296 



SCIENCE 



[Vol. LVI, No. 1446 



plants, for example, the cultivated legumes, 

 many of the eruciferse, and buckwheat, require 

 large amounts of calcium, much of which is 

 probably used for the second purpose. Mag- 

 nesium is probably used to a considerable ex- 

 tent for the third purpose as a carrier of 

 phosphorus. Calcium and potassium may, of 

 course, also be used for the third purpose, but 

 it is only with the first two purposes that the 

 present discussion is concerned. 



In connection with the first purpose it is 

 important to consider the foillowing: There 

 are always at least two important factors 

 which determine how completely an element 

 may be precipitated out of solution; viz., the 

 reaction of the solution and the solubility of 

 the precipitate formed. A proper regulation 

 aS. the reaction is the most important factor in 

 many precipitation processes. As a rule base- 

 forming elements are more completely precipi- 

 tated from a slightly acid or neutral solution 

 than from a more acid one. - 



On this basis plants with a slightly acid or 

 neutral sap especially of the leaves where the 

 most active processes take place should be 

 able to utilize potassium advantageously from 

 a more dilute solution and feed more strongly 

 on a slightly soluble potash mineral like feld- 

 spar than those plants with a more strongly 

 acid sap, providing the reaction of the nutrient 

 solution is favorable for the plants. Although 

 the data available along this line are very 

 meager, an examination of what there are 

 indicates that this is actually the case. The 

 sap of sweet clover leaves^ ranges from slightly 

 acid to slightly alkaline. Of the data known 

 to the writer this is the only case in which the 

 sap is sometimes alkaline. Theoretically the 

 plant should feed strongly on the potash in 

 feldspar and in actual test with quartz cultures 

 Bauer'' found it able to make a normal growth 

 when forced to obtain all of its potash from 

 feldspar. Of the common agricultural plants 

 for which there are data available buckwheat 

 has the most strongly acid sap of any in the 

 leaves and it feeds very poorly on feldspar as 

 should theoretically be the case. In tests with 



5 Haas, A. E. C: Soil Science, 9, (1920), pp. 

 341-368. 



oSoil Science, 12, (1921), pp. 21-41. 



quartz cultures the writer found that alfalfa 

 and sweet olover can obtain the necessary 

 potassium for normal growth from more dilute 

 solutions than corn and buckwheat which have 

 a more acid sap. Much more data are needed 

 before conclusions can be made definitely for 

 all cases. The available data indicate striking- 

 ly the importance of the internal acidity on 

 the feeding power of plants for potassium. 



What has just been said in regard to potas- 

 sium applies only when the nutrient or soil 

 solution has a reaction which is favorable for 

 the plant. If the nutrient solution is distinctly 

 more acid than the plant sap, it will tend to 

 make the plant sap more acid and the situation 

 in regard to feeding for potassium may be 

 greatly disturbed. In this connection it should 

 be noted that alfalfa and sweet clover require 

 relatively large amounts of potassium and 

 quickly suffer from a lack of it, if the even 

 dilute required concentration in the soil solu- 

 tion is not maintained due to a lack of the rela- 

 tively insoluble potash minerals. 



Undoubtedly the solubility of the potassium 

 compounds formed in different plants is also 

 a factor in the feeding power, but it seems 

 reasonable to believe that to some extent the 

 potassium compounds in different plants are 

 similar and hence have somewhat similar solu- 

 bilities. The solubility factor, therefore, be- 

 cause of its greater probable uniformity would 

 not cause as great differences in the feeding 

 power as the internal acidity factor which 

 varies a great deal. 



The relation of the feeding power of a plant 

 for calcium which is to be used for the first 

 purpose stated, to the acidity of the plant sap 

 is probably the same as in the case of potas- 

 sium. The amount of calcium required for 

 the first purpose is, however, usually relatively 

 small and the amount present in the soil solu- 

 tion relatively large so that the use of calcium 

 for the first purpose is not a critical factor in 

 the feeding power of a plant for calcium. 



When plants use high amounts of calcium, 

 the major portion is probably often used for 

 the second purpose previously stated. The 

 feeding power of a plant for calcium for this 

 purpose seems to be related to the acidity of 

 the plant sap, but the relation, as theoretically 

 should be the case, is opposite to what it is 



