KNOP'S NUTRIENT SOLUTION 
459 
Lack of evidence, from the analytical data, of adsorption of nutrient 
elements in measurable amounts by any of the solids employed here 
(unless we except the small decrease of calcium in the presence of 
ferric hydrate) is decidedly surprising. The results of True and 
Oglevee (2), Breazeale (3), and Parker (4), previously mentioned, and 
those of McCall (16) would seem to indicate that the amounts of 
materials added in the present investigation should have had some 
detectable adsorptive effect. The last-mentioned author found that 
contact with even relatively coarse solids affected the physiological 
balance of the nutrient solution as compared with water cultures. 
However, the ratio of solid to solution, and the composition of both, as 
employed by McCall, differed from those obtaining in our work. No 
explanation is advanced here for the depression of yield of barley by 
carbon black in the apparent absence of appreciable adsorption or 
toxicity. 
Although the results here given are based on too few data to per- 
mit of anything like conclusive deductions, some of the relations herein 
suggested appear to be worthy of further experimental consideration. 
It is suggested that such changes in the substratum as those here 
observed influence not merely the absorbing power of the roots but 
probably also the metabolism of the whole plant. 
Summary 
1. Barley and pea plants were grown twenty-one days in a Knop's 
nutrient solution to which had been added: (a) Fe(0H)3; (b) H2Si03; 
(c) carbon black; each upon two planes of application. 
2. The weight of the dry barley tops was increased approximately 
50 percent by the addition of Fe(0H)3 to the solution, was appreciably 
depressed by the addition of carbon black, and was unaffected by 
HaSiOs. 
3. The weight and length of barley roots were not seriously af- 
fected by any of these added substances. 
4. The weights of dry tops of barley were inversely proportional to 
the hydrogen-ion concentration of the solution, but the total range in 
the acidity of the cultures was comparatively small. 
5. The growing barley plants exerted, in all cases, a neutralizing 
effect upon the reaction of the solution. 
6. Over ninety percent of the phosphorus of the Knop's solution 
