Geoscience Reference
In-Depth Information
many fi ne roots and has a nearly neutral reaction and a very low value of bulk
density. Its profi le gradually changes color within the transition to parent volcanic
material. This transition, sometimes not properly denoted as B horizon, is gray
brown and maintains high porosity and a high hydraulic conductivity. However, if
there is a horizon with more intensive weathering below the A horizon, it is appro-
priately designated as B horizon. Below it at the bottom of the profi le, the parent
materials appear (C horizon). The profi le of Andosols is more complicated if there
is a repeated deposition of fresh ash that is forming different horizons. Andosols are
fertile and easy to cultivate and have good water storage and high infi ltration.
Moreover, they are easily penetrated by roots. Their single problem may be phos-
phate fi xation caused in certain instances by active Al and Fe. This unwanted prop-
erty is reduced by applications of lime and organic material. A great variety of crops
are planted on Andosols. Paddy rice cultivation is a major land use in lowlands
having a shallow water table. Some national classifi cations recognize them as Black
Dust Soils, Vitrisols, or Volcanic Ash Soils. In the US Taxonomy, Andosols are
known as Andisols.
Podzols are soils with an ash-light-gray horizon below a top humus horizon. The
name was already explained at the end of the introduction in this chapter, and if
roughly translated from the Russian, it would be ash-like soil. Since the original
vegetation was heather and coniferous forests, the organic material for humifi cation
already carried an acid reaction. With humid and cool climatic conditions both con-
tributing to existing acidifi cation processes, the acid reaction caused intensive
weathering with free release of Fe and Al. The soluble metal-humus complexes
commonly known as chelates were formed as humifi cation produced an abundance
of fulvic acids in the presence of mineral particles. The prevailing downward rain-
water percolation caused intensive leaching and the evolution of an ash-like eluvi-
ated horizon just below the humus horizon. Carboxylic and phenolic groups of low
molecular weight humic and fulvic acids act as pliers grabbing the Fe and Al until
they become “saturated” complexes. Being soluble, they are washed down until the
organic part of the molecule has no more “thirst” for ions, at which time the com-
plex material is no longer stable as it was during its transport. The eluviated material
is therefore deposited in the next illuvial horizon typically characterized by dark
brown or rusty brown hues. Dark gray up to black-gray streaks could appear with
accumulation of low molecular humus. In very humid tropical climates, Podzols can
also develop in a different way with their genesis starting and bounded by a perma-
nently washing and fl ushing out soil water regime. The parent rock of sandstones
and their weathering products are poor in clay minerals and plant nutrients. They are
washed downward into the profi le usually with many trace elements already being
captured into the transported metal-humus complexes. All Podzols have an acid
reaction of pH well below 6 and even down below 5 and hence manifest Al toxicity
and are also defi cient in P. Their amelioration must start with neutralization of the
acid reaction, deepening of their plowed topsoil, and drainage and removal of excess
water. Then and only then could fertilization begin. The great majority of national
systems use the name Podzol. The US Taxonomy refers to them as Spodosols.
