Iron Oxide Mineralogy of Well‐drained Ultisols and Oxisols: II. Influence on Color, Surface Area, and Phosphate Retention

Abstract

Subsoil samples from selected North Carolina Ultisols and Brazilian Oxisols were analyzed to determine how the colors of these materials were influenced by the nature and distribution of their constituent iron oxides. The effects of extraneous variables, i.e., other than the iron oxides, were minimized by utilizing pairs of red and yellow soils that were otherwise similar in their physical, morphological, and mineralogical properties.The iron oxides were found to be concentrated in the <0.2‐µm fractions, and the colors of these clays were the same as or similar to those of the parent soils. The spectral properties of the <0.2‐µm clays were primarily influenced by iron mineralogy. Goethite or mixtures of goethite and hematite were identified in all of the clays; however, Mössbauer analyses indicated that the red members of all sample pairs contained larger proportions of hematite than did their yellow counterparts. In addition, as the clays became redder in hue, the ratio of hematite to goethite generally increased. Calculated surface areas for the iron oxides ranged from 60 to 200 m2/g; values from the yellow clays were consistently higher than those obtained from their red counterparts. The yellow clays were also more efficient adsorbers of phosphate.

Rainfall-dependent transformations of iron oxides in a tropical saprolite transect of Hainan Island, South China: Spectral and magnetic measurements

“…Hm and Gt are the two main coloring agents in soils and are characterized by distinct colors, red and yellow, respectively. Hm is a more intense coloring agent than Gt [Bigham et al, 1978]. Redness, which is calculated as a sample's percentage reflectance in the red color band (630-700 nm) divided by the sample's total reflectance within the VIS (400-700 nm), is used to estimate Hm content [Torrent et al, 1980;Barrón and Torrent, 1986].…”

“…grained (submicron), similar to iron oxides found in soils [Bigham et al, 1978]. First, known quantities of Hm were added to the deferrated soil matrix produced by the CBD procedure to establish a primary regression equation between Hm content and redness.…”

Soils and agriculture in central-west and north Brazil

“…In the early 1970's research determined that the amount of surface area on the iron minerals was responsible for adsorbing large amounts of P added as fertilizer (Bigham et al, 1978). Field studies determined that initial applications as great as 838 kg P ha -1 sustained maximum corn yields through four crops (Yost et al, 1979).…”

Capacidade máxima de adsorção de fósforo relacionada a formas de ferro e alumínio em solos subtropicais

“…Já para a Hm, alguns autores atribuem a adsorção de P às faces não basais do mineral (Barrón et al, 1988;Torrent et al, 1994). Na maioria dos trabalhos publicados, a presença de Gt eleva mais a adsorção de P quando comparada a de Hm (Bigham et al, 1978;Karim & Adams, 1984;Fontes & Weed, 1996).…”