26 Liliana Suñer and Juan Galantini
and available, and produces more export P cycling and maintaining the
occluded fraction available. The soil organic matter and its fractions are
indicators of changes that are caused by the crop system, which may be
considered to evaluate differences relative to the nutrients that cycle. Not
only is the amount of your contribution but specifically in relation to the
forms of P, may show changes in their structure that will change the way
the nutrient interacts with the soil matrix. On the other hand it could be
detected, that the inclusion of pasture rotation maintains lower levels of
phosphorus available to plants, but this sequence maintains high levels of
P moderately labile, which would be likely to be available during the
cycle culture. Southwest of Buenos Aires in Argentina there is a sensible
equilibrium between the availability of N, P and soil water that entails
taking into account the three factors simultaneously when designing
strategies fertilization. To fertilize P deficient soils is achieved more
efficient water use, offsetting the adverse effects of a dry year.
Fractionating the soil by particle size and forms of P could be simplified
into a conceptual model for a better understanding of the balance of P in
soil.Keywords: P forms, P fractionation, cropping systems
INTRODUCTION
Phosphorus (P) is regarded as the most important soil nutrient after
nitrogen (N) for plant growth and development as it plays key roles in plant
metabolism, structure and energy transformation. The major part of total
phosphorus (Pt) in soil is not available to crop plants (Holford, 1997), but it is
in a dynamic equilibrium between the organic (Po) and inorganic (Pi)
phosphorous forms. Total P content in surface soil layer can oscillate between
200 (coarse textured soils) to 5000 (fine textured soils) mg kg-^1 (Kuo, 1996).
This variability depends on soil and climatic characteristics. While the Pt soil
content may be relatively high only a small and variable fraction is available
for crops. Soil solution contain P ready for plant uptake, which is in a dynamic
equilibrium with plants needs, inorganic P solubilization, organic P
mineralization and phosphate precipitation. Soil P availability depends both on
the initial amount of available P and on the soil’s ability to substitute the ions
taken in areas around plant roots. Thus the organic forms will have a different
degree of lability, depending on the characteristics of the material (from crop
residues to humic substances), the edaphic environment and the microbial
activity The P dynamics in soils and cycling in agro-ecosystems are of