DETERMINATION OF THE CONTENTS OF CLAY-SIZED PARTICLES IN SOILS

Edwards and Bremner's (1967) technique of dispersing soils by Bonification to form stable suspensions for particle-size analysis without the addition of dispersant has been tested on a range of soils and a chalk. Carbonate soils or acid soils, containing a suite of minerals which normally form flocculated suspensions in water, yielded stable suspensions in water if the soil organic matter content was sufficiently high. With similar soils, low in organic matter, dispersant had to be added to stop partial flocculation of the test suspensions. Any gypsum present in soils has to be removed to prevent flocculation. For soils very strongly aggregated by organic matter, all the < 2 μm particles present can be released only by sonic vibration in dispersant rather than water. On four soils studied in detail, the maximum amounts of < 2 μm particles obtained by sonic vibration, vigorous shaking, and the I.S.S.S. method have been compared. On three of the four soils, the values were in close agreement.     

MICROAGGREGATES IN SOILS1

Studies of the dispersion of compound soil particles by sonic vibration and cation-exchange resin techniques indicate that the difficultly dispersible particles in mineral soils of high base status are microaggregates (< 250μ diam) consisting largely of clay and humified organic material linked by polyvalent metals. The inter-particle bonds in these microaggregates can be disrupted by application of mechanical energy (sonic vibration or prolonged shaking with water), the amount of energy required for dispersion of clay-size mineral material being reduced by treatments that weaken or destroy these bonds (e.g. treatments leading to replacement of polyvalent metals by monovalent metals or to destruction of organic matter). A theory is proposed depicting microaggregate formation as a solid-phase reaction involving linkage of electrically neutral clay mineral and organic matter particles by polyvalent metals on exchange sites, and microaggregate disruption by sonic vibration as a reversal of this reaction. Experiments to evaluate this theory are described.     

STRUCTURAL FEATURES OF AGGREGATES IN SOME EAST ANGLIAN SILT SOILS

Samples from field plots of silty soils in East Anglia, which were high and low in organic matter, were subjected to mild ultrasonic vibration. In soils high in organic matter particles in the 2–20 μm or 20–50 μMm size range contained most humified organic matter, whereas in soils low in organic matter, most was present in the clay sized fraction. Scanning electron micrographs of the ultrasonically separated particles showed that those from the high organic matter soils had more extensive surface coatings. Clean fragments were left after peroxide and calgon dispersion treatment.     

Effect of cultivation on chemical characteristics and respiratory activity of crusting soil from Mazowe (Zimbabwe)

Abstract

Clearing and cultivation in crusting soils from Mazowe (Zimbabwe) has lead to severe changes in most physico‐chemical characteristics related to the concentration and distribution patterns of plant nutrients and to the total amount of soil organic matter. Nevertheless, the concentration of the different humus fractions showed lower intensity changes, as did the mineralization rates of the organic matter. The most significant effects of cultivation on the soil chemical characteristics coincided with those considered to favor clay dispersion and crusting phenomena, including generalized desaturation of the exchange complex and losses of divalent ions with a potential bridging effect between soil particles. Concerning the soil organic matter, the humic acid tended to concentrate in the cultivated soils as a probable consequence of selective biodegradation of the other humic fractions. The composition and activity of soil humus suggest low‐performance organo‐mineral interactions: in these soils the active turnover of the plant wastes is not regulated by intense physico‐chemical interactions with the soil mineral fractions, or by physical encapsulation of organic particles. In consequence, the mineralization rates were relatively constant in the soils studied and unrelated to soil organic matter concentration. The results suggests that there is a possibility to revert the early degradation stages of these soils through a rational management of suitable amounts of crop wastes.     

Separation of light and heavy organic matter fractions in soil — Testing for proper density cut-off and dispersion level

Density fractionation is frequently applied to separate soil organic matter according to the degree and the mode of interaction with minerals. Density fractions are operationally defined by density cut-off and sonication intensity, which determine the nature of the separated material. However, no tests or general agreements exist on the most appropriate density cut-off as well as on method and intensity of dispersion. Numerous variants have been proposed and applied, with results often contrasting each other and being hard to interpret. Here, we aimed at separating two light fractions (free and occluded into aggregates) composed of almost pure organic material, and one heavy fraction comprising the organic–mineral associations. We tested effects of different density cut-offs and sonication intensities, in combination and separately, on fraction yields, as well as on the fractions' organic carbon, total nitrogen and lignin-derived phenols. We tried to find optimum density cut-offs and sonication intensities, providing light fractions with maximum organic material and minimum contamination by mineral material. Under the test conditions, a density of 1.6 g cm^?3 gave best results for all test soils, allowing for separation of maximums amounts of almost pure organic material. The density cut-off at 1.6 g cm^?3 is well in line with previous studies and theoretical considerations, therefore we recommend the use of this density as most suitable for separation of organic debris. Sonication levels for aggregate disruption to achieve complete separation of occluded light organic matter varied amongst soils. The necessary intensity of dispersion relates to the type of soil, depending on the stability of contained aggregates. The application of one single dispersion energy level to different soils may result either in mineral contamination or in incomplete separation of light and heavy fractions as well as in redistribution of organic material amongst fractions. This means there is no single sonication level that can be applied to all soils. Thus, obtaining a meaningful light fraction residing within aggregates (occluded light fraction) requires assessment of the dispersion energy necessary to disrupt the aggregate system of a given soil without dispersion of organic–mineral associations. This can be done in pre-experiments where the soil is fractionated at different sonication levels. The appropriate dispersion is determined by mass yields and OC content of the obtained occluded fractions.     

ORGANO-MINERAL FRACTIONS OF A CLIMOSEQUENCE OF SOILS IN NEW ZEALAND TUSSOCK GRASSLANDS

The organo-mineral fractions of seven New Zealand topsoils, which are members of a climosequence, were isolated by ultrasonic dispersion in water and separated into sand-, silt- and clay-size fractions. Several soils contained stable aggregates of mainly silt-size, comprising cemented clay-size particles. The tendency of one soil (McKerrow) to form larger aggregates of sand-size was explained by its distinctive mineralogy. Total organic carbon and total nitrogen were used to indicate the amounts of organic matter in the organo-mineral fractions. The degree of humification of the organic matter was assessed by a pyrolysis-gas chromatography technique, which proved superior to the more conventional C/N method. In the sand-size fractions the climate, and especially annual precipitation, strongly influenced the amounts of organic matter, whereas in the clay- and silt-size fractions it was the composition of the organic matter that was influenced.     

Stability behavior of soil colloidal suspensions in relation to sequential reduction of soils

Flocculation and dispersion of colloidal particles of nine inorganic paddy soils were studied mainly based on turbidity measurements of the suspensions of soils which were previously incubated at 28°C under in vitro waterlogged conditions. After 1-week of incubation, the turbidity of the soils except for 1) two soils containing larger amounts of sodium salts and 2) one soil containing larger amounts of Fe and Al oxides, significantly decreased, and colloidal particles flocculated with 1) a decrease in soil Eh and 2) an increase in electric conductivity (EC). During the 3- to 4-week period of waterlogging, the turbidity of the three soils significantly increased with the 1) decrease in EC and 2) increase in pH of the soils although the Eh remained low. Infrared (IR) absorption analysis showed that the suspended colloidal particles consisted of layer silicates from respective soil clays. Oxidation of suspensions of waterlogged soils by air-bubbling led to an increase in turbidity with the 1) increase in Eh, and 2) decrease in pH, EC, and water-soluble Fe₂₊ concentration. It was suggested that the stability of the soil colloidal suspensions was affected by soil reduction with alterations in ionic species and their concentrations at clay surfaces and in soil solutions.     

Particle size fractionation of soil containing coal and combusted particles

Standard procedure for dispersing natural soils for particle size fractionation can be too aggressive for soil containing coal and other organic particles from coal industries. We have investigated ultrasonic dispersion for the latter in four soils differing in pedogenesis (Phaeozem, Podzol, reclaimed mine soils), carbon content (27.5–138.6 g kg^–1), clay content (80–153 g kg^–1) and sources of particles (airborne coal dust, combustion residues, lignite particles). As we found previously for natural soils, the ultrasonic energy needed for complete dispersion varies between 450 and 500 J ml^–1, but the resulting particle size distributions differ from those obtained by standard textural analysis. This is probably related to the different properties of native soil organic matter and coal and combusted particles. Coal and soot particles may partly resist oxidation with hydrogen peroxide, depending on material and particle size. The diameter of lignite particles, remaining after oxidation, is overestimated in sedimentation analysis by a factor of 1.66. Sand-sized lignite particles can be disrupted by ultrasonication and redistributed to finer particle size fractions. The ultrasonic dispersion and particle size fractionation procedure can be applied to soils containing coal and combusted particles, but caution is needed in interpreting the results if they contain large proportions of coal particles.     

Testing the Rothamsted Carbon Model against data from long-term experiments on upland soils in Thailand

We tested the Rothamsted Carbon Model (RothC) against three long‐term (27–28 years) experimental sites on Thai upland soils in order to see how this widely used ‘temperate’ soil carbon turnover model performed in a typical farming region in the tropics. We were able to verify – over a much longer period than had been examined in previous studies – that RothC performs well in a tropical region in plots used for continuous cropping experiments of maize and cassava without organic matter application. However, the model overestimated soil organic carbon (SOC) in some plots to which large amounts of organic matter (rice straw or cassava stalks) were applied. This overestimate could not be attributed to errors in estimating either the amount of C input to the soil or the ratio of decomposable plant materials to resistant plant materials entering the soil. Among many factors affecting SOC dynamics (e.g. weather conditions, soil characteristics, etc.), which are different in tropical regions from temperate regions, we conclude that the activity of soil fauna might be a major factor which makes the performance of RothC worse where much organic matter was applied. We suggest that care should be taken when applying RothC to tropical soils with large amounts of added organic matter.     

农田恢复措施对黑土母质发育的新成土壤团聚体微形态及孔隙结构的影响

土壤结构是土壤肥力的物质基础,水土流失等导致土壤结构退化成为威胁我国粮食安全的重要因素。本文利用8年田间定位试验,通过土壤切片技术,研究不同农田恢复措施(耕作和有机物料投入)对黑土母质发育的新成土壤团聚体微形态的影响,对理解成土过程和肥力快速恢复具有重要意义。结果表明:免耕土壤(自然、人工植被恢复)、不施肥耕作土壤和仅施化肥耕作土壤均为复合孔洞状微结构,主要孔隙形状均为面状或孔洞状,连通性及数量相当,土壤垒结和微结构类似;但土壤颗粒分布频度和均匀度及粗细粒质接触紧密度与耕作负相关,免耕土壤显著地高于耕作土壤(P0.05)。施肥耕作土壤中,施有机物料的土壤为海绵状微结构,施高量有机物料的土壤微结构发育最好,施低量有机物料的土壤微结构次之,不施有机物料土壤微结构最差;其土壤垒结和微结构发育特征与有机物料的投入正相关。     

The influence of sward species composition on the rate of organic matter decomposition in grassland soil

To examine the influence of different plant materials on the rate of organic matter (OM) decomposition in soil, respiration and N mineralization/immobilization were measured during incubation of a test soil to which the plant materials were added. Amendments consisted of sward material either from grassland plots with different amounts of OM accumulation, or material from plants associated with soils having markedly different OM contents. Evolution of CO₂ from the test soil plus herbage from an experimental plot showing OM accumulation was greater than that from the same soil amended with herbage from plots without accumulation. Apparently, grass species had no significant effect on OM turnover; differences in accumulation in grassland soils must be related to other environmental factors. Decomposition of young Calluna vulgaris was, however, slower than that of young grass material and in this case species could affect organic matter accumulation.     

配位体交换在减慢由酸雨造成的可变电荷土壤酸化速度中的意义

For the purpose of evaluating the role of ligand exchange of sulfate ions in retarding the rate of acidification of variable charge soils,the changes in pH after the addition of different amounts of HNO3 or H2SO4 to representative soils of China were measured .A difference between pH changes caused by the two kinds of acids was observed only for variable charge soils and kaolinite,but not for constant charge soils and bentonite,The larger the proportion of H2SO4 in the HNO3-H2SO4 mixture,the lower the calculated H^ ion activities remained in the suspension.The difference in H^ ion activities between H2SO4 systems and HNO3 systems was larger for soils with a low base-saturation(BS) percentage than those with a high BS percentage.The removal of free iron oxides from the soil led to a decrease in the difference,while the coating of Fe2O3 on a bentonite resulted in a remarkable appearance of the difference.The effect of ligand exchange on the acidity status of the soil varied with the soil type.Surface soils with a high organic matter content showed a less pronounced effect of ligand exchange than subsoils did.It was estimated that when acid rain chiefly containing H2SO4 was deposited on variable charge soils the acidification rate might be slower by 20%-40% than that when the acid rain chiefly contained HNO3 for soils with a high organic matter content,and that the rate might be half of that caused by HNO3 for soils with a low organic matter content,especially for latosols.     

Fate of diuron and terbuthylazine in soils amended with two-phase olive oil mill waste

The addition of organic amendments to soil increases soil organic matter content and stimulates soil microbial activity. Thus, processes affecting herbicide fate in the soil should be affected. The objective of this work was to investigate the effect of olive oil production industry organic waste (alperujo) on soil sorption-desorption, degradation, and leaching of diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea] and terbuthylazine [N2-tert-butyl-6-chloro-N4-ethyl-1,3,5-triazine-2,4-diamine], two herbicides widely used in olive crops. The soils used in this study were a sandy soil and a silty clay soil from two different olive groves. The sandy soil was amended in the laboratory with fresh (uncomposted) alperujo at the rate of 10% w/w, and the silty clay soil was amended in the field with fresh alperujo at the rate of 256 kg per tree during 4 years and in the laboratory with fresh or composted alperujo. Sorption of both herbicides increased in laboratory-amended soils as compared to unamended or field-amended soils, and this process was less reversible in laboratory-amended soils, except for diuron in amended sandy soil. Addition of alperujo to soils increased half-lives of the herbicides in most of the soils. Diuron and terbuthylazine leached through unamended sandy soil, but no herbicide was detected in laboratory-amended soil. Diuron did not leach through amended or unamended silty clay soil, whereas small amounts of terbuthylazine were detected in leachates from unamended soil. Despite their higher sorption capacity, greater amounts of terbuthylazine were found in the leachates from amended silty clay soils. The amounts of dissolved organic matter from alperujo and the degree of humification can affect sorption, degradation, and leaching of these two classes of herbicides in soils. It appears that adding alperujo to soil would not have adverse impacts on the behavior of herbicides in olive production.     

THE ORGANIC MATTER CONTENT OF THE SAVANNA SOILS OF WEST AFRICA

Published and unpublished data on the amounts of organic matter and nitrogen in the surface soils of the West African savanna are reviewed. In general, amounts are small; the mean carbon content of soils from 605 well-drained sites was 0.68 per cent. Two important factors governing amounts of organic matter in well-drained soils appear to be the clay content and a moisture factor related to the length of the wet season and represented here by mean annual rainfall. Multiple linear regression on soil clay content and rainfall accounted for 46.5 per cent and 57.2 per cent, respectively, of the observed variability of soil carbon and nitrogen contents. These findings suggest that the low levels of organic matter in savanna soils arise from their predominantly sandy nature and from the relatively low rainfall. In poorly drained soils organic matter levels are higher but are less significantly related to clay content and rainfall. The influence of human interference and of parent material and altitude on organic matter is demonstrated in the context of geographically limited areas within the savanna for which more detailed information was available.     

Dispersion of humic allophane soils with supersonic vibration

The difficulties in dispersing volcanic ash soils of Japan and New Zealand have been considered to be due to the association or aggregation of allophane (3, 5, 7–10). In particular, MIYAZAWA (9) has obtained evidence that stable microaggregates of Humic Allophane soils derived from volcanic ash have been formed by dehydration of allophane. Recently, several investigators (2, 4, 6, 8, 9, 12) have pointed out that ultrasonic vibration is effective in dispersing the fine particles of soils. With respect to the applicability of supersonic vibration to the particle-size distribution analysis of Humic Allophane soils, MIYAZAWA (9) stated that the maximum dispersion, as measured by the clay content, was obtained only with supersonic vibration using an acidic medium. KOBO and OBA (8) reported that calgon (sodium hexametaphosphate) as a dispersing agent was successfully applicable to most Humic Allophane soils, but the use of HCI was necessary for some highly allophanic subsoils, and that the effect of supersonic vibration on dispersion of the soils is attributed to the breakdown of aggregates larger than 20 microns in diameter. They also recommended a mixture of 10g of soil and 50 ml of water and 20 min. exposure for a supersonic vibrator (10 Kc, 300 W). With respect to the applicability of vibration treatment in the particle-size distribution analysis of Humic Allophane soils containing volcanic glasses in abundance, some apprehension may be entertained about the breakdown of primary minerals, especially of volcanic glasses (9, 10).     

DISPERSION, MECHANICAL COMPOSITION, AND FRACTIONATION OF WEST INDIAN VOLCANIC YELLOW EARTH SOILS (ANDEPTS)

Methods of dispersion used in Japan and New Zealand for soils formed on recent volcanic ash (Andepts) which involved adjusting the pH of organic matterfree soil suspensions to 4 or 10·5 were not effective on similar soils in the Caribbean. Partial dispersion was obtained by adjusting the pH to between 2 and 3 but lower (down to pH 1) and higher (up to pH 11·1) pH values were ineffective. Substantial amounts of Al dissolved at pH 1–3 probably as a result of some disintegration of the allophanoid minerals. Zirconium nitrate solutions were very efficient in causing dispersion, a concentration of 12 me Zr being necessary for complete dispersion of 20 g samples of soil. Excess Zr did not adversely affect dispersion but resulted in depression of pH of the suspensions. The high ionic charge, small atomic radius, and low ionization potential of Zr apparently resulted in saturation of the cation exchange capacity and some isomorphous substitution of Al leading to a net positive charge of the clay and an increase in anion exchange capacity. This was considered to be responsible for dispersion.     

DECOMPOSITION OF SOIL POLYSACCHARIDE

Polysaccharide material was isolated by absorption on charcoal from the acidified, non-humic fraction extracted by alkali from three soils. The polysaccharides were used as substrates in soil incubation, perfusion, and suspension experiments. Concordant results were obtained with freely drained Countess-wells and Insch Association soils derived from acidic and basic igneous parent materials respectively. Polysaccharide material added to soil at low concentration (I per cent) was apparently totally decomposed after 8 weeks when the amounts of polysaccharide in control and amended soils were statistically indistinguishable. At higher concentrations (2-3 per cent) a significant difference in reducing sugar, equivalent to about 30 per cent of the substrate, remained after 32 weeks. Partial neutralization of the polysaccharide material with calcium hydroxide increased the rate of decomposition in Countesswells Association soil but had an opposite, smaller effect in Insch Association soil. Soil polysaccharide material was decomposed slightly faster in perfusion and suspension experiments than in moist soil. Only 20 per cent of the carbohydrate in the unfractionated alkali–soluble organic matter of soil was decomposed during incubation in soil for up to 133 weeks. There was usually little change in the carbohydrate content of soil incubated alone. The soil microbial population showed a marked increase in response to added polysaccharide material but only slight qualitative changes were detected. It is concluded that the persistence of naturally occurring polysaccharide in soil is related to inaccessibility caused by chemical combination, complexing or insolubility but not to a biologically-stable molecular structure.     

Effect of organic amendments on the retention and mobility of imazaquin in soils

The influence of two organic amendments consisting of an urban waste compost (SUW) and a commercial amendment from olive mill wastes (OW) was assessed on the sorption properties and leaching of the ionizable herbicide imazaquin on four soils with different physicochemical characteristics. A loamy sand soil (CR), a loam soil (P44), a silt loam soil (AL), and a clay soil (TM), with low-medium organic matter contents, were chosen. Sorption-desorption experiments were performed on the original soils and on a mixture of these soils with the organic amendments at a rate of 6.25% (w/w). These mixtures were used just after preparation and after aging for 3 months. Imazaquin adsorption was higher on AL soil because of its high content of amorphous iron oxides, whereas it was related to the soils' organic matter (OM) contents on TM and CR soils and to acid pH on P44 soil. Addition of exogenous OM to soils caused a decrease in the adsorption of the herbicide with the only exception of CR soil, due to blocking of adsorptive surfaces and/or equilibrium pH rise. The extent of this decrease was dependent only on the nature of the added amendment on AL soil. The adsorbed amounts of imazaquin on aged organic fertilized soils were usually fairly close to that on original soils. Results of soil column experiments indicate that addition of exogenous organic matter cannot be considered as a regular practice for retarded movement of imazaquin.     

Exploring the effect of organic matter on the interactions between mineral particles and cations with Wien effect measurements

Purpose

Understanding of the interactions between cations, mineral particles, and organic matter (OM) in soils is of paramount importance in plant nutrition and environmental science, and thus, these phenomena have been studied extensively. At present, an effective and simple tool to investigate these interactions does not exist. Based on previous studies of Wien effect in suspensions, the interactions of cations with soil mineral particles, complicated by the presence of organic matter, can be easily determined by means of Wien effect measurements, which was the objective of this study.

Materials and methods

A paddy soil originating from a yellow-brown soil, rich in organic matter, served as a test sample, from which the clay fraction of less than 2 μm in diameter was separated. Organic matter of aliquots of the clay fraction was removed by the oxidation with hot H₂O₂, and the natural and OM-free samples were saturated with various cations: Na⁺, K⁺, Ca²⁺, and Cd²⁺. The effects of OM present in the paddy soil on the interactions between the cations and the soil mineral particles were investigated by measuring the suspension Wien effect with a homemade apparatus, SHP-2.

Results and discussion

The weak electrical field electrical conductivities (EC₀) of suspensions of the natural soils saturated with various cations were higher than those of the OM-free soils. The rate of increase in electrical conductivity of suspensions of the OM-free soil, except that of suspensions saturated with Na⁺, at electrical field strengths >50～100 kV?cm^?1 was higher than those of the natural soil suspensions. The presence of OM increased the mean free binding energies of cations other than Na⁺. The increasing binding energies for K⁺ and Ca²⁺ were 0.56 and 0.57 kJ?mol^?1, respectively, which were significantly larger than the increase for Cd²⁺ as only 0.03 kJ?mol^?1. The binding energies of various cations on both natural and OM-free soils were all in the order: Na⁺?<?K⁺?<?Ca²⁺≈Cd²⁺. As opposed to its effect on the binding energies, the presence of OM reduced the mean free adsorption energies of the cations. Except for Na⁺, the adsorption energies of K⁺, Ca²⁺, and Cd²⁺ at field strengths >50 kV?cm^?1 were lower in the natural soil as compared with the OM-free soil, and the differences between the adsorption energies became larger with increasing field strengths. The presence of OM made the zeta potential of the soil particles saturated with Na⁺ and K⁺ positive, and the particles saturated with Ca²⁺ and Cd²⁺ negative.

Conclusions

Organic matter affected the interactions of cations with soil mineral particles significantly. Binding and adsorption energies, which were quantitative measures of the interactions between cations and soil particles, could be determined by Wien effect measurements in suspensions. The binding energies on natural soils were larger than those on the corresponding OM-free soils, and the adsorption energies on the natural soils were lower than those on OM-free soils.     

有机质、全氮和可矿化氮在反映土壤供氮能力方面的意义

用 2 5个表层土壤样品和 6个土壤 3 6个不同层次的土壤样品研究了可矿化氮与有机质、全氮的关系。可矿化氮由通气培养法测定。研究结果表明 ,不论表层土壤或不同层次剖面土壤中的可矿化氮都与有机质、全氮高度正相关。但是由于可矿化氮与有机质、全氮有自相关存在 ,他们之间的相关有一定的不真实性。而且 ,土壤的可矿化氮并不与全氮或有机质成正比 ,可矿化氮与有机质或全氮的比值因土壤而不同 ,差别很大。特别重要的是 ,可矿化氮对有机质或全氮的比率几乎在一条直线上 ,它们之间的相关性明显高于可矿化氮与有机质或全氮的关系。这些结果显示 ,矿化氮的数量取决于有机质和全氮中的可矿化部分 ,而不是其总量。土壤剖面中累积的硝态氮数量低时 ,作物的吸氮量与可矿化氮的关系远较与有机质或全氮的关系密切 ,更证明了测定可矿化氮有其特定意义 ,有机质、全氮的测定并不能代替可矿化氮的测定。