366 



Journal of Agricultural Research voi. xviii. No. ^ 



of 9 high-yielding cultures 5 are corresponding cultures of the 3 series, 

 as is indicated on the diagrams representing high yields of tops, still 

 further points to the conclusion that good salt balance of nutrient 

 solutions for wheat tops is not markedly disturbed when the solutions 

 are diffused as films on the solid particles of an inert substratum in such a 

 manner as to produce even large differences in the degrees of moisture. 

 It is to be noted, however, that there is a slight but general shifting of 

 the physiological salt balance for the group of 9 high-yielding cultures, 

 as a whole, with each increase in moisture content, from a position in 

 the series characterized by lower partial concentrations of potassium 

 phosphate (KHjPOJ to one of higher partial concentrations of this 

 salt and correspondingly lower ones of calcium nitrate (Ca(N03)2) and 

 magnesium sulphate (MgSO^). Thus with the 36 different sets of salt 

 proportions here employed in sand cultures, and with approximately 

 constant total concentrations of the nutrient media, the best physiolog- 

 ical salt balance with the lowest moisture content used was also the 

 best with the medium and with the highest moisture content of the 

 solid substratum. 



Om. 



0.8 



0.7 



0.6 



0.5 



0.4 



0.3 



0.2 



H-65546746748436312112111E132582343321 

 C-23432S53141£31266 4326£738641674161 l_V 



Fig. a. — Average absolute yields of wheat tops for low, medium, and high moisture content of sand 



cultures. 



The foregoing consideiration of relative yields was not intended to show 

 the effect of differences in degrees of moisture content upon the actual 

 growth rates as indicated by the absolute dry-weight yields. To bring 

 out the relation in question, the average absolute dry-weight yield values 



