The second major great soil group of humid climates consists of the gray-brown podzolic soils. They differ from the podzols in that leaching is less intense and the soil colour is brownish. The various soil horizons correspond with those of the podzols. The, horizon is a moderately acid humus layer. The A2 horizon is a grayish-brown leached zone. It is less intensely leached than in the podzols and, consequently, is neither so light coloured nor so distinctly limited. The B. horizon is thick and yellowish brown to light reddish brown in colour. Like the podzols, it has concentrated colloids and bases.
The gray-brown podzolic soils contain more of the important bases than the podzols but are nevertheless somewhat acid, deciduous forests (maple, beech, oak) grow luxuriantly on the forest soils. These trees bring the bases up from the B-horizon, returning them to the surface as dead leaves and branches. Thus the soil is replenished by these bases in a way not found in the podzols.
When treated with lime and fertilizers, the gray-brown podzolic soils make highly productive farms on which diversified crop farming and dairying are well developed. Gray-brown podzolic soils are also found in the Pacific northwest and on summit areas in the Rocky Mountains.
The gray-brown podzolic soils are also found over much of western Europe in both the marine west coast climate and humid continental climate. Smaller areas occur in the humid continental climate of northern China and northern Japan, Summer-green deciduous forest is associated with these areas of gray-brown podzolic soils.
Of relatively minor extent are the brown podzolic soils, found in southern New England. These are transitional between podzols and gray-brown podzolic. The brown podzolic soils have only a very thin-leached 2 horizon. Although acid, they can be highly productive when heavily limed and fertilized.
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