Of the zonal soils of cool humid climates the most widely distributed are the podzol soils (or simply, the podzols), found closely associated with the sub arctic climate, the more northerly parts of the humid continental climate, and the cooler parts of the marine west coast climate. Podzol soils require a cold winter and adequate precipitation distributed throughout the year. The pedogenic regime is that of podzolization.
The podzol profile is the soil profile from, which the various soil horizons were originally named. At the very top, lying on the soil surface, is a layer of leaf mold and acid humus termed the A-horizon. Below this is the first soil layer, designated as the horizon. It is a thin, acid layer, rich in humus and varying in colour from gray through yellowish brown to reddish brown. The A1 horizon is rich in colloids and is a zone of interaction between acids and bases.
Below the A2 horizon is a distinctive light coloured zone called the A2 horizon. This is the strongly leached horizon from which colloids and bases have been carried down. It sometimes has a bleached ash-gray or whitish-gray colour because coloring agents such as the iron oxides and colloidal humual have been removed. Leaching such as occurs in the 2 horizon of the podzol constitutes eluviation.
Below the A2 horizon of the podzol profile is the B-horizon, a brownish zone that is enriched by the colloids and bases brought down from the A2 horizon. The process of accumulation is one of illuviation. The colloids give a heavy, clayey consistency to the B-horizon. Excessive deposit of oxides may cause the soil particles in this horizon to become strongly cemented into stony material known as hardpan, or, in European soil terminology, ortstein. Nodules of soils in the B-horizon formed by the same process as hardpan are termed concretions. They may be composed of clay cemented by limonite, a hydrous iron oxide compound. Altogether the A and B-horizons of the podzol profile total less that 3 ft (1 m) in thickness.
Podzol soils are low in fertility. Leaching of important plant constituents is shown by the association of coniferous forest with this soil. Coniferous need little of the calcium, magnesium, potassium, and phosphorus, which many other plants require. Consequently the trees do not bring these bases to the ground surface from which they can be restored to the upper soil horizons. Certain species of oaks have a similar lack of use of the bases.
The podzols cannot produce the crops to feed a large population. Addition of lime and fertilizers to the soil will largely correct soil acidity and replenish the leached bases, but the favorable areas for such treatment are limited by the effects of continental glaciation. Bouldry morainal topography interspersed with swamps and lakes still renders much of the podzol soil area unfit for farming.
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