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.
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