UC Davis

Extensive areas of Andosols in tropical Indonesia have been subjected to long-term (>100 years) alterations from native forest to agricultural land use. This study assessed the mineralogical and surface charge characteristics of Andosols in West Java, Indonesia and assessed their resilience upon conversion from rainforest (PF1) to tea plantation (TP1) and horticultural practices. Soils developed in basaltic-andesite volcanic ash (<15,000 years B.P.) under an isothermic/perudic soil climate and were classified as Silandic Andosols. The colloidal fraction of all soils was dominated by nanocrystalline/paracrystalline materials (e.g., allophone, imogolite, ferrihydrite) and Al/Fe-humic complexes. Crystalline minerals were a minor component of the clay fraction and followed kaolinite > hydroxyl-Al interlayered vermiculite (HIV) ≈ gibbsite. The colloidal fraction appeared relatively resilient to changes in land use, except for the tea plantation in which allophanic material content decreased and Al–humus complexes increased due to strong soil acidification, and decreased ferrihydrite in the horticultural soils, possibly due to liming and increased organic matter. In spite of the abundance of allophanic materials, Fe (hydr)oxides and organic matter appeared to regulate surface charge characteristics of the colloidal fraction. Net soil charge in PF1 and TP1 soils was positive (pHPZNC). Horse manure and lime amendments to the horticultural soils lowered the PZNC, increased negative charge (CEC) and decreased positive charge (AEC). At ambient soil conditions, CEC increased from <1.2 cmolc kg−1 in PF1 and TP1 soils to 6–20 cmolc kg−1 in soils under horticultural management, while positive charge (AEC) was appreciably higher (0.7–4.3 cmolc kg−1) in PF1 and TP1 soils than in horticultural soils (<0.6 cmolc kg−1). This study demonstrated that the colloidal fraction is relatively resilient to land-use change; however, charge characteristics of the variable-charged colloids can be readily altered by soil management practices.