1905.] 



On the Retention of Bases by the Soil. 



23 



or excreted by the root cells. The nutrient constituents pass through the cell 

 wall of the root hairs by osmosis until the concentration is the same on 

 either side ; such substances as are required by the plant are then withdrawn 

 from action by the protoplasm, thus lowering the concentration on the inside 

 and causing a fresh diffusion of that particular substance through the cell 

 wall. Looking at the question from the standpoint of the ionic hypothesis, 

 the soil water would be a highly ionised solution, in which any given ion will 

 pass by osmosis into the root-hairs as long as the plant maintains the tension 

 of that ion lower on the inside of the cell than in the solution outside. As 

 the plant is always transforming the nitrogen, sulphur and phosphorous from 

 the condition of inorganic acids in which they enter into neutral or even 

 basic organic compounds, and since it also as a rule utilises more of these 

 substances- than of the metallic bases, such acid ions will be withdrawn by 

 the protoplasm in greater quantity, and so must pass through the cell wall 

 from the external solution at a greater rate than the corresponding basic ions, 

 the necessary equilibrium being maintained by the carbonic acid excreted by 

 the root cells.* From this point of view, when the root is drawing nutriment 

 from a solution of a neutral nitrate, the nitric acid ions would be travelling 

 inwards to the protoplasm and the carbonic acid ions outwards, so that the 

 carbonate of the base might be considered as forming outside the cell wall. 

 If, on the other hand, the salt be considered to move into the cell 

 undissociated and there to lose its nitric acid to the protoplasm, the base must 

 then be supposed to diffuse out again as carbonate. The net result, however, 

 is the same under either hypothesis, viz., that after the plant has been 

 growing for some time in a neutral solution it will have taken up an excess 

 of acid and left a corresponding excess of base, now combined as carbonate, 

 in the solution representing the water of the soil. 



The following Table (XII) shows the composition of several crops as per- 

 centages and again recalculated as equivalents of hydrogen, phosphoric acid 

 being reckoned as tribasic, since the soil solution will be mainly derived from 

 tribasic phosphates in the soil. 



It will be seen that there is a considerable excess of acid in the plant, from 

 which it follows that an equivalent amount of base was left behind in the 

 soil. This base is in most cases nearly equivalent to the nitrogen taken in as 

 nitrate, and calculated as calcium carbonate will amount to between 100 and 

 300 lbs. of calcium carbonate per acre. In other words the normal growth of 

 farm crops leaves behind from the salts in the soil used for its nutrition about 

 as much base as would have been previously required for the nitrification of 



* ll'or a discussion of the electrical disturbances such an interchange would involve, see 

 Kohn, < Landw. Versuehs.,' vol. 52, 1899, p. 315. 



