6 WATER-RELATION BET^^EN PLANT AND SOIL. 



been announced.^ It is clear that this soil property must be expressed 

 as a rate; it is a dynamic rather than a static concept. It should be 

 properly expressed as a number of weight units of water possibly 

 delivered to (or through) unit cross-sectional area of the soil, per unit 

 time. Such a water-supplying power of the soil should take a place 

 in studies of subterranean plant environment, corresponding to that 

 occupied by atmometrical measurements in the characterization of 

 the water-extracting power of aerial environments. 



WHiile the dynamic aspect of the subterranean water-relation of 

 plants has thus begun to attract attention, the static properties of 

 soils have received more or less continued study for a long time and it 

 seems probable that, under certain more or less limited conditions, 

 the water-supplying power of a soil may eventually be derived from 

 the proper ones of its static properties. Such an outcome is still very 

 far from attainment, however, and the latter properties remain merely 

 as terms in the description of soils as such rather than as environmental 

 complexes influencing plant phenomena. 



The relative proportions of differently sized particles in the soil are 

 now to be determined with comparatively great precision.^ The 

 moisture-content of soils is determined by the method of weighing, 

 drying, and reweighing. The water-holding power is apparently best 

 found by the use of a one-centimeter column^ of soil in a suitable 

 container. Briggs and McLane'* have recently introduced the so-called 

 moisture equivalent in place of the water-holding power and the former 

 appears more useful than the latter in some ways.^ This is the water- 

 holding power of the soil when subjected to a water-removing, centrif- 

 ugal force equal to a thousand times that of gravitation. The aggre- 

 gate surface of the soil particles can be measured in several waj'S, the 

 most satisfactory being that of Mitscherlich,^ which determines calori- 

 metrically the "heat of wetting." 



Another physical constant of the soil has been brought forward by 

 Cameron and Gallagher,'^ the "critical moisture content," which bids 



^MacDouRal, D. T., Department of Botanical Research. Carnegie Inst. Wash. Year Book 12: 



78. 1913. [A report of this work, by Pulling and Livingston, appears as the second 



paper in the present publication.] 



^On the mechanical analysis of soils, as well as on the determination of other static characters, see 



treatises on soil physics, such as the following: 



Hilgard, E. W., Soils, their formation, properties, composition, and relations to climate and 



plant growth in the humid and arid regions. New York and London. 1912. 

 Mayer, Adolf, Die Bodenkunde. Heidelberg. 1905. 



Mitscherlich, Eilh. Alfred, Bodenkunde fiir Land- und Forstwirte. Berlin. 1905. 

 Wahnschaffc, F., Wissenschaftliche Bodenuntersuchung. Berlin. 1903. 



Also see: Briggs, L. J., F. O. Martin, and J. R. Pearce, The centrifugal method of mechanical 

 soil analysis. U. S. Dept. Agric, Bur. Soils Bull. 24. 1904. 

 ^Hilgard, 1912., page 209. Also, Wahn.schaffe, 1903, page 160 ct seq. 

 ^Briggs, L. J., and J. W. McLane, The moisture equivalents of soils. U. S. Dept. Agric, Bur. 



Soils Bull. 45. 1907. 

 ^For some brief but clear remarks in this connection, see Free, 1912. 

 ^Mitscherlich, 1905, page 51 ct acq. 



'Cameron, F. K., and F. E. Gallagher, Moisture content and physical condition of soils. U. S. 

 Dept. Agric, Bur. Soils Bull. 50. 1908. 



