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).