Phosphorus Dynamic in the Soil-Plant System ... 29system may provide useful information to avoid excessive fertilization with
undesirable environmental consequences.
Although P-extraction by farming may be low in comparison with the
total P-stock in soils, agricultural practices may strongly affect P-availability
by speeding up mineralization of labile forms and/or affecting the equilibrium
among the different forms.
Chemical and Physical Phosphorus Fractions
From the standpoint of availability to plants, soil P can be divided into
functional pools of differing bioavailability (Tiessen et al., 1982). The
information on soil P transformation between those pools is useful to predict
availability of P during crop cycle and the plant needs, as well as the risk of P
transfer from soil to surface waters. However, these soil P transformations
have received less attention than P stock calculation, attributable to the
difficulties associated with separation of Pi and Po pools, as well as interaction
among them. Complex chemical fractionation procedures have been developed
(Hedley et al., 1982; Olsen & Sommers, 1982) to evaluate the quantity,
availability and dynamics of the main P forms in soil. These procedures were
based on differences in the chemical resistance and biological susceptibility to
degradation, giving rise to extremely detail descriptive models which in some
cases exceed field validation possibilities (Parton et al., 1988).
More recently, physical fractionation methods based on SOM particle size
have been developed to separate organic fractions with different characteristics
and dynamics (Andriulo et al., 1990; Cambardella & Elliott, 1994; Galantini et
al., 2004). This development arises from an increasing awareness that the
turnover is depending on biological processes regulated by soil structure and
that the availability of substrates to decomposers depends on the chemical
nature of the substrate and, also, on the characteristics of its association with
the mineral fraction of soil. It is therefore now possible to distinguish between
SOM associated with the fine and with the coarse fractions, which have
different structures and roles. The fine soil fraction (clay, silt) corresponds to a
more humidified or mineral-associated organic matter (MOM), and the coarse
fraction (different sizes of sand) to less transformed, labile (Janssen, 1984;
Andriulo et al., 1990), light (Janzen et al., 1994) or particulate organic matter
(POM, Cambardella & Elliott, 1994, Galantini & Suñer, 2008). The MOM is
primarily responsible for the union with minerals in the soil, generating the
organo-mineral complexes basic to the formation of micro-aggregates that