In seeking a
suitable explanation of this phenomenon it must be recalled that the
proportion of available plant-food in arid soils is very uniform to
great depths, and that plants grown under proper dry-farm conditions
are deep rooted and gather much nourishment from the lower soil
layers. As a consequence, the drain of a heavy crop does not fall
upon the upper few feet as is usually the case in humid soils. The
dry-farmer has several farms, one upon the other, which permit even
improper methods of farming to go on longer than would be the case
on shallower soils.
The great depth of arid soils further permits the storage of rain
and snow water, as has been explained in previous chapters, to
depths of from ten to fifteen feet. As the growing season proceeds,
this water is gradually drawn towards the surface, and with it much
of the plant-food dissolved by the water in the lower soil layers.
This process repeated year after year results in a concentration in
the upper soil layers of fertility normally distributed in the soil
to the full depth reach by the soil-moisture. At certain seasons,
especially in the fall, this concentration may be detected with
greatest certainty.
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