September 15, 1922] 



SCIENCE 



295 



In order that this reaction continue indef- 

 initely, it is necessary that both products of 

 the reaction be removed in somewhat the pro- 

 portion that they are produced. This is the 

 condition that actually exists with plants like 

 buckwheat and sweet clover which use a large 

 amount of calcium. They are thus enabled to 

 feed strongly on rock phosphate, as is found 

 by experiment. In ease the calcium content of 

 a plant is low, and the calcium bicarbonate is 

 not removed in as high a proportion as the 

 soluble phosphate, the rate of solution of the 

 phosphate becomes slower and slower with 

 time and thus the plaiit is unable to feed ad- 

 vantageously on the rock phosphate. This is 

 the ease with plants like oats and corn which 

 have a low calcium content. 



This theory may be tested in other ways; 

 e. g., the immediate availability of rock phos- 

 phate to plants like corn is much greater on 

 acid soils than on the non-acid ones. This is 

 due to the removal and precipitation of the 

 calcium bicarbonate from solution by the soil 

 acids. The effect on the availability of the 

 phosphate is the same as though the calcium 

 bicarbonate were removed by the plant. Work- 

 ing with quan-tz cultures, Bauer* has shown 

 that the availability of rock phosphate to corn 

 may be increased by leaching the cultures oc- 

 casionally. This leaching removes the excess 

 of soluble calcium bicarbonate and the effect is 

 again the same as though it were removed by 

 the plant. The addition of ammonium salts 

 has also been found to increase the availa- 

 bility of rock phosphate. This is at least 

 partly due to the effect of aanmonium salts in 

 increasing the solubility of calcium bicar- 

 bonate, which has the same effect up to a cer- 

 tain point as is produced by removing the cal- 

 cium bicarbonate. 



There is no question but what the applica- 

 tion of the law of mass action to a study of 

 the conditions of solution of mineral matter 

 around the plant roots makes possible a correct 

 explanation of many differences in the feeding 

 power of plants. The conditions necessary for 

 the continued solution of mineral matter in 

 which two soluble products are foimied at the 



*Soil Science, 9, (1920), pp. 235-247. 



feeding points of roots are explained by this 

 means. When only one soluble product is 

 formed as is the ease in the solution of phos- 

 phorus from iron and aluminum phosphate by 

 hydrolysis and the solution of potassium from 

 orthoclase feldspar by either hydrolysis or car- 

 bonation, and in fact the solution of most 

 bases from the silicates of the soil, differences 

 in the feeding power of plants for these are 

 not explained directly by the foregoing prin- 

 ciple. The reaction of these minerals with the 

 solvent at the feeding points of roots may be 

 represented as follows : 



FePO^ _|_ 3H,^0 ^ Fe(OH).^ -f HPO . 



2KAlSi^0g + H^CO^ + H.O ^ 

 H Al si O -I- 4SiO ' -I- K CO . 



i 2 2 9 I 2 ' 2 3 



In both of the reactions only the last product 

 is soluble and hence is the only product that 

 can be removed either by the plant or in any 

 other way. The conditions of solution are 

 thus the same for all plants since water and 

 carbonic acid are present in all eases. Differ- 

 ences in the feeding power of plants for the 

 essentiail elements of these compounds must, 

 therefore, be due to differences in conditions 

 in the interior of the plants where the elements 

 are actually used by being precipitated out of 

 solution to form an essential part of plant 

 compounds, making it possible for some plants 

 to utilize more completely the elements from 

 dilute solutions than others. In other words 

 some plants can get along with more dilute 

 solutions of certain elements than others. 



The discussion in this connection will be 

 limited to the base-forming elements. These 

 elements are used by plants largely for at least 

 three rather distinct purposes: (1) They are 

 precipitated or held in physical and chemical 

 combination with important colloidal plant 

 compounds or complexes of which they may 

 form an essential part. (2) Jn the form of 

 the carbonate or bicarbonate they are used for 

 the regulation of the reaction of plant pro- 

 teins and other compounds, the plant sap, and 

 precipitation of acids like oxalic out of solu- 

 tion. (3) They may act as carriers of acid 

 forming elements. Potassium is used largely 

 for the first purpose. Caleimn is used for 

 both the first and second purpose. Some 



