34 Liliana Suñer and Juan Galantini
soils, as was subsequently corroborated by analysis of the non-weighted
results. Such enrichment gives rise to improved retention of humidity and
nutrient supply to crops by the smaller minerals (lime and clay) and their
associated organic matter (Hevia et al., 2003).
Soil pH
Soils sampled at different depths in the semiarid Pampas area were
incubated for 90, 180 and 360 days to assess the dynamics of P forms bounded
to Ca (P-Ca) and not occluded Fe+Al (P-Fe+Al) (Suñer & Galantini, 2013).
The pH content varied in the nine samples analyzed; it increased towards the
soil surface, from the deep (6.5) and medium levels (7.5) to the shallow layers
(8.0), and also in Ca-bound P. During incubation, the dynamics of P bound to
Ca and Fe+Al non-occluded was different across soils, and similar for similar
depths. The Fe+Al-bound P in shallow soils steadily decreased across the 360-
day incubation period; it did not vary in medium-depth soils and it decreased
in deep layers. Variations in Ca-P over time proved that this P-form is highly
dynamic (Figure 1). On average, it sharply dropped at the beginning but it then
increased by the end of the incubation period. Incubation affected the initial
equilibrium of the P-forms analyzed according to the soil depth. After the 360-
day incubation, a Ca-P and Fe+Al-P net decrease was observed for shallows
soils as a consequence of a significant drop in Fe+Al-P. The total sum of Ca-P
and Fe+Al-P content increased for medium-depth soils as a result of a Ca-P
rise. In deep soils, a higher Ca-P and Fe+Al-P content resulted from an
increase in both P-forms. Considering Fe+Al-P content alone, incubation
decreased this P-pool in shallow soils, kept it stable in medium-depth soils and
slightly increased it in deep soils. The Ca-P was the most abundant and
dynamic form. Changes were more pronounced in shallow soils, but the final
content was similar to the initial one. In deep and intermediate-depth soils,
changes were less sharp; however, Ca-P increased over time. In a nutshell,
incubation decreased Fe+Al-P in shallow soils, caused no substantial variation
in intermediate-depth soils and slightly increased it in deep soils. Incorporation
of P caused an initial drop in the analyzed forms. This decrease was sharper
and lengthier in shallow soils.