June, 1920.] SOME SOLUTION CULTURES OF WHEAT 75 



of the various component salts. It is clear that the total concentra- 

 tion may have any one of a very large number of magnitudes. 



A 3-salt solution may be said to have another important charac- 

 teristic besides its total concentration ; namely its particular set of 

 salt proportions. The total amount of salts in unit volume may be the 

 same for two different solutions, but nevertheless the proportions of 

 the three salts may not be at all alike in the two cases. The salt 

 proportions represent what has been called the physiological balance 

 of such solutions, although this term should include all other solutes 

 besides the salts — such as oxygen, carbon dioxide, etc. 



Whether a plant grows well or poorly in a given nutrient solution 

 depends as far as the salts are concerned, upon : (1) the total con- 

 centrations, (2) the salt proportions, and (3) the kinds of salts emplo}-- 

 ed. If we supply the six main essential mineral elements in the form 

 of just three salts, and if we consider these salts as composed of the 

 potential ions, K, Ca, Mg, N0 3 , H 2 P0 4 and S0 4 , we have to deal with 

 six different types of 3-salt solution, as has been pointed out by 

 Livingston and Tottingham, 1 for the potassium may be placed in the 

 solution as either the nitrate, the di-hydrogen-phosphate, or the sul- 

 phate, and similarly for the other two cations. The solutions employed 

 in the experiment dealt with in this paper were 3-salt solutions in this 

 general sense. 



The brief summary just given shows how very complex a nutrient 

 solution is, even when it is limited to three salts besides the trace of 

 iron salt, and how extremely difficult it is to determine experimentally 

 what may be the best set of salt-solution conditions for the growth of 

 any given plant in any stated phase of its development and under any 

 given complex of climatic conditions. In order to determine what sort 

 of solution complex may represent the best physiological balance for any 

 given set of non-solution conditions, it is necessary to test experimentally 

 a large number of different solutions. But if the solutions to be tested 

 are to be well chosen, so as to represent the range of physical, chemical, 

 and physiological possibilities, choice must be guided by some mathe- 

 matical system. The most satisfactory system for this sort of work is 

 the one introduced into culture experiments by Schreiner and Skinner. 2) 

 This is based on a triangular diagram, representing a 3-dimensional 



1) Livingston, B. E., and War. E. Tottingham. A new three-salt solution for 

 plant cultures. Amer. Jour. Bot. 5 : 337-346. 1918. 



2) Schreiner, O., and J. J. Skinner. Eatio of phosphate, nitrate and potas- 

 sium on absorption and growth. Bot. Gaz. SO : 1-30. 1910. Idem. Some effects of a 

 harmful organic soil constituent. S. U. Dept. Agric. Bur. Soils Bull. 70. 1910. 



