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 therefore 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 compacting 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 rainfall the conditions are modified in several respects. Grass sward rarely exists, nearly all grasses assuming the habit of growing in tufts or bunches some distance (a foot or two) apart; hence the name of “bunch grass” commonly used, which however means not any one definite kind of grass, but serves to distinguish the grasses of the uplands from those of the moist lowlands, where true sward may be found. Between these bunches of grass the soil is fully exposed, and being free from roots and leaf-covering is compacted, _unless its nature is such that the usually gentle rains do not produce a serious crusting of the surface_.