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• 30 • <br />distinct from those that forni naturally in a given microclimate. Anpng <br />these differences are the critical relationships beri.~een effective pre- <br />cipitation and evaporation which determine the zonation a~td character- <br />istics of soils that develop at a site. For example, a small increase <br />in effective precipitation during the season of peak saturation of the <br />soil by meteoric waters will result in an increased depth of soil satu- <br />ration and thus an increased depth to the base of the zone of illuviation <br />or downward migration of clay minerals and soil solutes. So long as ve- <br />getation is adapted to such an increase, the deeper moisture is available <br />for plant growth and mineral cycling pathways are established throughout <br />the active soil colu~. On the other ]land, if species are planted that <br />cannot utilize all the available soil moisture and the depth of saturation <br />is sufficient to carry moisture beyond the seasonal evaporative demands <br />of atmosphere and plants, then a totally 'inappropriate' soil could form <br />that could not sustain a diverse and stable native plant community at the <br />site if irrigation were to cease. If irrigation is controlled so as to <br />maintain a constant depth of moist soil during the growing season, then <br />other soil-forming processes are altered such that, for instance, carbo- <br />nates and other mineral salts would form a more distinct and discrete <br />basal B-horizon zonation in comparison to those more evenly disseminated <br />salts that one finds in naturally-developing local soils. <br />Another potentially critical consideration of effects of irrigation and <br />fertilization are changes in elemental mobility within the soil caused by <br />changes in soil pH. <br />