294 



SCIENCE 



[Vol. LVI, No. 1446 



THE FEEDING POWER OF PLANTSi 



Different species of plants vary greatly in 

 their feeding power or ability to secure the 

 required elements from the natural mineral 

 matter of the soil or from diificultly soluble 

 phosphate and potash minerals which may be 

 applied as fertilizers. The chai'<acter of the 

 native vegetation is in many eases determined 

 partly by differences in the feeding power of 

 plants. Of the cultivated plants it is well 

 known that buckwheat will feed much more 

 strongly on rock phosphate than corn. The 

 subject is thus not only of scientific interest 

 but also of great practical importance. 



It was formerly believed that the mineral 

 matter of soils was made soluble and available 

 to plants through the action of various acids 

 excreted by the plant roots. Later esperiments, 

 especially those by Czapex, indicated that, 

 other than carbonic acid, plants nonnally ex- 

 crete at the most only minute quantities of 

 acids. As is now well known practically all 

 plants excrete through their roots large quan- 

 tities of carbonic acid. Attempts to explain 

 differences in feeding power on the basis of 

 differences in amount of carbonic acid excreted 

 have not been successful. 



It might, however, still be argued that it is 

 not necessary for plants to actually give off or 

 excrete the acids in measurable amounts in 

 order that they exercise an influence on the 

 solution of mineral matter; it might be argued 

 that all that is necessary is for the acids to 

 saturate completely the walls of the root hairs 

 which come in intimate contact with the min- 

 eral particles. That this is not the correct ex- 

 planation is evident from what follows. 



During recent years, by aid of the hydi-ogen 

 electrode much valuable information has been 

 secured regarding the acidity of plant juices.- 



1 Published with the permission of the director 

 of the Wisconsin Agricultural Experiment Sta- 

 tion. 



- Truog, E., and Meacham, M. E., Soil Science, 

 7, (1919), pp. 469-474; Clevenger, C. B., Soil 

 Science, 8, (1919), 217-242; Bryan, 0. C, Soil 

 Science, 13, (1922), 271-302; Bauer, F. C, and 

 Haas, A. R. C, Soil Science, 13, (1922), pp. 461- 

 477. 



This information is aiding greatly to clarify 

 our conception regarding the feeding power of 

 plants and the relation of plant growth to soil 

 acidity and alkalinity. It indicates that the 

 excretion of other acids than carbonic or mere 

 presence of them in the walls of the root hairs 

 is not an important factor in the feeding power 

 of plants, for it is now known that a plant with 

 a nearly neutral sap may feed more strongly 

 on relatively insoluble minerals than one with 

 a decidedly acid sap; e. g., sweet clover and 

 alfalfa with relatively slightly acid root saps 

 of Ph 6 to 7 feed more strongly on feldspar 

 than buckwheat with a relatively strongly acid 

 root sap of Ph 4 to 5. . If plants made the 

 mineral matter of soils available through the 

 excretion or presence of acids other than car- 

 bonic, then the reverse should be true, that is, 

 the buckwheat should feed more strongly on 

 feldspar than ailfalfa and sweet clover because 

 it would excrete or have present much the 

 strongest acid. Similarly corn with a more 

 acid sap than either alfalfa or sweet clover 

 should feed more strongly than the other two 

 on rock phosphate and feldspar if it were a 

 matter of excretion or presence of acids, but 

 again the opposite holds true. Undoubtedly, 

 if data for more species of plants were avail- 

 able many more cases of this kind could be 

 cited. 



It is therefore necessary to find some other 

 explanation for certain differences in feeding 

 power than those thus far given, for evidently 

 there are other factors than the excretion of 

 acids which exercise a controlling influence on 

 the feeding power of plants. A number of 

 yeai-s ago the writer presented a new theory^ 

 regarding the feeding power of plants in which 

 the feeding power for rock phosphate was ex- 

 plained on the basis of the law of mass action 

 and chemical equilibrium. The reaction making 

 the phosphorus of rock phosphate available to 

 plants is one between carbonic acid and the 

 tricalcium phosphate in rock phosphate which 

 may be represented as follows : 



Ca^(PO), + 2H,C0^ ^ 

 Ca^H,(PO^), -f CaH,(COp,. 



3 Science, N. S., 41, (1915), pp. 616-618; Be- 

 search Bulletin 41, 1918, "Wis. Agr. Expt. Sta. 



