Phosphorus Dynamic in the Soil-Plant System ... 49understand P-equilibrium in soil. The equilibrium among all P-forms becomes
crucial; it is therefore be advisable to consider the system in a dynamic
equilibrium with organic and inorganic P-forms, and not just take into account
the P available to plants. Soil organic matter (SOM) and its fractions are
physical, chemical and biological indicators of the changes occurring between
systems, and they can be used to assess differences in the nutrients they cycle.
They are important not only in terms of their contribution, but also of their
relationship with P-forms, affecting the way P interacts with the soil matrix.
Also, inclusion of pasture in a rotation has been shown to maintain extractable
or available P to plants at low levels; however, this sequence keeps moderately
labile P at the highest levels, which is likely to become available during the
crop cycle. When water availability is far below the crop needs in the
southwest of Buenos Aires province in Argentina, efficacy of fertilizer
application is assessed in terms of total dry matter yield of wheat and not of
yield increase. Considering the dynamic sensitive equilibrium between N, P
and soil water, it is mandatory to take into account all three factors at the same
time when it comes to defining a fertilization strategy. Phosphorus fertilization
turns water use more efficient, and thus counteracts the adverse effects of a
drier year. Assessment of fertilizer efficiency should be used as an additional
tool, but it is not a decisive parameter in decision-making. No-till changes the
relationship between organic and available forms. It enhances P-cycling and
export and it keeps the occluded fraction unavailable.
REFERENCES
Anderson, C., (1996). Phosphate cycles in energy crop systems with emphasis
on the availability of different phosphate fractions in soil. Plant and Soil
184, 11-21.
Andriulo, A., Galantini, J., Pecorari, C., Torioni, E., (1990). Materia orgánica
del suelo en la región pampeana. I. Un método de fraccionamiento por
tamizado. Agrochimica 34, 475-489.
Balesdent, J., Chenu, C., Balabane, M., (2000). Relationship of soil organic
matter dynamics to physical protection and tillage. Soil tillage Res. 53,
215 - 230.
Baravalle, R., Rosell, R., Miglierina, A., Maiza, P, (1995). Soil fixation and
availability of phosphate fertilizer. Communication in Soil Science and
Plant Analysis 26, 2157-2165.