No. 545] PRESENT PROBLEMS IN SOIL PHYSICS 297 



a way that may lead to a scientific foundation, is con- 

 cerned with the maximum rates at which various soils 

 may furnish moisture to the root systems of whatever 

 plant forms with which we may be dealing. To such an 

 end, our knowledge of the physiology and ecology of 

 roots must be enormously increased, but with this phase 

 of the matter we need not here concern ourselves. It is 

 obvious, however, that the really crucial question with 

 regard to any soil, the properties of which we wish to 

 study with reference to plant behavior, is this : at what 

 rate, and for how long a time, can it deliver water to unit 

 area of a water-absorbing surface? This is a purely 

 physical question and one for which it ought not to be 

 very difficult to find adequate methods of attack. Indeed, 

 the method of studying evaporation from soil surfaces 

 already offers approximate results in this direction. 



This maximum rate of delivery per unit of cross sec- 

 tion must be related in some manner to the soil charac- 

 ters which are now often measured ; the power of water 

 delivery will vary with the percentage of water content 

 for any particular soil, and its graph will most likely ex- 

 hibit a critical point under about the same conditions as 

 those which accompany the critical points for evapora- 

 tion from the soil, the apparent specific gravity of the 

 latter, its penetrability (as recently brought out by Cam- 

 eron and Gallagher), its critical moisture content and 

 its moisture equivalent (as brought out by the centrif- 

 ugal method of Briggs and McLane). That the critical 

 point in maximum rate of delivery of moisture will be 

 found to correspond to the ordinarily observed optimum 

 water content for many plants is also to be expected, but 

 the physiologist will not make the mistake of supposing 

 that this optimum water content will not vary largely 

 with the nature and condition of the plant and also with 

 its rate of transpiration. That this critical point, with soils 

 of varying water content, will be found to be related to 

 the size, nature and arrangement of the soil particles is 

 likewise fairly certain, and it may confidently be ex- 



