Active aluminium and iron in volcanic soils

Angélica Sadzawka R.1 y Lissette Porte O.1

Forms of active Al and Fe, in 70 samples of volcanic ash soils, were estimated by selective dissolution analysis, using KCI 1 N, sodium pirophosphate 0.1 M, dithionite-citrate--bicarbonate, and acid oxalate 0.2 M as rgents. Total active Al increased with soil development and was chiefly constituted by Al bound to humus, in surface horizons, and by Al in allophane and imogolite, in B horizons. Valuable amounts of exchangeable Al were detected only in organic matter rich horizons and in crystalline minerals prevailing horizons. Amounts of total extracted Fe were lower than total active Al, except in crystalline horizons, and were mainly composed by Fe bound fo humus in surtace horizons, and by inorganic amorphous Fe compounds, in deeper horizons. Large amounts of crystalline iron oxides were found only in the most developed soils. Maximum concentrations of non-crystall me and para-crystalline aluminosilicates (allophane, allophane-like compounds and imogolite) ranged from 2 fo 24%, in less than 2 mm soil fractions. Calculated Al/Si mole ratios were near to 1.0, when non-crystalline and para-crystalline aluminosilicate amounts were low, but they increased to around 2.5, with ihe highest amounts of these compounds.

1 Estación Experimental La Platina (INIA), Casilla 5427, Santiago, Chile.