No. .14:,] PRLSEXT PROBLEMS IN SOIL PHYSICS 299 



sequently passing to the real point of interest by means 

 of an interpretation, bnt such a possibility is at present 

 so far removed from actuality that it seems highly desir- 

 able to begin with attempts to measure the soil property 

 which directly influences plants. In any event, it can not 

 be too strongly emphasized that such soil studies as I 

 am suggesting must always be carried on simultaneously 

 with studies on the behavior of plants, and also with ade- 

 quate determinations of the water-extracting power of 

 the aerial environment. It seems quite likely that we 

 shall be able empirically to determine some highly im- 

 portant principles bearing upon the water relations which 

 exist between plants and soils, without having yet suc- 

 ceeded in analyzing the mode of manifestation of these 

 into its elementary physical propositions— just as it has 

 recently been possible to work out exceedingly valuable 

 principles with reference to the relation of plants to 

 evaporation, without any one's having yet succeeded in 

 determining the quantitative dependence of this climatic 

 factor upon its components, water and air temperature, 

 air humidity and air movement. 



When a little headway has been gained in the dynamic 

 study of the soil in relation to plant processes, we shall 

 probably begin to be able to interpret and correct, and 

 place upon a proper quantitative basis, some of the eco- 

 logical classifications of plants and the physical classi- 

 fications of soils, which already occupy so much of our 

 literature. 



Another aspect of this whole question of the water 

 relations of the subterranean parts of the terrestrial 

 plant may be worthy of attention. The majority of the 

 physical soil studies which have so far been made depend 

 upon the removal of the soil sample from its natural 

 position, with consequent and usually profound altera- 

 tions in the arrangement of its component grains, upon 

 which arrangement assuredly depend some of the most 

 fundamental of the soil qualities which we need to know 

 about. Various methods have been devised aiming to 



