3.3.3 pH
Figure 5 shows pH results in the six SUs for the three depths assessed.
Natural vegetation had a lower pH at the surface that slightly increased
with depth, with a variation of 0.3 units between 0 and 25 cm. This
pattern in pH could be related to three factors that lead to the
transformation of interchangeable bases at the soil surface: rainfall
(leachate of bases), SOM (formation of soluble bicarbonates dragged in
depth, generating the elimination of bases), microbial activity
(mineralization of organic matter by transforming N from ammonia,
nitrites and nitrates) and parent material (Cárdenas-Fonseca, 2015). In
agricultural SUs, pH was approximately constant in the profile, possibly
because of the turning of the soil which homogenizes its
characteristics.
[Insert Figure 5]
The natural vegetation had the most acidic soil in the study area (4.6),
being lower for Ds, with 3.9 at the surface level. The low pH in Ds
could be associated to the accumulation of weak organic acids (fulvic
and humic) in the degradation process of fresh organic matter (Martínez,
Fuentes, & Acevedo, 2008). In general, páramo soils are characterized
for being acidic due to the high concentration of aluminum (Estupiñán et
al., 2009) and organic acids (carboxyls and phenolics) (Jaramillo, 2002;
Martínez et al., 2008).
pH in agricultural SUs was low, associated to the application of
amendments (Estupiñán et al., 2009), such as agricultural lime to
increase productivity and to the retention and cation exchange
(Hernández, Triana, & Daza-Torres, 2009) in addition to nutrient
assimilation. The plot under Oc had a high variability in pH, possibly
associated to a heterogeneous application of fertilizers and amendments.
This variability was not observed in Pc, probably because of its recent
transformation from natural vegetation to crops and lower use of
agrochemicals as reported by the farmers. Our results for F and Pc are
similar to those from Daza-Torres et al. (2014) in another páramo
complex (Sumapaz).