ROOT ABSORPTION FROM SOLUTIONS AT MINIMUM 
CONCENTRATIONS 
R. B. Harvey and R. H. True 
In former papers by R. H. True and H. H. Bartlett (i, 2) there 
was found to be a point in the absorption curve for the lupine grown 
in various salt solutions, at which the absorption and excretion of 
electrolytes were in equilibrium. For the lupine this value was 
found to be about 16 x io~*^ normal expressed as NaCl. The conditions 
which exist at this point have been further studied in the squash, 
peanut, and soy bean. The values have been found to differ slightly 
for the various plants used. 
The absorption minimum for the same plant seems to have nearly 
the same value independent of the volume of the solution, the con- 
centration of the salt, or the kind of nutrient salt used. The quantity 
of salt solution offered must, of course, be within the requirements of 
the plant during its growth period, so that the minimum may be 
reached, and the concentration must be below the toxic limit. The 
constancy of the equilibrium concentration when once attained is 
shown by the graph for sweet corn in calcium nitrate (fig. i). The 
value of 12-15/7 X io~^ Ca(N03)2 was held for 24 days by five cultures 
in this series. In cultures of this plant grown in KNO3 and Mg 
(N03)2, practically the same value was obtained. The graph for the 
absorption of the squash in Ca(N03)2 (fig. 2) gives the average values 
for twelve cultures at each concentration indicated. The squash seems 
unable to maintain a concentration as low as corn, giving a value of 
35-40?^ X io~^ Ca(N03)2 as its absorption minimum. The peanut 
is able to maintain a concentration of ^on x io~®; the soy bean one of 
about 2on x lo"*' expressed as Ca(N03)2. Sweet corn gave a more 
nearly constant value than the other plants tested. 
The specific minimum concentration for absorption is controlled 
by at least two classes of factors. One external factor, the concen- 
tration of CO2 in the air and in the solution saturated with it at the 
given temperature, like other similar purely external factors, must be 
the same for different plants growing under the same conditions. 
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