74 THE BOTANICAL MAGAZINE. LVoi. xxxrv. No. m. 



chemical and physical properties of the given solution are subject to 

 profound alteration in the presence of the insoluble particles, so that a 

 nutrient solution of known make-up cannot be expected to remain 

 unchanged when suspended in the interstices of a finely divided solid, 

 even though the solid is really insoluble in the solution. In such an 

 artificial soil the solid phase is not without influence upon the liquid 

 phase, even though the kinds of solutes may not be altered. The 

 partial concentration of every solute in the original solution is general- 

 ly greatly changed when the solid phase (particles of quartz, etc.) is 

 brought into contact with it. Furthermore, the packing of the solid 

 phase greatly influences the rate at which water, oxygen and carbon 

 dioxide,— as well as the dissolved salts, ions, etc., of the original liquid, 

 — may come to the root surfaces of the plant. Since no means have 

 been devised by which the extremely complicated 3 phase system of 

 even such a simple soil as moist quartz sand may be analyzed and 

 understood, and since it is often possible to obtain well-grown plants 

 in the single-phase media offered by aqueous solutions alone, it is 

 highly desirable that the physiological relations between such liquid 

 media and plants rooted therein should be well worked out. Only in 

 this manner can the first steps be made toward an understanding of 

 the simpler principles of the salt nutrition of plants. 



Nutrient solutions for physiological experiments with plants have 

 generally been prepared with four or five salts besides the iron salt, 

 but Shive's 1 studies made it clear that satisfactory growth may be 

 obtained, with wheat and buckwheat at least, when the six main 

 mineral elements are supplied as the nitrate, phosphate and sulphate, 

 of potassium, calcium and magnesium — that is, with only three salts 

 besides the very small amount of the one containing iron. 



Plants growing in aqueous solution are influenced by several fea- 

 tures or properties of the solution, as far as its salt content is con- 

 cerned. The first of these features, which all work together to produce 

 what may be called the solution-complex, is the total concentration. 

 This may be measured in terms of the lowering of the freezing point, 

 the osmotic value, the vapor tension, the ratio of salt molecules to 

 water molecules used, the number of gram-molecules of salts contained 

 in unit volume, etc. Each dissolved salt, of course, has its own partial 

 concentration, and the total concentration of the medium is simply the 

 sum of all the partial concentrations (however these may be measured) 



1) Shive, J. W. A study of physiological balance in nutrient media. Phy- 

 siological Researches 1:327—397 1916. 



