ASSOCIATION OF DCB-EXTRACTABLE IRON WITH MINERALS IN COARSE SOIL CLAYS

Abstract

Preferential association of iron, manganese, and titanium oxides with specific minerals in soils has often been assumed, but not clearly demonstrated. The association of the free oxides in different high-gradient magnetic separation (HGMS) fractions of 2− to 0.2-μm clay from different horizons of a Boralf (Gray Luvisol) was studied. HGMS fractions showed contrasting change in color from white to reddish brown with decreasing levels of magnetic flux density (MFD). The unfractionated 2-to 0.2-μm clay samples from different horizons contained <2% total DCB-soluble constituents (Fed, Sid, Mnd, and Tid) extracted by one 10-min DCB treatment; some structural cations are included, as shown from data for standard minerals treated similarly with DCB. HGMS fractionation showed that most of these constituents were present in the magnetic (<1.38 T) fractions. Fed constituted 51 to 88% of the total DCB-extractable components. Although substantial amounts of Fe, Ti, and Mn are extracted by DCB, the amounts of Si and Al are much lower. The molar ratios of Fed/Ald, Fed/Sid, Mnd,Ald, and Tid/Ald increased, without exception, with decreasing levels of MFD, suggesting the preferential association of Fe with iron-bearing minerals. Sid/Ald molar ratios were fairly constant in all the HGMS fractions of the coarse clay samples from all the horizons, suggesting their nonpreferential distribution among the iron-bearing and iron-free minerals. Removal of the DCB-extractable constituents resulted in an increase in CEC. A portion of this increase is attributed to the reduction of iron in the crystal structure by DCB. The increase in CEC in the non-iron-bearing mineral fraction (>1.38 T) was <5%, whereas the iron-bearing mineral fractions show an increase of 17 to 29%. The results show that considerable Fe, Mn, and Ti lost from the structure of iron-bearing minerals remain attached to the mineral grains from which they are lost.