summers, or in time of drouth. It can- 
not, however, maintain normal growth, save in the case of some 
desert plants. 
2. "High moisture absorption prevents the rapid and undue 
heating of the surface soil to the danger point, and thus often 
saves crops that are lost in soils of low hygroscopic power." 
On Hilgard and Loughridge 3 being associated a second time 
they seem for the first time to have applied to the study of soils 
under drouth conditions their knowledge of the significance of 
the hygroscopic coefficient. "In the following pages the hygro- 
scopic moisture and the free water of the soil are frequently 
spoken of, the latter being that contained over and above what is 
held in the hygroscopic condition, and representing that which 
is considered as free to enter the plant roots and upon which 
the plant chiefly depends." 4 "The actual amounts (of water) 
required for particular cultures * * * are found by elimi- 
nating the hygroscopic moisture and ascertaining the amount of 
free water present in soils where cultures grew and where they 
suffered and comparing the results.'' 5 In this study Loughridge 
reports upwards of 100 determinations of the moisture in the 
1 Hilgard, E. W. Cotton Production in the U. S., vols. 5 and 6, Tenth 
Census of the U. S., 1880. 
2 Hilgard, E. W. Proceedings of the American Society for the Promo- 
tion of Agricultural Science, vol. 1, 1882, p. 118. Ueber die Bedeutung 
der hygroskopischen Bodenfeuchtigkeit fur die Vegetation. Forschungen 
auf dem Gebiete der Agrikulturphysik, vol. 8, 1885, pp. 93-100. Soils, New 
York, 1906, pp. 199-201. 
3 Hilgard, E. W., and Loughridge, R. H. Endurance of Drouth in Soils 
oc the Arid Region. Report of the Agricultural Experiment Station of the 
University of California for the year 1897-8, pp. 40-64. — Loughridge, R. H. 
Moisture in California Soils During the Dry Season of 1898, ibid., pp. 
65-96. 
♦Ibid., p. 66. 
5 Ibid., p. 95. Quoted also in Hilgard's "Soils," New York, 1906, pp. 
212-218. 
