F De Coninck

Major mechanisms in formation of spodic horizons

Abstract Current explanations of the formation of spodic horizons do not accomodate all features of the horizons in their natural state. In this paper, a more complete explanation of major mechanisms is proposed, using two principles of colloid chemistry: (1) organic substances may form hydrophylic colloids with surface charges, and (2) the hydrophylic character and negative surface charges determine the dispersibility of the colloids. The hydrophylic character is due to the presence of hydrophylic radicals as parts of the organic compounds in soils. The surface charges are the result of dissociation of -COOH and possibly phenol-OH radicals. The neutralization of the surface charge can in principle occur: (1) through electrostatic or physical adsorption and (2) through chemisorption. The first case is typical for monovalent alkali cations. The adsorbed cations are distributed in a double layer, which favours dispersion. Chemisorption occurs mostly with polyvalent cations. This process corresponds in reality to the formation of organo-metallic compounds. It results in a relatively complete disappearance of the double layer and in the formation of large immobile “polymerized” organo-metallic compounds. Because these compounds contain much hydrophylic water, they form a gel. Transition into the solid state is accompanied by the loss of most of the hydration water. The dehydration may be induced by a decrease in thickness of the double layer. At a certain stage of the dehydration process, Van der Waals bonds and protonic bridges can form and bring about a certain degree of hydrophoby. In soils, mobile organic substances are formed during breakdown of plant remains. If at the top of the mineral soil enough polyvalent cations, especially Al and Fe, are available, the mobile organic substances formed are immobilized immediately and no migration occurs. In case insufficient amounts of Al and/or Fe are available to completely immobilize the mobile compounds, these cations are complexed by the mobile compounds and transported downward. Immobilization may occur at some depth through supplementary fixation of cations, through dessication or on arrival at a level with different ionic concentration. In nature, spodic horizons range from loose, with many roots, to very cemented with few roots. These differences can be related to changes in microstructure. Loose spodic horizons have a predominance of polymorphic pellets and aggregates, whereas organans or monomorphic coatings prevail in cemented horizons. The former horizons have many features suggesting major biological influences during their formation, viz., high numbers of roots, thorough mixing of the organic units with clay and silt, the presence of pedotubules and relatively young mean residence times. The latter horizons have features consistent with organo-metallic compounds immobilized in a gel-state, viz., the coatings are strongly cracked, indicating the transition of a gel into a solid; they contain much Al or Al plus Fe but very little or no Si, and the mean residence time is considerably higher than in loose horizons. The two processes seem to operate simultaneously during the formation of spodic horizons and their relative intensities determine the composition of each spodic horizon at any moment in its evolution. As long as the biological activity predominates, the horizon remains loose; if the accumulation of mobile organo-metallic compounds starts to prevail, the horizon is gradually cemented and fossilized.

Characterization of some spodic horizons of the Campine (Belgium) with dithionite-citrate, pyrophosphate and sodium hydroxide-tetraborate

Solutions of dithionite-citrate-bicarbonate, Na4P2O7 and NaOHNa2B4O7 have been used to extract sixteen subhorizons of four Spodosols, forming a drainage sequence in the Antwerp Campine (Belgium). The purpose of these analyses was to test the extractability of Al, Fe and C by those solutions and to determine the nature of the extracted organic substances using the value of Δ log K, which represents log K400 – log K600, i.e., the difference between the logarithms of the absorption coefficients at 400 and 600 μm. Some conclusions drawn from this study are: 1. (1) The extractable fraction of the organic matter increases and the extractable compounds become less polymerized downward in the spodic horizons, but this trend is absent in subhorizons with high amounts of Fe soluble in dithionite-citrate. 2. (2) In the B21h horizons of all profiles low in Fe soluble in dithionite, the three solutions extract similar amounts of Fe and Al. 3. (3) In the B22h and B3h horizons, the amounts of Fe and Al extracted decrease in the order dithionite-citrate > Na4P2O7 > NaOHNa2B4O7. The differences are more pronounced for Fe than for Al. 4. (4) From the atomic ratios of Al, Fe and C extracted in Na4P2O7 and in NaOHNa2B4O7, it is evident that in some samples the amount of extracted organic matter is too low to bind all extracted Fe and Al. Moreover, the atomic ratios of Fe and Al extracted by dithionite-citrate and Na4P2O7 to total organic carbon suggest that the amount of organic matter is insufficient to bind all Fe + Al extracted. Both featurs indicatethat Na4P2O7 is able to extract amounts of Fe and possibly of Al not bound to organic matter. 5. (5) The atomic ratios (Al + Fe) extracted in NaOHNa2B4O7 to total organic carbon suggest that this solution extracts only Al and Fe bound to organic matter and that these cations may be extracted from organic compounds without solubilizing the organic fraction.

Dissolution characteristics of hectorite in inorganic acids

The effect of acid type (HCl, HNO3 and H2SO4) and concentration on the dissolution rate of hectorite were monitored through chemical analyses and XRD. The rate of dissolution increased with increasing acid concentration during the first 2–4 h of contact. After that time, the correlation between acid concentration and the amounts of dissolved elements strongly decreased and often higher concentrations were found in 0.25 M solutions than in 1 M solutions. The monitoring of the amount of Si is somewhat more complex since it is the result of two processes: release from the mineral and reprecipitation as an amorphous end product. In the case of HCl, the behavior of Li, Mg and Fe differed from their behavior in the other acids, but further research is necessary to characterize the reactions that may occur. From the half times of dissolution, the dissolution curves and the XRD data, it could be concluded that the dissolution rate of hectorite decreased in the order H2SO4>HNO3≥HCl at the same molar concentration, which is the reverse of what was found by other investigators. After 8 h, for example, the 1 M H2SO4 treatment dissolved more than 70% of all Li present in the hectorite, whereas equal molar HNO3 and HCl dissolved 58% and 53%, respectively. Oriented XRD patterns only showed a background scatter after 6 h of contact with 0.25 M H2SO4 and after 4 h using a concentration of 1 M H2SO4. Treatments with 0.25 M HNO3 and 0.25 M HCl still gave reflections after 6 h and even after 8 h, the d(001) XRD peak could still be observed. In 1 M HNO3 and 1 M HCl, no more reflections could be seen after 4–6 h. At that time, XRD powder patterns showed that the crystal structure was still partly preserved in the a- and b-direction of the mineral. It must be stressed that absence or a decrease in intensity of the d(001) peaks may not be fully assigned to the breakdown of the mineral structure since the Si, extracted from the lattice reprecipitates as amorphous silica, which may disrupt the layer stacking and decrease the coherence of the reflections. For the comparison of the three acids on a normal basis, the quarter times of dissolution were found to be a more appropriate tool than the half times of dissolution. The results showed that the differences among the three acids were rather small and that the concentration of the acids was the main parameter affecting the dissolution rate of the hectorite, in particular, in the beginning of the treatment.

Role of Plinthite and Related Forms in Soil Degradation

According to Alexander and Cady (1962), laterite is a highly weathered material rich in secondary oxides of iron, aluminum, or both. It is nearly void of bases and primary silicates, but it may contain large amounts of quartz and kaolinite. It is either hard or capable of hardening upon exposure to wetting and drying. This general definition is based on the original work of Buchanan (1807), modified by the work of many others since then. However, the term laterite is perhaps one of the most misused terms in earth sciences. Many inconsistencies and confusion exist in the literature about the term laterite and the process of laterization. The term has been adopted by geologists, mineralogists, mining engineers, and pedologists, and today it encompasses materials that show some kind of sesquioxide accumulation and ranges from weathered rock to hard and cemented ironstone. Further complication was introduced in tropical soils literature, when terms such as laterite profile and lateritic soils were introduced. Due to this confusion in soils literature, Kellogg (1949) introduced the term latosol—LAT derived from laterite—to distinguish them from laterites, but later Maignien (1966) proposed to abandon the term laterite. The reader is referred to this excellent review by Maignien (1966) for information on the early work.

Mineralogy of clay fractions of some soils on loess in northern France

Abstract Important field characteristics of soils on loess deposits in northern France result primarily from processes of clay migration. Two successive steps can be distinguished. The first has uniformly distributed brown “primary” clay coatings in a brown matrix (Hapludalfs), the second has features of hydromorphy and degradation, especially bleached tongues, penetrating into the horizon of clay accumulation, with gray “secondary” clay coatings in the lower parts. The mineralogy of fine and coarse clay in the two kinds of soils shows an identical mixture of different materials: quartz, kaolinite, mica, chlorite, an heterogeneous complex of minerals having micaceous, smectitic, vermiculitic and chloritic layers, and smectite. The first four minerals predominate in the coarse clay, the complex and the smectite in the fine clay. The only fundamental difference between primary and secondary stages is a difference in distribution of free iron, the latter being characterized by a strong local redistribution due to redox processes. The coarse clay in the upper part of the glossic soils has an obvious increase in chlorite, relative to the other minerals. The chemical composition indicates that this chlorite is trioctahedral, excluding a process of secondary chloritization.