Oct. IS. 1919 Relation of Nutrient Medium to Plant Absorption 113 



(5) The optimum total concentration of the nutrient solution, if 

 defined as the least concentration giving a yield equal to any higher 

 concentration, was not found to be greater than that represented by 0.6 

 atmosphere osmotic pressure; and it may be less than o.i atmosphere. 

 For the solutions used in these experiments, inhibitive concentrations 

 were not higher than those represented by 2 to 2.5 atmospheres osmotic 

 pressure. 



(6) In the interpretation of the results of solution- and sand- culture ex- 

 periments in terms of concentration and ionic ratios, emphasis is placed on 

 the necessity of clearly distinguishing between the concentration and com- 

 position of the solution used and the total supply of the various elements 

 provided for each plant. In the periods between changes of solution 

 the concentration and composition may be markedly altered because of 

 absorption by the plant. In many experiments the number of plants 

 and size of culture vessels have been chosen arbitrarily without reference 

 to these facts. 



(7) From a consideration of previous experiments it is concluded that 

 there is no sufficient evidence to prove that the plant requires for optimum 

 yield any very specific ratio of ions or elements within wide limits, pro- 

 vided the total supply and concentration of essential elements are 

 adequate. 



(8) In solutions with an acid reaction (Ph 5 to 5.5) the absorption of 

 several ions was greater than from a neutral solution (Ph 6.8) of similar 

 composition and the same total concentration. 



(9) One experiment suggested the possibility that at certain periods 

 of growth excretion of electrolytes may take place, and that this phe- 

 nomenon is dependent on the reaction and concentration of the nutrient 

 solution. 



(10) An acid reaction represented by Ph 5 was not found to be injurious 

 to the barley plant at any period. There was a tendency toward the 

 production of a neutral reaction in the solution as a result of changes in 

 the equilibria due to absorption. 



(11) When the plant was placed in a very dilute nutrient solution, 

 excretion of electrolytes, especially Ca, Mg, PO4, took place at first. This 

 was followed by absorption which continued until the solution had a 

 resistance comparable to that of the distilled water used in making the 

 solutions. 



(12) There was no evidence that the sand used significantly altered 

 the concentration or composition of the nutrient solution. Other reasons 

 are suggested for differences between sand and solution cultures. 



(13) The mode of expressing the composition and concentration of the 

 nutrient solution is discussed, and it is suggested that the theoretical 

 total osmotic pressure of the nutrient solution is not necessarily significant 

 in its relation to the plant. It is also concluded that the interpretation 

 of results should be based on the composition of the solution in terms of 

 ions or radicles rather than of the salts used in preparing the solution. 



