Organic matter and exchangeable aluminum interactions in soils cultivated with coffee (Coffea arabica)

The use of shading trees to improve soil quality on coffee plantations has been studied. A field experiment was established to evaluate the effect of shading densities of P. carbonarium on chemical properties of an Alonso soil planted with coffee. Treatments included a full sunlight plot (FS), a high density shaded plot (HDS), and a low density shaded plot (LDS). Soil samples were collected at 0-20 and 20-40 cm depths in all plots. All plots presented a clayey texture and similar mineralogy, consisting primarily of kaolinite (1:1 clay), and iron and aluminum oxides. The iron oxides presented the sharpest peaks when determined by X-ray diffraction (XRD), indicating more crystallinity, and also was the dominant fraction as determined by the citrate- dithionate - bicarbonate (CDB) and acid oxalate (AO) extraction methods. Soil pH was acid (<5) and decreased at 20-40 cm depth. Available phosphorus was higher at the HDS plot and decreased considerably at the 20-40 cm depth. The FS plot presented the lowest organic matter (OM) content at both depths, while the LDS plot presented the highest, decreasing with increasing depth. The percent of aggregate stability was higher at the HDS plot and decreased at 20-40 cm depth. The cation exchange capacity (CEC) was higher at the HDS plot (10.89 cmolc kg-1) and lower at the LDS plot (7.27 cmolc kg-1), decreasing with increasing depth. The highest amount of exchangeable aluminum (Al3+) was observed at the HDS plot, with 6.47 cmolc kg-1at the 0-20 cm depth and 9.23 cmolc kg-1 at the 20-40 cm depth. The Al3+ was significantly lower at the LDS plot (1.83 at 0-20 cm and 3.69 cmolc kg-1 at 20-40 cm). The percentage of base saturation (% BS) was significantly lower at the FS and HDS plots, but the amount of exchangeable bases was similar. The LDS plot presented about half the effective CEC (CECe) of the other two plots. Since these are nearby plots, with similar slopes and similar mineralogy and chemical properties, the difference in exchangeable Al3+ can be related to organic matter complexation. There is the possibility that Al could be complexed by soil organic matter in a less available form at the LDS plot. Exchangeable and organically bound Al fractions were extracted with 1 M KCl, 0.33 M LaCl3 and 0.2 M CuCl2. Aluminum extraction was higher at the 20-40 cm depth in all plots. Less Al3+ was extracted from the LDS plot at both depths when compared to the other plots, but presented the highest increment between the fractions extracted by the LaCl3 and CuCl 2 solutions (33% at 0-20 cm depth and 25% at 20-40 cm depth). This indicates that a higher percentage of Al3+ may be complexed to OM in the LDS plot.