8 WATER-RELATION BETWEEN PLANT AND SOIL. 



It is readily recognized that the physical properties of the soil, as 

 this term is usually employed, depend in great measure on the state of 

 aggregation of the ultimate soil units. Only when these units alone are 

 dealt with is the mechanical analysis of any value in defining a soil. 

 The state of aggregation, or of packing, is clearly of profound import- 

 ance in determining such properties as water-holding power, and the 

 state of packing becomes frequently the prime controlling condition 

 in the determination of the optimum or critical water content of 

 Cameron and Gallagher. The moisture content of the soil, on the 

 other hand, is a property that fluctuates with climatic and drainage 

 conditions, and, within rather widely separated limits, bears no direct 

 relation to the nature and size of the soil particles. Thus, it must be 

 necessary to measure at least two physical properties (one of which is 

 the moisture content) before the water-supplying power of the soil 

 may possibly be determined indirectly. Such indirect determination 

 is of course quite impossible at present, and the only means so far 

 proposed for comparing water-supplying powers of different soils, or 

 of the same soil at different times, depends upon direct measurement. 

 Two such methods have been suggested. One proposes to measure 

 the highest possible rate of evaporation attainable from unit surface 

 of the soil. 1 The other proposes to determine the highest possible 

 rate at which an osmometer may withdraw moisture from the soil, 

 per unit of surface. 2 Neither of these methods appears as yet to have 

 been subjected to an actual working test under field conditions. Nor 

 does the " artificial root-hair" of Briggs and McCall, 3 by which water 

 is removed from the soil through a porous clay filter-candle attached 

 to a vacuum bottle, seem to have received the attention, in this con- 

 nection, which it apparently deserves. 



The water-supplying power of the soil (which, as has been pointed 

 out, is the time rate of possible water delivery at unit cross-sectional 

 area in the soil) depends, obviously, upon the resistance to water 

 absorption offered by the soil layer adjacent to the absorbing surface. 

 This, in turn, is a function (1) of the nature and arrangement of the 

 soil particles in this active layer, and (2) of the water content of the 

 latter. The nature and arrangement of the soil particles may be 

 assumed to remain practically constant in this layer after the soil mass 

 as a whole has once settled into an approximate static equilibrium, but 

 root absorption itself necessarily decreases (or tends to decrease) the 

 water content of the soil from which water is removed. Whatever 



'Livingston, 1906, page 37. 



2 Whitney, M., and F. K. Cameron, The chemistry of the soil as related to crop production. 



U. S. Dept. Agric., Bur. Soils Bull. 22. 1903. 

 Livingston, 1906, page 20 et seq. 

 MacDougal, 1914, page 78. 



Pulling and Livingston, the second paper of the present publication. 



3 Briggs, L. J., and A. G. McCall, An artificial root for inducing capillary movement of soil mois- 

 ture. Science N. S. 20: 566-9. 1904. 



