PHYSICAL COMPOSITION OF SOILS. 85 



It must be distinctly understood, however, that these figures make no 

 claim to accuracy or invariability. For, the tilling qualities of a soil 

 containing one and the same amount of such " clay " may be very 

 materially modified according to the kind and amount of each of the 

 several grain- sizes of rock powder or sand they contain. 



Influence of fine powders on plasticity and adhesiveness. An 

 admixture of a large amount of fine powders diminishes mater- 

 ially the adhesiveness of a clay soil, even though it may render 

 it even more " heavy " in tillage; while the admixture of coarse 

 sand, even in very considerable proportions, does not greatly 

 influence the adhesiveness of the clay. The latter alone can- 

 not therefore serve as a proper guide or basis for the classifica- 

 tion of soils in respect to tillage; we must also take into con- 

 sideration the nature and amount of the several granular sedi- 

 ments mixed with it. 



Moreover, the nature and especially the adhesiveness of the 

 clay substance as obtained by analysis may vary considerably 

 in the presence of a very large amount of the finest grain-sizes ; 

 among which ferric hydrate or iron rust is especially apt to 

 accumulate predominantly in the clay, considerably increasing 

 its apparent weight and greatly diminishing its adhesiveness. 1 

 In strongly ferruginous soils, therefore, it becomes necessary 

 to take into special consideration the amount of the ferric 

 hydrate or rust which accumulates in the clay substance. The 

 presence of large amounts of humus or vegetable mold also 

 influences materially the adhesiveness and physical properties 

 of the clay obtained by the method described, although most of 

 it remains with the finer powdery sediments or grain-sizes. 

 There are, besides, other colloidal or at least amorphous sub- 

 stances present in all soils, such as silicic, aluminic and zeolitic 

 hydrates, which are all non-plastic, and yet sufficiently fine to 

 form part of the " clay " obtained as above specified. 



Despite these imperfections, (which however can in a meas- 

 ure be taken into consideration in judging of a soil's tilling 

 qualities by its clay content), the figures given in the above 

 table approximate much more nearly to a tangible basis for 

 such estimate, than the utterly indefinite mixtures which under 

 the older methods of analysis have been, and still are to some 

 extent, used as a basis for soil classification by writers on 

 agriculture. 



' This fact emphasizes the impossibility of explaining the plasticity and adhesive* 

 ness of clay simply as a function of fineness of grain. 



