48 Liliana Suñer and Juan Galantini
fraction represents an active pool. According to this finding, a conceptual
model can be proposed where P could be linked to SOM fractionation
schemes.
The principal flows in this model are
a) weathering and physico-chemical processes, which reduce the size of
coarse minerals until mineral particles reach a size below 50 μm and
are then included in the fine fraction;
b) humification, by which organic inputs are transformed into more
complex molecules with a lower size;
c) Mineralization of SOM fractions, which produce nutrient release and
make P available for plants;
d) physico-chemical equilibrium,
e) P-uptake by plants;
f) recycled P, by which crop residue returned to the soil can improve Po
by a POM increase.Finally, Texture determines P reserves and the equilibrium of its different
forms in the soils of the region under study. The available P, estimated with Pe
content, comes predominantly from the inorganic pool and it is associated to
the fine fraction of the soil. The level of Po is higher in the coarse fraction of
soils with a higher content of fine fractions. Phosphorus content in its different
forms is closely associated with soil size fractions. Size fractionation of SOM
and P forms determination can be in a conceptual model to a better
understanding of soil P equilibriums.
CONCLUSION
Assessing the effect of natural factors and the extent of changes resulting
from agricultural management on soil P-equilibrium can be a useful tool to
make P-management productive and environmentally efficient. Under the soil
and climate conditions of the sites in the semi-arid and sub-humid Argentine
Pampas, soil texture determines P-stock and the equilibrium of the different P-
forms in the area. Available P, estimated from Pe, is mainly associated with the
inorganic P-pool and with fine fraction of soil. Po levels are higher in the
coarse fraction in soils rich in fine fractions. Phosphorus content in its
different forms is closely related to soil particle size fractions. Particle size
fractionation and P-forms can be summarized in a conceptual model to better