Similarly in localities where the relative humidity is low, the sunshine abundant, and the temperature high, evaporation may go on so rapidly that the lower soil layers cannot supply the demands made, and the topsoil then dries out so completely as to form a protective covering against further evaporation.  It is on this principle that the native desert soils of the United States, untouched by the plow, and the surfaces of which are sun-baked, are often found to possess large percentages of water at lower depths.  Whitney recorded this observation with considerable surprise, many years ago, and other observers have found the same conditions at nearly all points of the arid region.  This matter has been subjected to further study by Buckingham, who placed a variety of soils under artificially arid and humid conditions.  It was found in every case that, the initial evaporation was greater under arid conditions, but as the process went on and the topsoil of the arid soil became dry, more water was lost under humid conditions.  For the whole experimental period, also, more water was lost under humid conditions.  It was notable that the dry protective layer was formed more slowly on alkali soils, which would point to the inadvisability of using alkali lands for dry-farm purposes.  All in all, however, it appears “that under very arid conditions a soil automatically protects itself from drying by the formation of a natural mulch on the surface.”

Naturally, dry-farm soils differ greatly in their power of forming such a mulch.  A heavy clay or a light sandy soil appears to have less power of such automatic protection than a loamy soil.  An admixture of limestone seems to favor the formation of such a natural protective mulch.  Ordinarily, the farmer can further the formation of a dry topsoil layer by stirring the soil thoroughly.  This assists the sunshine and the air to evaporate the water very quickly.  Such cultivation is very desirable for other reasons also, as will soon be discussed.  Meanwhile, the water-dissipating forces of the dry-farm section are not wholly objectionable, for whether the land be cultivated or not, they tend to hasten the formation of dry surface layers of soil which guard against excessive evaporation.  It is in moist cloudy weather, when the drying process is slow, that evaporation causes the greatest losses of soil-moisture.

The effect of shading

Direct sunshine is, next to temperature, the most active cause of rapid evaporation from moist soil surfaces.  Whenever, therefore, evaporation is not rapid enough to form a dry protective layer of topsoil, shading helps materially in reducing surface losses of soil-water.  Under very arid conditions, however, it is questionable whether in all cases shading has a really beneficial effect, though under semiarid or sub-humid conditions the benefits derived from shading are increased largely.  Ebermayer showed in 1873 that the shading due to the forest cover reduced