These deductions as to the depth to which plant roots will penetrate the soil in arid regions are fully corroborated by experiments and general observation.  The workers of the Utah Station have repeatedly observed plant roots on dry-farms to a depth of ten feet.  Lucerne roots from thirty to fifty feet in length are frequently exposed in the gullies formed by the mountain torrents.  Roots of trees, similarly, go down to great depths.  Hilgard observes that he has found roots of grapevines at a depth of twenty-two feet below the surface, and quotes Aughey as having found roots of the native Shepherdia in Nebraska to a depth of fifty feet.  Hilgard further declares that in California fibrous-rooted plants, such as wheat and barley, may descend in sandy soils from four to seven feet.  Orchard trees in the arid West, grown properly, are similarly observed to send their roots down to great depths.  In fact, it has become a custom in many arid regions where the soils are easily penetrable to say that the root system of a tree corresponds in extent and branching to the part of the tree above ground.

Now, it is to be observed that, generally, plants grown in dry climates send their roots straight down into the soil; whereas in humid climates, where the topsoil is quite moist and the subsoil is hard, roots branch out laterally and fill the upper foot or two of the soil.  A great deal has been said and written about the danger of deep cultivation, because it tends to injure the roots that feed near the surface.  However true this may be in humid countries, it is not vital in the districts primarily interested in dry-farming; and it is doubtful if the objection is as valid in humid countries as is often declared.  True, deep cultivation, especially when performed near the plant or tree, destroys the surface-feeding roots, but this only tends to compel the deeper lying roots to make better use of the subsoil.

When, as in arid regions, the subsoil is fertile and furnishes a sufficient amount of water, destroying the surface roots is no handicap whatever.  On the contrary, in times of drouth, the deep-lying roots feed and drink at their leisure far from the hot sun or withering winds, and the plants survive and arrive at rich maturity, while the plants with shallow roots wither and die or are so seriously injured as to produce an inferior crop.  Therefore, in the system of dry-farming as developed in this volume, it must be understood that so far as the farmer has power, the roots must be driven downward into the soil, and that no injury needs to be apprehended from deep and vigorous cultivation.

One of the chief attempts of the dry-farmer must be to see to it that the plants root deeply.  This can be done only by preparing the right kind of seed-bed and by having the soil in its lower depths well-stored with moisture, so that the plants may be invited to descend.  For that reason, an excess of moisture in the upper soil when the young plants are rooting is really an injury to them.