OCTOBEE 8, 1915] 



SCIENCE 



505 



samples of milk from the same barns, it is clear 

 that much remains to be done in the perfec- 

 tion of the technique of sampling and bac- 

 teriological analyses of milk. 



These data show how flimsy is the basis for 

 the common belief that there is a relation be- 

 tween the score of a dairy and the quality of 

 the milk produced by it, and how premature 

 the official sanction for the grading of milk by 

 means of dairy scores. 



J. Arthur Harris 



" SOIL ACIDITY AND METHODS FOR ITS DETECTION " ^ 



In a previous issue of Science, J. E. Harris^ 

 published an article entitled similarly as above. 

 In this article Harris states that two theories 

 have been advanced to explain soil acidity, 

 viz., the humic-acid theory and the colloid ab- 

 sorption theory. This same investigator also 

 makes reference to an article published by the 

 present writer on a new method for the de- 

 termination of soil acidity,^ and after quoting 

 the writer in regard to the use of calcium 

 chloride in this method, says: 



This statement brings out very clearly the ab- 

 surdity of the position of those who accept the 

 humic acid theory. These humie acids are sup- 

 posed to be strong enough and soluble enough to 

 liberate hydrochloric acid from calcium chloride, 

 but not strong enough or soluble enough to liber- 

 ate hydrogen sulphide from zinc sulphide. It is 

 also suggested that this method be made the basis 

 for a quantitative determination of the lime re- 

 quirements of the soil. The writer does not be- 

 lieve this is possible because he has shown* that 

 acid soils do not absorb equivalent amounts of dif- 

 ferent ions. 



Although Harris apparently assumes that 

 the vTriter believes soil acidity in upland soils 

 is due largely to the so-called humic acids, yet 

 the writer has never published such views or 

 believed that such was the case. The writer 

 also, wishes to state that he is even more ad- 

 verse to accepting the colloid absorption theory 

 as an explanation of soil acidity, than he is 



1 Published with the permission of the director 

 of the Wis. Expt. Station. 



2 Science, 40 (1914), 49. 



3 Science, 40 (1914), 246. 



iJour. Thy. Chem., 18 (1914), 355. 



to accepting the so-caUed humic acid theory. 

 The acidity of peat and muck soils is undoubt- 

 edly due in part to organic acids. There are 

 upland soils, however, which are practically 

 free of organic matter and still they react 

 strongly acid. Similar soils containing con- 

 siderable organic matter appear to retain all 

 their acid properties even after the organic 

 matter is destroyed with hydrogen peroxide. 

 A¥hat is this inorganic acidity due to ? Harris 

 and many other investigators have assumed 

 that it is due to absorption of bases by soil 

 colloids. They have arrived at this conclu- 

 sion because by their methods of experimenta- 

 tion, acid soils do not take up chemically 

 equivalent amounts of the different bases. 

 Colloids exhibit similar properties as to the 

 absorption of bases, and hence they conclude 

 that soil acidity is due to colloids. Let us 

 carefully examine the facts and draw our con- 

 clusions accordingly: 



Upland soils consist of from 75 per cent, to 

 over 95 per cent, of silicates and silica. Sili- 

 cates are salts of various silicic and alumino- 

 silicic acids. The water solution in the soil 

 slowly reacts with these silicates, forming with 

 the bases of the silicate a soluble hydroxide or 

 salt, which is taken up by plants or removed 

 in the drainage water. The other product, an 

 acid silicate, being comparatively insoluble, 

 accumulates in the soil and gives rise to an 

 acid condition. The writer and assistants 

 have treated powdered basalt, granite, feld- 

 spar and other minerals with carbonated 

 water, and after filtering have obtained resi- 

 dues which are acid to litmus and other tests. 

 This is essentially comparable to the weather- 

 ing process going on in soils. Acid soils 

 treated in this way are made more acid. 



n soil acidity is due to true acids and acid 

 salts such as acid silicates, why have investi- 

 gators not been able to show that acid soils 

 take up equivalent amounts of the different 

 bases from salt solutions? This is due to the 

 fact that the acid silicates and their neutral- 

 ized products are only very slightly soluble, 

 and the solubility of the neutralized silicates 

 varies according to the base that effects the 

 neutralization. The law on which the ad- 



