344 SOIL CONDITIONS AND PLANT GROWTH 



But, on the other hand, mechanical analysis is restricted 

 in its application and breaks down altogether on chalk soils, 

 acid humus or peat soils, and neutral humus soils, while it 

 gives useful indications only on the mineral soils, i.e. sands, 

 loams and clays. Agricultural soils belong so largely to this 

 group that the method is really applicable in by far the great 

 majority of cases. 



Among the mineral soils the chemical grouping cuts across 

 the mechanical classification. When 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 XXIX. (p. 102). Further 

 evidence of dissimilarity 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 other 

 cases, however, very different relationships obtain. 



For the comparison or characterisation of types chemical 

 analysis becomes of considerable importance. For this pur- 

 pose Hissink (136) and von 'Sigmond (266^) have each em- 

 phasised the value of extraction by strong HCl, though they 

 do not agree as to the interpretation of the results, Hissink 

 claims to be able, by suitable modification of the analytical 

 process, to differentiate van Bemmelen's three groups : — 



(i) "Complex A," the "weathered silicates" capable of 

 absorption and exchange of bases. 



(2) "Complex B," the less active "kaolin silicates."^ 



(3) Inert "un weathered silicates" incapable of absorption 



and base exchange effects. 

 The advantages of such discrimination are manifest, but 

 von 'Sigmond is not prepared to admit that it can be done. 

 He considers, however, that strong hydrochloric acid has a 

 practical value for describing soils. Other chemists do not 

 altogether agree : E. A. Mitscherlich sees no advantage in the 

 method ^ (see p. 234). 



^ For a discussion of these see R. Gans, Int. Mitt. Bodenktinde, 1913, 3, 546. 

 ^Ibid.. 1914, 4, 327. 



