CALIFORNIA SOIL CHARACTERS 29 



as the original surface soil. The unconcern 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 region, 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 ''vitalized" by the fallowing effect of the atmos- 

 phere, and to acquire the needful amount of humus or vegetable 

 mold. Thus the surface 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 disinte- 

 gration 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 uncommonly 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 permit 

 of the penetration of roots to depth 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 perennials 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 deciduous fruit trees 

 to grow thriftily and bear fine fruit through six months of drouth 

 while as many weeks of drouth may bring distress and loss of 

 fruit to surface-rooting trees on the shallow soils of the humid 

 region. 



Richness. The foregoing conditions are rendered the more sig- 

 nificant and effective through the third characteristic of soils 

 formed in arid climates. The average aggregate amounts of plant- 



