IIO SOILS. 



in volume from consolidated clay to crumb-structure is given by 



-wr>n 



1L. 



; -^.?;;,i8>?iL__ Tr"*-^-/ '''': '-\; 1 i>-^: y -1:$' !:S^^. > .Va'i';'^.^_'.vj'i' / l 



;y-("";--;'v;:-^ ^~'7^T^V?>. >''?T?^- 'T^TT? {"J^S 



FIG. n. Land before and after plowing. The compactness of the soil is indicated by the density 

 of dotting. Before plowing there is a compact surface crust (s), below which the soil becomes less 

 and less compact as we go deeper. After plowing we find the soil (fs, furrow-slice) converted into a 

 loose mass of crumbs (floccules), with increase of bulk. Compacted plow-sole at pi 



Wollny (Forsch., vol. 20, p. 13, 1897) at 41.9%, to powder as 



33%. On moistening dry clay 

 increased 36.9%, quartz powder 

 8.01%. When land is plowed 

 in the proper moisture-condition 

 the crumbs of floccules are held 

 together by the surface tension of 

 the capillary films (menisci) of 

 water at the points of contact. In 

 the case of sands, the crumbs will 

 collapse into single grains when- 



F, G ., 2 .-A soil-crumb magnified to h water _ films evaporate, tin- 



show the particles of which it is com- 

 posed. The particlesare held together by less some cementing substance was 



h bhThrvtte i r t Th a rwtue^p7c dissolved or suspended in the 



between the particles represent air. Water. ( See figure 12). LitTlC 



carbonate is one of the substances most commonly found per- 

 manently cementing the floccules ; hence the ready tillage of 

 most calcareous soils, and especially the loose texture of the 

 " loess " of the western United States, and of Europe and 

 Asia. In these deposits we find sandy and silt aggregates or 

 concretions ranging from ten or more inches in length (loess 

 puppets) to microscopic size, held together by lime carbonate, 

 but collapsing into silt and sand when the material is treated 

 with acid so as to dissolve the cement. The rough surfaces 

 of these aggregates, gripping into each other, explain the 

 stability of the steep loess cliffs in the United States, as well 

 as in northeastern China, as observed by Von Richthofen and 

 Pumpelly. 



Clay is most frequently the substance which imparts at least 

 temporary stability to the crumbs and crumb-structure; this is 



