Thus, although exchangeable base cations seemed
high at the top depth, these large leaching losses require compensation
through soil amendments, suggesting dependency of these smallholders
on nutrient additions through fertilization. The high exchangeable
Na in the oil palm plantations was due to borate fertilizer
application during the years prior to our study; however, Na was also
displaced to the lower depths via leaching (Kurniawan, 2016) as indicated
by the higher exchangeable Na concentrations in the lowest two
depth intervals, especially in the loam Acrisol soil (Fig. 1c). Hydrated
Na ions are weakly adsorbed onto soil exchange sites, causing Na ions
to be easily displaced with downward water movement (Schlesinger,
1997). Altogether, these results show that although fertilization in
these oil palm plantations abated the decrease in soil nutrient stocks,
this was only short-term and confined in the topsoil.
The differences in management practices between the two-monoculture
plantations illustrate that fertilization in the oil palmplantations
not only contributes to the higher base saturation in the topsoil in this
land use (described above), but also contributes to the higher extractable
P in the topsoil, particularly in the clay Acrisol soil (Figs. 1b, d).
The low extractable P in the rubber plantations signifies the eventual
decrease in rock-derived nutrients after years of cultivation in unfertilized
agricultural land uses (Ngoze et al., 2008). The lower extractable
P and exchangeable bases present in the lower depths (N0.5 m) of the
oil palm and rubber plantations compared to the forest may also be