THE DENSITY AND VOLUME-WEIGHT OF SOILS. 



109 



tent proportionate to the diminution of solid mass thus brought 

 about. The pore-space might in that case, with the oblique 

 arrangement of the globules as shown in Fig. 10, be as high as 

 74.05 per cent. But since the soil particles may be of all 

 shapes and sizes within the same soil, and usually fit much 

 more closely than would globular grains, the empty space rarely 

 approaches (only in certain alluvial soils and in loose mulches) 

 to the figure last named. In sandy soils it may fall as low as 

 20%, and in coarse gravelly soils even as low as 10%. Most 

 cultivated soils range between 35 and $0% of empty space. 



Effects of Tillage. That these figures can be only approxi- 

 mations is obvious from the consideration that one and the 

 same soil will vary materially in its volume-weight according 

 to its temporary condition of greater or less compactness. 

 After land has been beaten by winter rains, its volume-weight 

 will be found to have materially increased from the well-tilled 

 condition brought about by thorough cultivation. This differ- 

 ence is strikingly seen when, in plowing, the height of the 

 ground on the land side is compared with that of the turned 

 furrow-slice in well conditioned loamy land. This loose con- 

 dition is called tilth, and it results from the formation of 

 relatively large, complex crumbs l or floccules, between which 

 there are large air spaces that were wholly absent in the un- 

 tilled land ; the floccules themselves being also more loosely 

 aggregated than was the case before tillage. 



Crumb or Flocculated structure. Figure 1 1 illustrates the 

 difference between the unplowed land, consolidated especially 

 on the surface by winter rains, and in its upper portion con- 

 sisting largely of single grains ; while the plowed land, toward 

 which the furrow-slices have been turned, is greatly increased 

 in height and volume and consists almost wholly of variously- 

 shaj>ed and-sized aggregates or iloccules, loosely piled upon 

 one another and separated by large interspaces. The increase 



1 The word crumbs, which is generally understood as meaning a relatively large, 

 loose aggregate, seems preferable to the word kernels, suggested for the same by 

 King (Physics of the Soil, p. 110). Kernels are understood to be bodies rather 

 more solid than the surrounding mass, and do not convey the idea of loose aggre- 

 gates. The word " Krumelstructur " (crumb-structure), adopted by Wollny for 

 this phenomenon, has both fitness and priority in its favor. 



