212 § 111. ANALYSIS OF SOILS AXD ROCKS. 



CLAY. 



111. This should be taken for analysis in its natural, 

 air-dried condition, and examined by the same processes 

 as have been given for the mechanical analysis of soils, 

 and their treatment Avith acids. Different clays are very 

 differently affected by these agents. 



Conclusions in regard to their agricultural value must 

 be based upon their relative solubility in the acids used, 

 and the composition of the part that is soluble in hydro- 

 chloric acid, and of that which is made soluble by treat- 

 ment with sulphuric acid. 



For technical purposes it will usually answer to treat 

 10-15 grms. of the clay with 6-8 times its weight of con- 

 centrated sulphuric acid, evaporate the mixture to com- 

 plete dryness, exhaust the residue w^ith dilute hydrochloric 

 acid, eliminate silicic acid from this solution, and estimate 

 alumina, ferric oxide, manganese, lime, magnesia, and the 

 alkalies, according to Scheme I., a and c, § 94, omitting 

 the estimation of phosphoric acid, and consequently the 

 addition of ferric chloride ; determine also the silica solu- 

 ble in alkaline carbonate in the residue that is insoluble in 

 hydrochloric acid above. 



Burnt clay may sometimes bo profitably applied as a 

 fertilizer, or an amendment. The examination of such a 

 clay should be conducted in the same manner as described 

 for burnt marl (§ 109,/); particular attention should be 

 paid to the amount of alkalies soluble in water, and in 

 cold and hot hydrochloric acid. 



The proportion of phosphoric acid is not usually any 

 larger than in arable soils. 



