THE INFLUENCE OF IONIC ENVIRONMENT ON THE NATURE OF IRON OXIDES IN SOILS

The colour in a sequence of soils formed on basalt in the Burnie area of Tasmania changes from red to red-brown near sea level to a yellow-brown in the more elevated areas. This colour change reflects the variations in the goethite/haematite ratio which in turn has been positively correlated to the proportions of Ca plus Mg on the exchange complex. The results support the laboratory observations of Schellmann that the adsorption of Ca and Mg ions by freshly precipitated iron oxide gel directs the ageing towards haematite.     

MECHANISMS OF PHOSPHATE SORPTION BY SOILS AND HYDROUS FERRIC OXIDE GEL

Data for the sorption of added inorganic phosphate (P) by contrasting soils and iron oxide gel were resolved by a method of successive approximation of the Langmuir sorption constants. Three distinct Langmuir equations described the overall isotherm irrespective of the experimental conditions used. Free energies of sorption (ΔG) for a particular region were similar for each soil and for iron oxide gel under all experimental conditions. In contrast, the sorption maximum for each region was influenced by the sorbent and experimental conditions. Because of the pronounced similarities in their P sorption characteristics iron oxide gel was used as a model surface for P sorption by soils. Three mechanisms for P sorption by iron oxide gel and by soils are proposed: (i) chemisorption at protonated surface sites, (ii) chemisorption by replacement of surface hydroxyls, and (iii) a more-physical sorption of P as a potential-determining ion.     

水稻土的氧化还原过程及其与水稻生长的关系

水稻土的主要特点是在一年之中有季节性的淹水和落干,而各种水稻土的干湿程度和时间都有不同,因而影响土壤性质发生复杂的变化。在这些性质中,氧化还原状况是变化最剧烈而指示最明显的性质,并且直接或间接影响水稻的生长。为此,作者等从1954年起,开始了这方面的研究工作,下面是关于几个主要简题的筒单概括。     

Characterization of commercial iron chelates and their behavior in an alkaline and calcareous soil

Iron deficiency is a common problem for many plants grown in alkaline and calcareous soils. To correct this problem, iron is supplied to plants as chelates. Several iron chelates are sold under diverse trademarks with different characteristics. This work evaluated 18 commercial products containing the most representative chelated iron sources used in agricultural practice in Spain when the study was done, namely the ferric chelates of EDDHA, EDDHMA, EDDCHA, EDDHSA, EDTA, and DTPA. The chelates were comprehensively characterized and quantitated by several techniques, including several chromatographic methods. Iron and chelate dynamics in soil were also studied in a model alkaline and calcareous soil. Results indicate that, in this model soil, among the different iron compounds studied only FeEDDHA and analogues have the capacity to maintain soluble iron in soil solution over time. These results are in agreement with general experience under field conditions. Furthermore, among the different ortho-ortho isomers of FeEDDHA's, FeEDDHSA and FeEDDCHA showed greater capacity than FeEDDHA and FeEDDHMA to maintain the chelated iron in soil solution over time.     

THE MOBILIZATION OF IRON BY EXTRACTS OF EUCALYPTUS LEAF LITTER

When aqueous extracts of leaf litter from four closely related Eucalyptus species were reacted with soil material under aerobic conditions their iron mobilizing activity was found to be inversely related to the productivity of the sites on which the species grew. The activity of litter extracts of the four species grown in similar soils in the field was found to increase in the order E. regnans, E. obliqua, E. radiata, E. sieberiana. The results indicate that species-soil interactions could be as important as inherent species characteristics in determining whether or not a species is a ‘podzol former’. The effect on the activity of the extracts of altering their pH before reaction with either soil or with prepared iron oxides suggests that, whereas organic acids could be mainly responsible for mobilizing iron from soil and from anhydrous ferric oxide, polyphenols could be more important than organic acids in mobilizing iron from hydrous ferric oxide. The pH and E_h curves obtained when litter extracts were titrated with a ferric chloride solution showed that iron from this source was strongly reduced by the extracts at low pH.     

Iron hydroxides in soils: A review of publications

Iron hydroxides are subdivided into thermodynamically unstable (ferrihydrite, feroxyhyte, and lepidocrocite) and stable (goethite) minerals. Hydroxides are formed either from Fe³⁺ (as ferrihydrite) or Fe²⁺ (as feroxyhyte and lepidocrocite). The high amount of feroxyhyte in ferromanganic concretions is proved, which points to the leading role of variable redox conditions in the synthesis of hydroxides. The structure of iron hydroxides is stabilized by inorganic elements, i.e., ferrihydrite, by silicon; feroxyhyte, by manganese; lepidocrocite, by phosphorus; and goethite, by aluminum. Ferrihydrite and feroxyhyte are formed with the participation of biota, whereas the abiotic formation of lepidocrocite and goethite is possible. The iron hydroxidogenesis is more pronounced in podzolic soils than in chernozems, and it is more pronounced in iron-manganic nodules than in the fine earth. Upon the dissolution of iron hydroxides, iron isotopes are fractioned with light-weight ⁵⁴Fe atoms being dissolved more readily. Unstable hydroxides are transformed into stable (hydr)oxides, i.e., feroxyhyte is spontaneously converted to goethite, and ferrihydrite, to hematite or goethite.     

Solubility Relationships of Iron and Evaluation of its Fertility Status in Degraded Soils

This study was conducted to determine the solubility of iron (Fe) and identify the solid phases responsible for controlling its solubility in these soils by using Baker soil test (BST) computer program. The results indicated that the ferric ion (Fe³⁺) activity in all the soils, except the acidic ones, nearly approached the theoretical solubility line of known minerals, namely soil-Fe, amorphous-Fe, maghemite, and lepidocrocite. Solubility of Fe in acid soils of Ranchi (soil 3) and Cooch-Behar (soil 6) did not match the theoretical solubility lines of any of the known minerals. The acidic soils of Ranchi and Cooch-Behar were sufficient for Fe with respect to both quantity and intensity factor based on BST rating, while alkaline (soil 1) and calcareous (soil 5) were sufficient in reserve Fe; deficiency of iron still could be suspected because of high pH. These findings elucidate the role of solid phase controlling iron solubility in soil solution of degraded soils.     

与有机物质的分解相关的还原作用对红壤磁性质的影响

Five soils derived from different parent materials were sampled from red soil region of southern China and studied by magnetic methodology to understand to what extent iron reduction would affect soil magnetic properties and how iron reduction would affect the magnetic minerals in soils.Reduction associated with organic matter decomposition strongly affected soil magnetic parameters at low pH.The losses of original soil magnetic signals in terms of magnetic susceptibility(χ),‘Soft’ isothermal remanent magnetization (‘Soft’ IRM),anhysteretic remanent magnetization(ARM) and saturation isothermal remanent magnetization(SIRM) at pH 4-6 during the two-month saturation treatment,were 66%-94%,54%-90%,64%-95% and 33%-83%,respectively.These changes were interpreted as a consequence of substantial dissolution of maghaemite and haematite in the soils.At pH 10,however,there was no significant magnetic change observed.Moreover,stable single domain soil maghaemite grains were also sensitive to reduction,which suggested that both pedogenic and detrital maghaemite were not stable in acid and reducing environments.Goethite,instead,was the most stable iron form under reducing conditions.     

对黄壤发生分类的看法

长期以来我们对黄壤的发生、特性和分布总觉得模糊不清。在土壤调查制图时;因红壤黄壤在形态性质上不易区分,所以在确定南方山地红壤带和黄壤带的垂直分布界线时,只有按海拔高度来划分。1982年10月在重庆召开的第三次全国森林土壤学术讨论会上有两篇都是有关广东鼎湖山的土壤论文,其中一篇把红壤黄壤界线划在300米,而另一篇划在700米,说明划分标准还不统一。云贵高原是我国黄壤集中分布区,根据已有的资料看来,那里的黄壤是否典型、黄壤是否为湿润气候的产物,都值得讨论。本文就多年来的考察,结合有关论著,谈一点不成熟的看法,希望能起到抛砖引玉的作用。     

EVALUATION OF POTENTIAL SUBSTRATES TO MONITOR RESPIRATORY NITRATE REDUCTASE ACTIVITY IN SOILS

Different inorganic ions were tested for their ability to replace nitrate as the terminal oxidant for bacterial respiration in anaerobic soils. Chlorate, bromate. selenate. tellurite and ferric ions were all unsuitable substitutes, lodate reduction in soils was similar to nitrate reduction because it required anaerobic conditions, was increased by glucose amendment and decreased by phenylmercuric acetate. Amendment of soils with iodate and measurement of iodide production can be used to measure nitrate reductase activity in a soil and so give a qualitative indication, and possibly a quantitative estimate, of the denitrification capacity of soils.     

Inhibition of iron uptake from iron salts and chelates by divalent metal cations in intestinal epithelial cells

Iron chelates, namely, ferrous bisglycinate and ferric EDTA, are promising alternatives to iron salts for food fortification. The objectives of this study were to compare iron uptake from radiolabeled ferrous sulfate, ferrous ascorbate, ferrous bisglycinate, ferric chloride, ferric citrate, and ferric EDTA by Caco-2 cells with different iron status and in the presence of divalent metal cations. Iron-loaded Caco-2 cells, with reduced DMT-1 and elevated HFE mRNA levels, down-regulated uptake from ferrous ascorbate and bisglycinate but not from ferric compounds. Nevertheless, iron uptake from all compounds was markedly inhibited in the presence of 100-fold molar excess of Co2+ and Mn2+ cations, with ferrous compounds showing a greater percent reduction. Our results suggest that ferrous iron is the predominant form of iron taken up by intestinal epithelial cells and the DMT-1 pathway is the major pathway for uptake. Iron uptake from chelates appears to follow the same pathway as uptake from salts.     

THE OXIDATION OF IRON SULPHIDES IN SOILS IN RELATION TO THE FORMATION OF ACID SULPHATE SOILS, AND OF OCHRE DEPOSITS IN FIELD DRAINS

Aerating pyritic soils causes acidification and the forrnation of acid sulphate soils, or cat-clay. The Oxidation of pyrite in soils is associated with the deposition in tile drains of a form of ochre quite distinct from that formed by the action of filamentous iron bacteria. Pyrite-derived ochre results from the action of Thiobacillus ferrooxidans, which, below pH 3.5–4.0, catalyses the Oxidation of Fe²⁺ and pyrite. In soils less acid than c. pH 4, pyrite oxidizes relatively slowly by chemical reactions to Fe²⁺ and SO²₄^?. Under these conditions iron enters the drains as Fe²⁺ and is there oxidized by T. ferrooicidans and deposited as hydrated ferric oxide. Once the soil becomes acid enough for T. ferrooxidans to multiply, the rate at which pyrite oxidizes increases several-fold, and at c. pH 3 iron appears in the drainage water in the ferric form. Liming seems to decrease the rate of Oxidation.     

A new method for determining free iron in paddy soils

For pedological or edaphological study, it is very important to find a suitable method for determining free iron in soils. The removal of free iron also facilitates the identification and determination of minerals present in soils and clays by X-ray and thermal analysis and microscopic observation. Up to the presnt time many methods have been proposed for the removal or the determination of free iron in soils and clays. With some exceptions these methods are all based on the principle that the free iron compounds existing mainly as ferric iron in soils are reduced to ferrous iron and extracted with suitable reagents. A short historical review is as follows.     

台湾红壤及森林土壤中之氧化铁

本文综合汇整近年来在台湾，应用高梯度磁场分离技术，配合一般化学分析，Ｘ－射线衍射法，磁测分析，电子显微镜观察与电子衍射鉴定，以及铁－５７穆期堡尔谱学分析等方法，研究红壤与森林土壤中，氧化铁的分布及其结构性质之成果回顾；关于氧化铁之分布情形，其研究结果显示：台湾红壤中普遍存在有赤铁矿与针铁矿等氧化铁矿物，而在化育自大屯山更新世纪安山岩母质与澎湖列岛更新世纪玄武岩母质的红壤中，以及兰屿绿岛海边之银白色     

Aggregation of soil particles by iron oxides in various size fractions of soil B horizons

Quantitative relationships between aggregation of soil particles and the content of haematite and goethite were studied by removing iron oxides, with dithionitetrate-bicarbonate (DCB) and oxalate reagents, from various sizes of soil separates which were stable to sodium hexametaphosphate, and then determining the particle-size distribution. Significant quantitative relationships were found between DCB-extractable oxides and particles obtained from the separates up to 20 pm, whereas oxalate-extractable oxides were correlated only with < 0.2-μm particles. Oxalate-extractable oxides were observed to aggregate a greater quantity of fine clay and a larger surface area of particles per gram of oxide than DCB-extractable oxides, A more efficient mechanism of aggregation was postulated for the oxalate-extractable oxides than for the DCB-extractable oxides. There were fewer aggregated particles in soils containing haematite and goethite than in soils containing only goethite. This was attributed to differences in pedoenvironment, rather than to a difference in the behaviour of the oxides. Although various clay minerals were identified in the soils studied, no preferential aggregation of any mineral was observed.     

Use of the Kubelka—Munk theory to study the influence of iron oxides on soil colour

The Kubelka–Munk theory, which is commonly applied to ‘pigment mixtures’, adequately predicted the colour of mixtures of synthetic haematite, synthetic goethite and deferrated soil powders. The theory illustrated the higher pigmenting effect of haematite compared with goethite. In mixtures containing haematite the calculated colour coordinates could be combined into simple ‘redness’ indices that were highly correlated with haematite content and were not appreciably influenced by goethite. The theory was also applied to the study of the chromatic characteristics of 98 soils, differing widely in their origin but having an organic matter content <2% in which haematite or goethite was the dominant Fe oxide. The theory showed that the average ‘soil’ haematite and ‘soil’ goethite had colours similar to those of their synthetic counter-parts, whereas the rest of the soil components could be considered as an essentially ‘grey’ matrix. It was also useful in predicting the haematite and goethite contents of soils either from several reflectance measurements of soil-white standard mixtures or from the indices of redness developed for the synthetic mixtures.     

Highly weathered soils in the east coast hinterland of Southern Africa with thick, humus-rich A1 horizons

Strongly weathered red and yellow soils with thick (490–900 mm) humic Al horizons (Haplohumox and Palchumults) derived from sandstones and basic igneous rocks, and occurring near the east coast of Southern Africa, are described and discussed in terms of their distribution, morphology, texture, mineralogy, chemistry, genesis and classification. The high organic matter content (2–5%C) of the Al contributes significantly to a high pH-dependent negative charge, probably to poorer crystallinity of goethite and kaolinite and to the transformation of haematite to goethite. Varying proportions of kaolinite and gibbsite reflect different soil ages within these old landscapes. The yellow colour of aluminous goethite, the main pedogenic iron oxide, is masked in the Al by organic matter and in red B2 horizons by haematite. Temperature may have influenced the broad pattern of occurrence of red and yellow B2 horizons. These soils need not, as previously suggested, have developed from the weathering products of an ancient laterite. Neither Soil Taxonomy nor the South African soil classification system accommodates the soils entirely satisfactorily and possible improvements to the latter are discussed.     

EDAPHOIDS AND PALEOSOLS OF BASALTIC ORIGIN IN THE GALILEE, ISRAEL

Red soil-like layers intercalated amid basaltic flows from the Miocene-Pleistocene in the Galilee, Israel, were examined. Micromorphological features which can be related to (a) translocation of materials, and (b) biological activity, and are therefore of distinct pedological origin, identify two of the layers as paleosols. Absence of these features and preservation of the fabric of the parent basalt serve to identify the third layer as an edaphoid. A fourth layer consists of an edaphoid superimposed on a paleosol. The edaphoids are composed almost entirely of a dioctahedral smectite. In two of the paleosols dioctahedral smectites predominate. In the third kaolinite and haematite are present also in considerable amounts. The presence of quartz in the paleosols is attributed partly to precipitation from Si-rich solutions percolating the upper basalt layer and partly to aeolian contamination. In all the layers the free iron is predominantly in an anhydrous form. Similarity in the clay mineral composition of the Plio-Pleistocene paleosols with that of contemporaneous basaltic soils is taken to indicate similarity in climatic conditions of formation. Differentiation between paleosols and edaphoids cannot be made by the use of a single criterion but must be based on corroborative evidence derived from micromorphological and mineralogical examinations.     

南方水稻土中的磷酸铁对水稻磷素营养的意义

研究了我国南方18个不同类型水稻土标本的无机磷的形态。结果表明,非闭蓄态的磷酸盐,包括磷酸铝、磷酸铁和磷酸钙三种形态,一般占土壤无机磷总量的30—60%,其中磷酸铁一极占非闭蓄态总量的50—80%。土壤无机磷中另外的40-70%主要是被氧化铁胶膜包蔽的磷酸铁和磷酸铝。应用同位素P³²示踪法进行的水稻盆栽试验表明:在水稻不同生育期,土壤有效磷量“A”值与水稻土中非闭蓄态的磷酸铁化合物具有极显著的相关性(成熟期的r=0.95)。用P³²人工合成的两种不同晶形的磷酸铁进行了水稻盆栽试验;进一步指出:在水稻土的条件下,晶形的粉红磷铁矿对水稻具有良好的磷素供应能力。这些结果说明,水稻土中非闭蓄态的磷酸铁化合物,即使是以比较稳定的结晶状态存在,但却有可能作为水稻磷素营养的重要给源。室内试验表明,粉红磷铁矿所以对水稻磷素营养具有良好供应能力,以及酸性和中性水稻土中的非闭蓄态磷酸铁化合物,所以能作为水稻磷素营养的重要给源,主要是受渍水还原过程的影响,这一过程,使原来晶形的磷酸铁化合物转化为无定形的状态。     

Evolution of porosity and changes in heat treated lepidocrocite

The thermal decomposition of lepidocrocite in air has been studied by X–ray powder diffraction, IR spectroscopy, transmission electron microscopy and N₂ adsorption isotherms. Three consecutive processes may be differentiated with increasing temperature: (a)<200°C; no structural change is observed, but a surface dehydration gives rise to an increase in surface area besides a change of mesopore shape from cylindrical to slit–shaped, (b)200–300°C; the maghaemite phase is obtained with a high increase in surface area mainly due to the formation of micropores, because of water losses from inner positions that produce lattice disruption, and (c)≥ 350°C; the transformation of maghaemite to haematite takes place decreasing the specific surface area as the temperature increased. At 450°C microporosity is destroyed and a change of slit–shaped to cylindrical mesoporosity is observed. Porosity dissappeared at ≥600°C, mainly due to interparticle sintering which produces well crystallized haematite samples of low surface area.