Agriculture Reference
In-Depth Information
processes, greater reserves of most nutrient elements occur near the soil surface and,
at a smaller scale, close to the roots. Nutrient reserves are high near roots because of
exudation and their continual death and replacement. Close to the surface, such reserves
are a consequence of the greater inputs of litter and are associated with the higher organic
matter status of the A horizon. Figure I.36 shows the similar and decreasing patterns with
depth in the concentrations of carbon, exchangeable calcium, magnesium and potassium
in a tropical oxisol (Spain
et al.,
1989a). In contrast to the other elements presented, the
concentration of total phosphorus varies little down the profile, probably due to fixation
on the considerable amounts of iron- (haematite, goethite) and aluminium-containing
(gibbsite) minerals present in these soils (Isbell
et al.,
1976). However, as considered
below (see also section I.3.1.2.3), a substantial proportion of this element occurs in organ-
ic form in most soils.
Where the rate of accession of organic matter by the soil exceeds its rate of decom-
position or where decomposition rates remain depressed, as occurs with waterlogging,
little-modified plant remains may accumulate as a litter or peat layer (O or H horizons).
The presence of higher organic matter concentrations near the surface of the mineral
horizons has a number of consequences. Because of the high cation exchange capacity
of soil organic matter (see below), the cation exchange capacity of the whole soil
will normally be greatest near the surface and in locations where the roots are most
densely distributed. Such concentrations of organic matter are normally associated with
higher microbial and faunal activities and thus a more rapid turnover of the contained
nutrient elements.
