THE DENSITY AND VOLUME-WEIGHT OF SOILS. 



117 



Loamy and Sandy Soils. It is largely the absence of these 

 extreme changes of volume that renders the cultivation of 

 loamy or even sandy lands so much more easy, and the success 

 of crops so much more safe, than is the case in clay soils. 

 Whenever the content of colloidal clay diminishes below 15%, 

 the shrinkage in drying from the wet condition becomes so 

 slight as to cause no inconvenience; while in sandy soils prop- 

 erly speaking, no perceptible change in volume occurs. 



Peaty soils, however, and all those containing a relatively 

 large amount of humus, are also liable to visible shrinkage 

 when passing from the wet to the dry condition. But on ac- 

 count of their looseness and porosity such shrinkage does not 

 usually result in the formation of cracks or rupture of the 

 roots, as is the case in heavy clay lands. The entire mass of 

 the soil then shrinks downwards, but rarely forms cracks on the 

 surface. Hence the introduction of humus into " heavy " soils 

 is among the best means of improving their tilling qualities. 



Formation of Surface Crusts. Some soils, especially those 

 of a clay-loam character, are very liable to the formation of 

 hard surface crusts from the beating of rains, and from sur- 

 face irrigation ; owing, doubtless, to the ready deflocculation 

 of their clay substance. It is not easy to define the precise 

 physical composition conducive to this crust formation ; but 

 the subjoined physical analyses show examples of soils in 

 which this tendency is very prominent and is frequently an- 

 noying, in that when they occur in the regions of frequent 

 summer rains, it becomes necessary after each one to till the 

 surface in hoed crops (c. g., in cotton-fields) in order to pre- 

 vent the injurious effects of such consolidation of the surface. 

 It may, of course, be prevented by mulching, or on the large 

 scale by green-manuring, to such extent as to prevent con- 

 traction. 



The subjoined physical analyses of two soils from the Brown-Loam 

 region of Northern Mississippi (see chap. 24), shows the composi- 

 tion of lands excellent in every respect other than the tendency to 

 crust after each rain : 



