4.3. Balance of soil nutrient reserves and aboveground
nutrient content
Nutrients with very unstable indices (>100%) are P and K,
except for P in V. ferruginea (87%) and K in P. caribaea (59%). Ca
and Mg resulted in unstable stability indices that range from
16% to 68% depending on species, and N is the nutrient with
the less unstable values ranging from 1.5 to 4. Hence, given
that these sites are growing on Ultisols and Oxisols, with
minimal to no weatherable minerals that can resupply
bioavailable forms of soil nutrients; the soil nutrients reserves
on these sites may not support many rotations of trees
without nutrient amendments. In general terms P. caribaea
resulted with the better nutrient stability indices in comparisons
with broadleaf species.This species has the lowest
concentrations of nutrients in the bark and steam, with the
exception of P (V. ferruginea has the lowest value). At present,
forest plantations in Costa Rica concentrates in two species T.
grandis and G. arborea and are often planted on infertile and
degraded soils that are notable for their low nutrients reserves
to sustain a good tree growth under intensive management.
Taking this into account, if managers are seeking productivity
models that guarantee sustainability, plans should be
considered that includes residues management and nutrient
amendments to replenish soil nutrient reserves during rotation cycle. If these plantations are taking place on medium
to high fertility soils, nutrient losses by harvesting could be
relatively small and probably be replaced by soil weathering
and natural inputs such as rainfall.