SOIL TYPES 257 



1. Arid soils poor in humus: 



a. Extremely arid desert soils. 



b. Arid and semiarid saline soils (solonetz,' solontschak, white alkah, black 

 alkali). 



c. Semiarid steppe and loess soils (brown, gray, and reddish steppe soils). 



2. Humid soils of warm regions poor in humus: 



a. Soils with considerable moisture; tropics (laterite). 



h. Soils with moderate to little moisture; subtropics (red soil; terra rossa. 

 Pro]>ably also the little-known yellow soils). 



3. Humid soils of temperate and cold regions, rich in humus: 



a. Soils with moderate to little moisture (black soils (tschernosems), degraded 

 black soils, chestnut soils). 



b. Soils with medium moisture and leaching (brown and gray forest soils, brown 

 soils proper). 



c. Soils with medium to high moisture and severe leaching (pale earth, podsol; 

 and podsol-like soils). 



d. Soils with very high moisture (climatically conditioned peat and moor soils: 

 tundra soils). 



e. Soils with very high moisture and humus (alpine humus soils); 



Besides these main soil types, probably a large number of subtypes 

 of more local distribution are to be distinguished, whose definition must 

 remain in the hands of the soil scientists. 



In general, decreasing salt content of the top soil corresponds with 

 increasing humus content in the above arrangement. In arid climates 

 more water is evaporated than is received. Consequently, a rising 

 stream of water exists, and soil salts accumulate in the upper horizons. 

 The humus acids derived from decomposing plant matter are neutral- 

 ized almost immediately in the Mediterranean climate, so that even 

 upon silicate soil, on the surface, high pH values are found. Con- 

 versely, in humid climates the precipitation exceeds evaporation. 

 The flow of water is downward; the top-soil layer is leached out more or 

 less, and the soluble matter is carried away with the water or deposited 

 in lower soil layers. Inasmuch as organic remains are decomposed 

 only slowly at low temperature, the humus accumulates more and more, 

 which leads to acidification of the soil and to low pH. 



The relation between precipitation and evaporation and climatic 

 soil formation has been investigated by Meyer (1926). On account of 

 the paucity of evaporation data, he has considered the best expression 

 of the relative moisture of a habitat to be the quotient of precipitation 

 and saturation deficit, which has come to be known as the "N. S. 

 quotient" (see p. 143). 



1 The solonetz soil under special local conditions may appear extrazonally in 

 adjoining climatic regions. This also applies to the other climatic soil types. 



