Oct. IS, I9I9 Relation of Nutrient Medium to Plant Absorption 109 



mous fluctuations may be due to these factors. Possibly other condi- 

 tions related to the physical nature of the medium may have an effect. 

 There is also a strong presumption in certain experiments that the total 

 supply of nutrients may limit the yield in the manner just outlined. In 

 any case it would seem necessary to determine what are the limiting 

 factors that prevent the production of an optimum plant. It is possible 

 to obtain in sand and water cultures plants very similar in size to those 

 given in productive fields under equal climatic conditions. The follow- 

 ing data are evidence of this statement. 



AIR-DRY YIELD PER PLANT. 



The plants of the water- and sand-culture experiments referred to above 

 were grown out of doors under light conditions similar but inferior to 

 those obtaining in the soil experiments. In some instances the grain 

 presented a more shrunken appearance in the sand and water cultures, 

 though the total yield and proportion of heads to straw are not very 

 different. 



As a corollary to the foregoing discussion the conclusion is unavoidable 

 that no sufficient evidence has yet been adduced to show that varying 

 salt proportions within a wide range have any significant effect on yield. 

 The validity of such deductions can not be established until the control 

 of the nutrient solutions is more definite and until the interpretation of 

 the data is made with due regard to the significance of variability studies, 

 such as those proposed by Waynick (55) . 



In connection with any critical discussion of the interpretation of data 

 from plant nutrition studies, one other factor must be considered — the 

 relation of solution cultures to sand cultures. Upon this point the liter- 

 ature is in disagreement. Bouyoucos {2) found that solution cultures 

 gave a higher yield than sand cultures with the same solution. Lyon 

 and Bizzell (^5) observed in certain experiments that sand cultures 

 were superior to solution cultures and advanced the hypothesis that 

 absorption took place around the solid particles, so that the plant 

 really obtained its nutriment from a solution of higher concentration than 

 that added to the sand. McCall (27) reached an opposite conclusion. 

 He found that a smaller yield was obtained in sand cultures and that 

 optimum ratio of ions was changed materially. This he attributed to 

 absorption by the sand, changing the composition and concentration of 

 the solution available to the plant. On the other hand, Shive {40, 41) 



