86 The Substratum 



of the ground. For these various reasons calcium tends to accumulate 

 in the upper layers of the soil, and from this fact the name "ped-o-cal" 

 is derived. The retention of calcium carbonate and magnesium car- 

 bonate helps to prevent pedocals from becoming acid. 



An outstanding example of the interaction of climatic, edaphic, and 

 biological agents in the development of a pedocal soil is furnished by 

 the chernozems, or blackerths, that occur from the Dakotas south- 

 ward. The climate is less arid than it is farther west, and a good 

 grass cover is typically present. The action of the ground water and 

 of the vegetation in retaining salts and other nutrient materials in the 

 upper layers and in maintaining a neuti'al or slightly alkaline condi- 

 tion is indicated schematically in Fig. 3.11. Organic matter tends to 

 accumulate in the A horizon, resulting in the development of the rich, 

 dark soil condition familiar in the region. Chernozems are conse- 

 quently especially valuable for grazing or for farming because when 

 left undisturbed, or properly managed, the ecological processes pres- 

 ent tend to perpetuate soil fertility. 



The pedolfer division consists of soil groups found principally in 

 the more humid regions of the eastern half of the United States. 

 Here the rainfall is heavier with usually more than 75 cm falling per 

 year. The solubles of the soil tend to be carried beyond the reach of 

 the roots by the large amount of water percolating tluough the upper 

 horizons. Much of this water filtering through the soil reaches the 

 water table and drains off valuable nutrients,. The roots of the typical 

 forest vegetation growing on such soils extract salts but do not remove 

 relatively as much calcium as do the grasses. The annual leaf-fall is 

 on the surface, and some of the organic matter resulting from decay 

 is carried away by the run-off, particularly in hilly regions. This loss 

 of nutrient materials is in marked contrast to the situation with the 

 pedocals and the grass vegetation of the prairies. 



In the north-central and northeastern parts of the United States 

 the increased rainfall and lower temperatures tend to accentuate the 

 processes just described. Soils of the podzol group, which occur in 

 these regions, represent an extreme development of the conditions 

 found in the pedalfer division (Fig. 3.12). In the podzols the 

 solubles are rapidly lost by leaching. The A horizon becomes acid 

 because of the accumulation of leaves, needles, and other organic 

 debris on and in the surface strata. Calcium is dissolved from the 

 upper layers and is often almost completely extracted from the soil. 

 Aluminum and iron are similarly carried down but tend to be precipi- 

 tated again as silicates in the less acid B horizon, and may cause the 

 formation of hard-pan at this level. The tendency toward relative 



