506 



SCIENCE 



[N. S. Vol. XLII. No. 1084 



herents of the colloid theory base their con- 

 clusion reads as follows: The relative affinity 

 of the acids is independent of the nature of 

 the base. It must he carefully noted that this 

 law only holds when all the reacting sub- 

 stances are in a true solution, or if there are 

 partially soluile substances formed, then in 

 any series of comparisons, the solubility of 

 the corresponding substances must be of the 

 same order. The opportunity for secondary 

 or side reactions must also be eliminated or 

 made comparable. In the soil there is almost 

 unlimited opportunity for these side reactions 

 to occur. Most previous investigators of soil 

 acidity and absorption have entirely over- 

 looked and ignored these most important con- 

 siderations and hence have accordingly ar- 

 rived at erroneous conclusions. 



In order to overcome these difficulties and 

 make the conditions more comparable, the 

 writer has proceeded as follows : Small amounts 

 of very finely powdered soil were thoroughly 

 shaken with comparatively large amounts of 

 the respective salt solutions for a short period 

 and then quickly filtered, and the acidity of the 

 filtrate determined. When this was done the 

 soil tooh up very nearly equivalent amounts 

 of different bases from salts having a common 

 acid ion. The use of very finely powdered soil 

 and the procedure of thoroughly shaking made 

 the conditions more like those of a true solu- 

 tion than was the case of other investigators. 

 The use of an overwhelming mass of salt solu- 

 tion in comparison to the soil used equalizes 

 the side reactions to a large extent. The main 

 reaction takes place almost instantly, and hence 

 the treatment should be short in order to pre- 

 vent further side reactions. The results of 

 these experiments point strongly to the exist- 

 ence of true acid substances as the cause of 

 soil acidity. 



Besides this active acidity, there is in soils 

 usually a considerable amount of inactive 

 acidity, which comes into play on continued 

 treatment with salt solutions or basic mate- 

 rials. The writer has evidence, though not 

 conclusive, which indicates that kaolin is a 

 substance which may exist as either an active 

 or an inactive acid. It is possible that rear- 



rangements of the molecule, or polymeriza- 

 tion may be the cause of this. In contact with 

 basic material, kaolin gradually takes up more 

 and more base, which may be due to the grad- 

 ual change of kaolin to the active acidic 

 form. Treatment with carbon dioxide or 

 other acids also seems to cause a change to 

 the active form. 



The writer and assistants have perfected a 

 new quantitative method for soil acidity. In 

 this method 25 g. of soil are moistened in a 

 casserole with 35 c.c. of water and then 

 treated with an excess of barium hydroxide 

 solution. This is allowed to act for one min- 

 ute with constant stirring and then carbon 

 dioxide is immediately passed in, changing 

 the excess of hydroxide to the carbonate. The 

 material is evaporated to complete dryness on 

 a steam bath, and then the excess of hydroxide 

 is measured by determining the carbonates 

 present. A special form of apparatus has 

 been devised for making this carbonate de- 

 termination. It can also be used for determin- 

 ing carbonates in non-acid soils. When the de- 

 termination is made by this method, it makes 

 little difference whether calcium, barium or 

 sodium hydroxide is used to neutralize the 

 acidity. If soil acidity is due to colloids, then 

 according to the properties of colloids it should 

 take different chemical equivalent amounts 

 of sodium, barium and calcium hydroxide. 

 This, again, indicates very strongly that soil 

 acidity is due to true acids and not colloids. 

 The writer has a considerable amount of other 

 data which bears on the subject at various 

 angles, but always supports the existence of 

 true acids as the cause of soil acidity. 



There is no question but what colloids exist 

 in soils. Some colloids absorb bases and others 

 absorb the acid ion. The amount of absorp- 

 tion, however, in the case of the pure colloids 

 which have been worked with in the labora- 

 tory is very small in comparison to the phe- 

 nomena going on in acid soils. The absorption 

 capacity of these colloids is practically negli- 

 gible as far as soil acidity is concerned. The 

 wrongly so-called selective absorption of bases 

 in soils has received a large amount of atten- 

 tion during the past fifty years, but the ex- 



