Potassium to Growth in Plants. 
9i 
change occurring in concentration or elemental composition. We there- 
fore chose for our working basis a full nutritive solution that has been much 
and very successfully used at Rothamstead. 1 The composition of this 
solution is given in Table I so that the reader may properly appraise the 
changes that we found it necessary to introduce. The type of full nutrient 
(solution A) that we have used was prepared as shown in Table II. The 
difference between solution A and the Rothamstead solution lies in the 
, elemental composition, not the concentration. Solution A contains more 
calcium and nitrogen, but less sodium and chlorine. The increase in the 
amount of calcium used causes the calcium-magnesium ratio to change 
from 2*3 : 1 to 3*8 : 1, but the calcium-magnesium ratio can, as we shall see, be 
changed even to a greater extent without injury resulting ; and the reduc- 
tion in sodium and chlorine cannot be considered a matter of great 
importance since plants may be successfully grown over considerable 
periods of time in their absence. 
The nutritive solution less potassium was prepared by substituting 
calcium nitrate for potassium nitrate on the basis of the nitrogen content of 
the two salts and adding calcium sulphate in the amount required to bring 
up the concentration to 3*04 parts per litre. The method of preparation and 
the elemental composition of this solution (solution B) are given in Table III. 
It will be noticed that the nutritive solution less potassium has the same 
concentration as solution A, and, with the exception of the potassium, differs 
in elemental composition from it only in greater richness of calcium and 
sulphur. The calcium-magnesium ratio is, therefore, also changed, and 
instead of being 3-8 : 1 as in solution A, has become 10*9 : 1. Owing to the 
importance that has been attached to the calcium-magnesium ratio, we did 
not feel justified in carrying the experiment through without at the same 
time verifying the fact that the effect on growth of solution B could not 
be attributed to the change in ratio introduced. We therefore prepared 
a full nutritive solution (solution C) containing a calcium-magnesium ratio 
of 10-9:1. The method of preparation and elemental composition of this 
solution are given in Table IV. It will be immediately noticed that solution 
C has a concentration of 1*18 parts per litre greater than either solutions 
A or B, and that the sulphur content is very nearly twice that of solu- 
tion B. Solution C, however, possesses the same elemental composition as 
solution A, with the exception of the elements calcium and sulphur, which 
are present in larger amounts, and, unless the increase in sulphur or the 
change in the calcium-magnesium ratio has a disturbing influence, should 
produce plants in all points similar. 
The experiment was begun on August 28, and on September 18 the 
plants were removed and the roots and tops separated, the plants from each 
1 Hall, A. D., Brenchley, W. E., and Underwood, L. M. : The Soil Solution and the Mineral 
Constituents of the Soil. Phil. Trans. Royal Soc., Ser. B, cciv, 1914. 
