48 jenny. ORIGIN OF SOILS [Ch. 2 



porary or permanent ground-water influences produce peat and bog 

 conditions. In arid regions, alkali soils and saline soils may result. 

 Figure 5 illustrates such a hydrosequence in the semi-arid Coalinga 

 area of California. On an expansive, gently sloping, colluvial fan, 

 consisting of outwash from softly consolidated calcareous sandstone 

 and shales, all soil-forming factors — save one — are constant. This 



^2£he. Oxalic 

 Land^^~-^ 2 -^----__LeWs River 



Fig. 5. Schematic illustration of a toposequence of soils influenced by ground 



water in arid California. 



variable factor is represented by the plumb distance from the surface 

 of the soil to the temporary water table which forms during the rainy 

 season. As a result of capillary rise of the salty ground water, the 

 uniform fan material became differentiated into Panoche soil, Oxalis 

 soil, and Levis soil. The Panoche soil, lying on the upper portions of 

 the fan, is nearly free of alkali. The wet end of this hydrologic se- 

 quence, represented by the Levis soil, is strongly impregnated with 

 salt. The Oxalis soil, situated between the two extremes, possesses a 

 nearly salt-free surface soil but has a slight to moderate salt content 

 in the subsoil. 



Combinations of slope and ground-water sequences are often classi- 

 fied as catenas (Bushnell, 1944). 



Climate (Climofunctions and Climoseqtjences) 



Hilgard (1912) in this country and Dokuchaev in Russia showed 

 that soils derived from the same parent material may have widely 

 different properties, depending on the climate in which the soils are 

 formed. Hilgard's comparison of chemical analyses of soils of arid 

 and humid regions (Table 2) has become a classic. In general, soils 

 from arid regions contain more acid-soluble materials than soils from 

 humid regions. The differences are especially pronounced for Ca, Mg, 

 K, and Na. 



The fundamental difference in mode of formation of soils of arid 

 and humid regions is conditioned by the moisture regime. In regions 

 of low rainfall, water penetrates the soil to a limited depth only; 

 weathering and soil formation do take place, but the products are not 



