THE DENSITY AND VOLUME-WEIGHT OF SOILS. m 



one of its most important functions in soils, as it serves to 

 maintain tilth once imparted by cultivation, even after the land 

 dries out. Beating rains, and cultivation while too wet, will 

 in this case of course destroy the crumbs and the loose tilth. 



Other substances which greatly aid the maintenance of tilth 

 are the several humates (of lime, magnesia, iron), which when 

 fresh are colloidal (jelly-like) like clay itself, but unlike the 

 latter, when once dried do not resume their plastic form by 

 wetting (Schloesing). The crumbs thus formed are there- 

 fore quite permanent and contribute to the looseness of soils 

 rich in humus. One part of lime humate is said by Schloesing 

 to be equal in cementing power to eleven parts of clay. 



Silica, silicates and ferric hydrate are sometimes found 

 cementing soil crumbs, wholly or in part. 



The importance of the ready penetration of air, water and 

 roots thus rendered possible is obvious; and the question arises 

 how it happens that wild plants are able to do without tillage. 



How Nature Tills. When we examine the undisturbed soil 

 of woods or prairie in the humid region, we will as a rule find 

 the natural surface soil in a very good condition of tilth ; the 

 obvious cause being the presence in it of an abundant network 

 of surface roots and rootlets of grasses and herbs, which in 

 connection with the fallen foliage prevent the beating and com- 

 pacting of the soil surface ; which can be seen to happen before 

 the observer's eyes whenever a heavy rain falls on a bare land 

 surface, however well tilled. 



Crusting of Soils. In some soils, especially of the Gulf 

 States, the beating of rain followed by warm sunshine so 

 effectually compacts the surface that in the case of taprooted 

 plants like cotton, it becomes necessary to cultivate after each 

 rain, so as to break the crust that would otherwise not only 

 prevent the proper circulation of air, but would also serve to 

 waste the moisture of the land. The same land in the wild 

 condition suffered no such change, being protected by the 

 native vegetation, and by fallen leaves. (See chapt. 8). 



Soils of the arid region. In the regions of deficient rain- 

 fall the conditions are modified in several respects. Grass 

 sward rarely exists, nearly all grasses assuming the habit grow- 

 ing in tufts or bunches some distance (a foot or two) apart; 



