Phosphorus Dynamic in the Soil-Plant System ... 35Figure 1. Dynamics of P bounded to Ca (P-Ca) and not occluded Fe+Al (P-Fe+Al)
without added P (P0, control) with the addition of 50 μg P g-^1 in shallow soils, medium
and deep.
INFLUENCE OF MANAGEMENT PRACTICES ON P STATUS
Soil Tillage
The impacts of tillage on P forms and availability have been the subject of
many investigations. There was an increase in total and labile P contents in
surface soil layer due to no-till operations (Selles et al., 1999). This was
attributed to the enhanced microbial activity and mineralization of soil Po
(Zibilske & Bradford, 2003). The first effects of tillage are observed in the
declines of organic matter of the soil surface layer, which rapidly decreases
when soils are cultivated (Galantini et al., 2006). Since the amount of SOM
decrease, its P content (Po) is changing to most labile forms, and finally to
available phosphato to crops (1% of MO loss can represent a loss of 80-120 kg
ha-^1 of P). In the semiarid Pampean region, Galantini et al. (2005) observed a
sharp decrease in Pe during the initial 4 years under continuous wheat and a
slow increase after the 5th or 6th year (Figure 2). In similar conditions, others