8 THE PHYSICAL ACTION OF LIME ON CLAY SOILS 



The use of dyes iii the identification of minerals has been undertaken 

 by Pelet and Grand, Hundeshagen, Dittler and Cornu. Certain dyes are 

 used which will be adsorbed by specific substances, and thus an attempt 

 is made to identify the materials present. Unfortunately, it is not always 

 possible to differentiate between the colloidal and crystalloidal matter. 

 Furthermore, it is possible that colloids of approximately the same chem- 

 ical composition may vary in their adsorptive powers for dyes. Such 

 factors as the amount of water of hydration may quantitatively intluence 

 the results. Kohland and von Possanner find that talc and kaolin vary 

 widely among themselves as to their adsorptive properties, and according 

 to Bancroft, the nature of hydrous ferric oxide varies with the method 

 of preparation. 



It is possible that in the soil we have processes which tend to simplify 

 the nature of the colloidal material. Lacroix, in a study of the decompo- 

 sition products of the aluminum silicate rocks, concluded that the end 

 product was hydrous aluminum oxide. Rohland holds the same view. In 

 fact, it seems necessary to assume an hydrolysis of the silicates in order to 

 explain the beneficial action following the application of calcium silicates 

 to the soil. Whether hydrolysis takes place or- not, the probabilities, 

 nevertheless, are that in most soils we have a vast series of colloidal 

 materials present, each varying somewhat in its qualitative and quanti- 

 tative adsorptive power. It is, therefore, apparent that any dyestuff is 

 only very roughly specific with regard to its adsorbent. 



In view of the insolved nature of the subject, there has existed in the 

 literature the greatest confusion with regard to the use of dyes on soils 

 and the interpretation of the results obtained. However, the fact that 

 certain dyes are adsorbed only by certain colloids when prepared in the 

 pure state, permits our obtaining some idea as to the nature of the 

 adsorbing material in the field. The weakness of the method consists 

 in the fact that, owing to the variation in the properties of the colloidal 

 matter, our evidence is circumstantial at best. 



Before proceeding further, it seems necessary to discuss certain factors 

 influencing dyestuff adsorption, and indicate their relation to soils work. 



1. Nature of the dye. The opinion has existed in the soils literature 

 that all dyes were equally valuable, as long as they were adsorbed. No 

 idea could be more erroneous. Lyollema in 1905 found that certain dyes 

 were specific for certain materials. The specificity of dyes has been 

 worked out further by Rohland, and Beaumont; subject, of course, to 

 the limitations reviewed in the preceding pages. We find that the azo 

 dyes are adsorbed practically not at all. Rohland has tried to correlate 

 this peculiarity of the azo dyes with certain molecular configurations. 

 He finds safranine and indigo (the leuco indigo white?) to be specific 

 for humus. Beaumont thinks diamine sky blue is adsorbed by colloidal 

 aluminum. The tri-phenyl methane dyes are taken up by both humus and 



