This condition is of the greatest importance in developing the principles upon which successful dry-farming rests.  Further, it may be said that while in the humid East the farmer must be extremely careful not to turn up with his plow too much of the inert subsoil, no such fear need possess the western farmer.  On the contrary, he should use his utmost endeavor to plow as deeply as possible in order to prepare the very best reservoir for the falling waters and a place for the development of plant roots.

Gravel seams.—­It need be said, however, that in a number of localities in the dry-farm territory the soils have been deposited by the action of running water in such a way that the otherwise uniform structure of the soil is broken by occasional layers of loose gravel.  While this is not a very serious obstacle to the downward penetration of roots, it is very serious in dry-farming, since any break in the continuity of the soil mass prevents the upward movement of water stored in the lower soil depths.  The dry-farmer should investigate the soil which he intends to use to a depth of at least 8 to 10 feet to make sure, first of all, that he has a continuous soil mass, not too clayey in the lower depths, nor broken by deposits of gravel.

Hardpan.—­Instead of the heavy clay subsoil of humid regions, the so-called hardpan occurs in regions of limited rainfall.  The annual rainfall, which is approximately constant, penetrates from year to year very nearly to the same depth.  Some of the lime found so abundantly in arid soils is dissolved and worked down yearly to the lower limit of the rainfall and left there to enter into combination with other soil ingredients.  Continued through long periods of time this results in the formation of a layer of calcareous material at the average depth to which the rainfall has penetrated the soil.  Not only is the lime thus carried down, but the finer particles are carried down in like manner.  Especially where the soil is poor in lime is the clay worked down to form a somewhat clayey hardpan.  A hardpan formed in such a manner is frequently a serious obstacle to the downward movement of the roots, and also prevents the annual precipitation from moving down far enough to be beyond the influence of the sunshine and winds.  It is fortunate, however, that in the great majority of instances this hardpan gradually disappears under the influence of proper methods of dry-farm tillage.  Deep plowing and proper tillage, which allow the rain waters to penetrate the soil, gradually break up and destroy the hardpan, even when it is 10 feet below the surface.  Nevertheless, the farmer should make sure whether or not the hardpan does exist in the soil and plan his methods accordingly.  If a hardpan is present, the land must be fallowed more carefully every other year, so that a large quantity of water may be stored in the soil to open and destroy the hardpan.

Of course, in arid as in humid countries, it often happens that a soil is underlaid, more or less near the surface, by layers of rock, marl deposits, and similar impervious or hurtful substances.  Such deposits are not to be classed with the hardpans that occur normally wherever the rainfall is small.