36 Liliana Suñer and Juan Galantini
studies (Magid & Nielsen, 1992) found that after ploughing pasture soils,
available P increased over the plant requirements suggesting physicochemical
equilibrium could result in precipitation of inorganic P forms such as apatite
(Ca 5 (PO 4 ) 3 (OH,F,Cl)) or brushite (CaHPO 4 ·2(H 2 O)). Physicochemical
equilibrium of soil P were climatic (water regime) and management
(phosphate status and tillage) dependent. As observed by others, this dynamic
could mask biological effects on soil P changes.
Figure 2. Variation of soil extractable P (Pe) content under continuous wheat (WW)
with (f) and without (nf) fertilizer (N+P) applications.
In soils of the Argentina Pampas region two systems after 18 years of
tillage were compared. These systems were: conservation tillage (CT) and no
tillage (NT). The differences observed in both tillage systems were relative
and they result from the net balance between the positive and negative effects
of each management. The differences would thus be associated with the
increase and redistribution of the organic fraction under NT and with the loss
of the organic fraction under CT, where the differences were more pronounced
(Galantini et al., 2006).
In general, the differences between systems in the A-horizon over the
study period were 63, 31 and 169 kg ha-^1 Po, Pi and Pt, respectively. As
mentioned above, the extent of the P-changes and redistribution across the
depths studied was different for Po in comparison with Pi. The content of Po
(63 kg P ha-^1 ) was twice as much between tillage systems as that of Pi (31 kg
P ha-^1 ). Pt content, though, was comparatively much lower. Even though