Value of Lightness and Depth. 29 
portant in regions of abundant rainfall, is largely obliterated in 
arid climates. Very commonly hardly a perceptible change of 
tint or texture is found for depths of several feet; and what is 
more important, material from such depths, when thrown on the 
surface, oftentimes subserves the agricultural uses of a soil 
nearly or quite as well as the original surface soil. The uncon- 
cern with which irrigators proceed to level or otherwise grade 
their land, even though this may involve covering up large areas 
of surface soil with subsoil from several feet depth; the rapidity 
with which the red loam of the placer mines of the Sierra Nevada 
foot-hills is re-covered with the natural forest growth of the re- 
gion, etc., are examples familiar to the residents but surprising 
to newcomers, who are accustomed to dread the upturning of 
the subsoil as likely to deprive them of remunerative crops for 
several years, until the ‘raw’ subsoil has had time to be “vital- 
ized” by the fallowing effect of the atmosphere, and to acquire 
the needful amount of humus or vegetable mold. Thus the sur- 
face soil, which in the humid regions supplies the bulk of the 
nourishment, becomes here of minor importance, serving chiefly 
as a mulch to prevent waste of moisture; while the active process 
of nutrition occurs in the deeper portion of the soil stratum, 
whose composition, as well as condition of disintegration and 
aeration, is substantially the same as above. The second foot is 
rarely found to differ materially from the first, even as to humus 
content; for the latter, being almost exclusively derived from the 
humification of roots, the leaves and herbage on the surface 
being mostly oxidized away under the intense heat of summer; 
it not unconimonly happens in very porous soils that the first six 
inches of surface soil are poorer in humus than the second foot. 
Practical Results of Lightness and Depth—The “lightness” 
and perviousness of the prevailing soils of the arid region per- 
mit of the penetration of roots to depths which in the humid 
region are inaccessible to them on account of the dense subsoils, 
which prevent the needful access of air. This deep penetration 
enables even annual plants to avail themselves directly of the 
stores of moisture in the substrata, at depths which in the humid 
region are scarcely reached save by the tap-roots of some per- 
ennials and trees; while the latter themselves reach depths never 
approached by them in the region of summer rains. Professor 
Hilgard has personally found the ends of the roots of grape-vines 
at a depth of twenty-three feet, in a gravelly clay-loam; and from 
ten to fifteen feet are ordinary depths reached by the root system 
of fruit trees. Such depth of rooting, when conservation of 
moisture is secured by proper surface cultivation, enables decid- 
uous fruit trees to grow thriftily and bear fine fruit through six 
months of drouth while as many weeks of drouth may bring 
