EXPERIMENT STATION BULLETINS. S47 



pound, the depression would vary according to the degree of dissociation 

 of this compound. Again, if the substance were in solution, then the 

 same kind of curve should be obtained if only the soil solution alone was 

 titrated with Ca(0H)2. When the soil extract was titrated with Ca(OH). 

 the absorptive curve was not obtained. The type of curve that was ob- 

 tained, consisted of a no-lime absorption or alkaline curve, i. e., the de- 

 pression began to rise almost immediately upon the addition of a small 

 amount of Ca(0H)2 to the soil extract. 



Hence the evidences are strongly in favor of the view that the Ca{OE)o 

 is taken up or ahsorhed ty a substance which is not in solution and does 

 not affect the freezing point depression. Consequently it must he in the 

 solid form. 



The question now is, if the substance which causes the disappearance 

 of the Ca(0H)2 from the solution is solid, what is its chemical and 

 physical composition? An attempt will now be made to answer this 

 question. 



As it is well known, the mineral soils consist largely of silica, alumi- 

 num, and iron, followed in smaller amounts by calcium, potassium, 

 sodium and magnesium. These elements, together with others, combine to 

 form polysilicates. The mineral constituents of the soil, therefore, con- 

 sist of salts of polysilicic acids. These polysilicates being salts of weak 

 acids with strong bases hydrolyse upon being dissolved in water, and the 

 products of hydrolysis consist mainly of an unhydrolyzed salt, a base, 

 and an acid. The acid, which consists probably of silicic acid, alumina- 

 silicates, and silica is more or less insoluble. The bases, however, being 

 more soluble, are carried away by the drainage system, or consumed by 

 the plants, or are dissolved by inorganic and organic acids that might 

 temporarily be formed in the soil. The residue left behind, therefore, con- 

 sists mainly of silica, acid alumino-silicates, and silicic acid. These com- 

 pounds possessing a high reactivity for Ca(0H)2 will absorb it imme- 

 diately upon its being offered to them, and combine with it to form cal- 

 cium silicates. The calcium silicates are difficultly soluble. 



Hence, it appears from all evidences, that the substance which causes 

 the disappearance of the Ca(0H)2 from solution consists mainly of 

 silica, acid alumino-silicates, and silicic acid. This view is strongly sup- 

 ported by the investigation of Mclntire, Willis and Hardy^. These in- 

 vestigators in conducting a study to determine the factors influencing the 

 lime requirement determinations, found that when different soils were 

 treated with large amounts of CaCOig and MgCOg, these compounds 

 were soon decomposed by the silica, acid silicates and titanium oxide, 

 the OO2 being dissipated, and the Ca and Mg radicals combined with the 

 above acids to form calcium, and magnesium silicates and calcium and 

 magnesium titanate. 



On the other hand, these silica, acid silicates, and silicic acid, must be 

 in some particular physical form, probably in the hydrated and colloidal 

 condition, otherwise they do not seem to show any reactivity towards 

 the Ca(0H)2. Thus, for instance, quartz sand both in medium size and 

 in extremely fine powdered form takes up practically no Ca(0H)2. 

 Again, any of the mineral soils which shows a very high lime requirement 

 absorbs practically no Ca(0H)2 after it is burned. 



While the absorption of the Ca(0H)2 may be attributed mainly to the 



(8) Univ. Tenn. Agr. Expt. Sta. BuU. 107. 1914. 



