148 SOIL CONDITIONS AND PLANT GROWTH 



so largely to this group that the method is really applicable in by far 

 the great majority of cases. 



Among the mineral soils there are indications of chemical groups 

 cutting across the mechanical classification, but it is not easy to trace 

 them because so much of every soil is silica. When, however, the clays 

 are separated out and subjected to hydrofluoric acid treatment or fusion 

 with alkalis and then analysed, they are seen to fall into two or more 

 types as shown in Table XXIV. (p. 53). Further evidence of dissimi- 

 larity among the clays is obtained by a study of the results of the acid 

 extraction of the soil ; in general the alumina is about one-third of the 

 clay in amount and the potash is roughly one-tenth of the alumina ; in 

 exceptional cases, however, and usually where the abnormal clay occurs, 

 very different relationships obtain. It will be necessary to accumulate 

 many more analyses of clays before we shall have the material for 

 chemical classifications. 



