Agriculture Reference
In-Depth Information
Stocks of nutrient and other elements may be calculated for specific areas by summing
the products of their concentrations and the bulk densities for successive horizons or
layers sampled within the soil. These values provide an estimate of the total amounts of
the elements in this soil and, for the nutrient elements, an estimate of overall reserves.
However, they provide little insight into biogeochemical processes, as considered below.
For many elements in many environments, nutrient element stocks included in the
vegetation biomass are small in relation to those held within the rooting depth of
the plant community. However, this may not be true of highly-weathered soils. This is
illustrated in Table I.18 which presents the distributions of four selected nutrient elements
in a Amazonian rainforest ecosystem formed on an extremely acid (pH 3.5) oxisol
(Klinge in Salati and Vose, 1984). The stocks of total nitrogen and phosphorus held
in the phytomass are equivalent to approximately 37 and 47
per cent.
of those present in
the soil. In contrast, soil calcium concentrations were below detection limits and
the amount of magnesium in this highly-weathered soil comprised approximately eight
per cent.
of total ecosystem stocks. Therefore, the stocks of these last two elements,
together with those of potassium and sulphur, occur predominantly in the phytomass.
