DISPERSION OF SOIL PARTICLES BY SONIC VIBRATION1

Dispersion of soil particles is achieved without use of oxidants, acids, or peptizing reagents, by subjecting an aqueous suspension of the soil sample (10 g soil, 25 ml water) to sonic vibration at 13 to 15°C using a Raytheon (9 kc, 50 w) vibrator. Studies using soils of widely different textures and organic matter contents showed that the dispersion caused by sonic vibration for 30 min, as evaluated by pipette analysis for < 2 μ material, was similar to that obtained by chemical methods currently used for dispersion of soils. The dispersion effected by ultrasonic vibration of soil-water suspensions using a probe-type vibrator (18–20 kc, 60 w) is comparable to that achieved by sonic vibration. The vibration method of dispersing soil particles yields stable suspensions and is effective with highly calcareous soils and with soils containing large amounts of montmorillonite and organic matter. It permits dispersion of soil particles without dissolution of more than trace amounts of organic or inorganic material and does not significantly affect the pH or conductivity of the soil suspension.     

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.     

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.     

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.     

Dynamics of soil organic matter associated with particle-size fractions of water-stable aggregates

Stable macroaggregates (> 200 μm) of cultivated soils are reported in the literature to be richer in organic carbon, and in young organic carbon in particular, than microaggregates (< 200 μm). However, the nature of this additional carbon is not yet known. To determine it, we compared the composition of organic matter in stable macroaggregates with that in unstable ones. Macroaggregates 2–3 mm in diameter were separated from two silty cultivated soils from the Paris basin. They were slaked, and the primary particle composition of the resulting fractions was analysed. We used the natural abundance of ¹³C to quantify the amount of young carbon, derived from a maize crop, in the various size fractions. The stable macroaggregates were richer in total C and in young C (younger than 6 and 23 years respectively in the two soils studied) than the unstable ones. This young C comprised 50% particulate organic matter, 20% associated with silt and 30% with clay particles. We propose a schematic composition of aggregates in these soils in which stable aggregates are formed by the binding of microaggregates by additional young organic matter, predominantly plant debris. Young organic matter is preferentially incorporated and is responsible for aggregation, though it is eventually redistributed among aggregate classes through the destruction and re‐formation of the aggregates. We have developed a model to simulate this redistribution. The model shows that stable macroaggregates have a life of a few years, but that microaggregates may exist for decades. We suggest that the stabilization and de‐stabilization of macroaggregates in soils is linked to the incorporation and biodegradation of plant debris.     

Modification of soil physical properties by manipulating the net surface charge on colloids through addition of Fe(III) polycations

The addition of 0.07 per cent Fe in the form of polycation of molecular weight 10000–50000 flocculated soil suspensions. Higher concentrations of Fe(III) caused redispersion of the clay. Electrophoretic and electron microscopic studies confirmed the flocculation-dispersion phenomena. The soil suspensions with higher concentrations of Fe(III) gave points of zero charge (PZC) between pH values 5.0 and 6.0. The flocculation resulted in microaggregation and created pores 40–100 μm in diameter. This led to an increased water-holding capacity and hydraulic conductivity and lower bulk densities and modulus of rupture. The soils treated with Fe(III) polycations were shown to be more friable than untreated soils.     

Soil microaggregation as influenced by uncharged organic conditioners

Abstract

To assess the interaction of water soluble stabilizing agents with soil particles, soil microaggregation was studied after treatments with two uncharged organic conditioners, Polyvinyl alcohols (PVAs) and Dextrans, of different molecular weight. The size distribution of microaggregates (diameter <250 pm) was determined on two soils of low organic matter contents with differing texture by means of a laser light technique. PVAs and Dextrans modified the microaggregate size distribution, increasing the proportion of >75 μm aggregates. The extent of this modification increased with the molecular weight of the conditioners and, for the same molecular weight, decreased with increasing amount of conditioner used. The aggregating effect of PVAs was stronger on particles smaller then 5 μm, which formed microaggregates in the range 10–100 μm which were not broken down even by ultrasonication. S.E.M. micrographs of soil aggregates confirmed the results reported above. PVAs and Dextrans appeared to produce a more porus structure with more aggregates of about 100 μm size, both in the clay soil and, to a lesser extent, in the sandy soil.     

OPALINE SILICA OF RECENT VOLCANIC ASH SOILS IN JAPAN

Soil samples were investigated from three different climatic regions of Japan. The soil profiles were well drained and consisted of a number of buried soils. Organic matter and free iron oxides were removed from the samples which were then fractionated. Laminar opaline silica particles occurring in the 0.2 to 5 μm fractions were examined electron-optically following ultrasonic and cold 5 per cent sodium carbonate treatment. Four types of opaline silica particles were distinguished: circular, elliptical, rectangular, and rhombic. The opaline silica particles are extremely thin. They show fine-grained uneven surfaces and weathered and alkali-treated opaline silica particles appear to be very porous suggesting that the particles are actually composed of extremely fine silica spheres. Opaline silica panicles occur only in the 0.2 to 5 μm fractions and are most abundant in the 0.4 to 2 μm fractions. Of the four types the circular and elliptical types predominate. The elliptical type is the most common in the fine fraction and the circular type in the coarse fraction. Compared with B- or C-horizons, the A-horizon of a profile tends to be relatively rich in opaline silica. Opaline silica particles are abundant in recent volcanic ash soils, particularly in Hokkaido soils < 500 years old. Soils of this area more than 7000 years old contain opaline silica particles whereas they are rare in Kanto soils > 6000 years old and in Kyushu soils > 4000 years old. Hokkaido has much lower mean annual temperatures and rainfall than Kanto or Kyushu. This may have caused slower weathering of opaline silica in the Hokkaido soils. Conditions for formation of opaline silica may be less favourable in subsoils because supersaturation of silica and concentration of bases is less likely to occur in subsoils than in surface soils.     

Buffer capacity and Cu affinity of soil particulate organic matter (POM) size fractions

Particulate organic matter fractions (POM), defined as sand‐sized organic separates in soils, are known to be labile organic components with a rapid turnover. Recently, POM fractions were identified to be metal‐enriched in both metal‐contaminated and uncontaminated soils. However, mechanisms for such metal‐enrichment are poorly understood, because of the paucity of information on the chemical properties of POM. The aim of this study was to quantify the reactivity of POM towards Cu and to show a POM‐size effect on this reactivity. POM was isolated from soils with different organic amendment managements: straw (S), conifer compost (CC), and non‐amended (NA). Two POM size fractions were isolated by density‐fractionation in water: 50–200 μm and 200–2000 μm. These fractions were studied for their metal contents, acid‐base properties and affinity toward Cu. The buffer capacity and Cu affinity were modeled by FITEQL 4.0 software and compared between the two POM size fractions. Each POM size fraction provided a buffer capacity due to the presence of reactive sites, the greatest being for the 50–200 μm POM fractions. A signature of organic inputs as seen by the buffer capacities was observed for the 50–200 μm but not for the 200–2000 μm POM fractions. But Cu affinity was comparable between the coarse and fine POM fractions and no significant differences were found between NA, S and CC samples. We checked the hypothesis that decreasing POM size due to degradation processes generates more reactive surface sites. Results confirmed that soil POM plays a key role as a metal sink, due to its chemical properties.     

Organische Substanz und Benetzbarkeit von Bodenproben ‐ eine Interpretation

Organic matter and wetting properties of soil samples — an interpretation Samples were taken from A horizons of Udands and Humults at four 4 sites under forest and pasture vegetation in Southern Chile. Aggregates of 4—6 and 10—12 mm diameter were obtained by sieving and separated into aggregate surface and core fraction by peeling. The fraction 38—63 μm was separated by sieving. Organic matter was determined in all samples. Wetting angles were determined by direct reading at drops of saturated KCl solution on a layer of particles fixed on microscope glass slides by double‐sided adhesive tape. Values were correlated with contents of organic matter. The results showed an increase of wetting angles with increasing percentage of organic matter in the overall samples as well as in all separate groups (depth, vegetation, size, and parts of aggregates). The correlations, however, were not close (r ～ 0.5), irrespective of the significance level. Wetting angles in soils are caused by combined effect of different relative amounts and properties of free organic matter particles and organic films on mineral particles.     

The behavior of particulate material in the treatment lagoons of a bleached kraft pulp mill

The effect of treatment lagoons on the suspended particulate matter contained in Bleached Kraft Mill Effluent (B.K.M.E.) has been studied. Good settling characteristics for coarse material (> 10 μm diam) were found, but fine particles (< 1 μm diam) remain in suspension throughout the treatment system. When the treated effluent is discharged into the brackish water of a narrow tidal cove, the increased ionic strength of the medium promotes the combination of fine particles. This flocculation effect produces a larger concentration of coarse material in the treated effluent than was contained in the raw mill discharge. A procedure for artificial flocculation, within the treatment system, is suggested as a method of improving both the treatment efficiency and the physical properties of the effluent released to the marine environment.     

Effects of organic carbon and clay contents on structure-related properties of arable soils with high clay content

Understanding of factors governing soil structural features is necessary for managing key processes affecting crop productivity and environmental impacts of agriculture, for example, soil water balance, aeration, and root penetration. Organic matter is known to act as a major binding agent in soil aggregation and thus constitutes a central pillar in soil structure formation. However, knowledge of the structural role of organic matter or carbon (OC) in soils highly rich in clay-sized particles (<0.002 mm) is limited. In this study, the effects of clay and OC contents on aggregate stability, water holding capacity, near-saturated hydraulic conductivity, total porosity, and pore size distribution were assessed in cultivated fields with high clay content located in private crop production farms in southern Finland. Significant positive correlations were found between OC content and proportion of water stable aggregates and specific pore sizes from the range of 30 μm up to 1 mm diameter determined by image analysis. Porosities on a smaller size range derived from water retention measurements likewise showed a positive correlation with OC in <0.2 μm sizes. On the range of 0.2–1 μm, a negative relationship was observed, which induced a negative effect of OC on soil plant available water reserves. In line with the positive correlation between OC and larger soil pores, free water, representing the amount of water that can be drained by gravity, exhibited a positive relationship with OC suggesting that OC content can enhance aeration of soils with high clay content. Compared to OC, clay content tended to have an adverse effect on soil structural properties. Clay correlated negatively with pores larger than 30 μm, free water content, and extrapolated field saturated hydraulic conductivity. Further, our imaging results showed how saturated hydraulic conductivity was controlled by pore morphology, and there was a power law relationship between the conductivity and critical pore diameter. $K\propto d_c^2$ in agreement with the percolation theory. Overall, the structural impacts and hydrological implications of OC and clay in heavy clay soils vary by pore size ranges and their emergent practical impacts are thus not straightforward.     

A re‐evaluation of the enriched labile soil organic matter fraction

Identifying ‘functional' pools of soil organic matter and understanding their response to tillage remains elusive. We have studied the effect of tillage on the enriched labile fraction, thought to derive from microbes and having an intermediate turnover time. Four soils, each under three regimes, long‐term arable use without tillage (NT), long‐term arable under conventional tillage (CT), and native vegetation (NV), were separated into four aggregate size classes. Particle size fractions of macro‐ (250–2000 μm) and microaggregates (53–250 μm) were isolated by sonication and sieving. Subsequently, densiometric and chemical analyses were made on fine‐silt‐sized (2–20 μm) particles to isolate and identify the enriched labile fraction. Across soils, the amounts of C and N in the particle size fractions were highly variable and were strongly influenced by mineralogy, specifically by the contents of Fe and Al oxides. This evidence indicates that the fractionation procedure cannot be standardized across soils. In one soil, C associated with fine‐silt‐sized particles derived from macroaggregates was 567 g C m^?2 under NV, 541 g C m^?2 under NT, and 135 g C m^?2 under CT, whereas C associated with fine‐silt‐sized particles derived from microaggregates was 552, 1018, 1302 g C m^?2 in NV, NT and CT, respectively. These and other data indicate that carbon associated with fine‐silt‐sized particles is not significantly affected by tillage. Its location is simply shifted from macroaggregates to microaggregates with increasing tillage intensity. Natural abundance ¹³C analyses indicated that the enriched labile fraction was the oldest fraction isolated from both macro‐ and microaggregates. We conclude that the enriched labile fraction is a ‘passive' pool of soil organic matter in the soil and is not derived from microbes nor sensitive to cultivation.     

TOWARDS AN OBJECTIVE CLASSIFICATION OF SOIL STRUCTURE

A classification of structural condition in surface soils is proposed, based on the volumes of two categories of pore size, termed air capacity (pores greater than 60 μm diameter) and available water (pores of 60 to 0.2 μm diameter. Relationships of pore volumes to particle size class, organic carbon content and soil water regime are examined. Soil structural conditions are mainly affected by water regime and organic carbon and, apart from the extremes of sandy or clayey textures, less influenced by particle size distribution.     

Size Characterization by Laser Granulometry of Colloids Extracted from Compost at Different Temperatures

The objective of this work was to characterize colloids extracted from composts and their potential retention in soils. Compost made of sludge and green wastes was sampled (i) during the fermentation phase and (ii) after maturation. The same kind of compost was used in a long-term field experiment at Feucherolles (France), near Paris where amended and nonamended soils were sampled. The colloidal fraction was extracted from composts in water at room temperature (20°C) and compared to the colloidal fraction extracted from the soil. Composts were also extracted by pressurized hot liquid water at 50, 125 and 175°C. The total organic carbon of the extracts was measured and the particle size distribution (PSD) of colloidal extracts was analyzed by laser granulometry. The diameters of the colloids extracted from the soil ranged between 0.040 and 0.300 μm, independently of the temperature. For composts, it varied from 0.040 to 3.200 μm when extraction was done at 20°C, while at higher temperatures, much more organic matter was extracted, and colloid diameters ranged from 0.040 μm to 0.200 μm. The water-soluble C decreased and the size of colloids recovered in water at temperatures below 50°C increased when compost maturity increased. The adsorption on soils of colloidal particles extracted from composts was characterized. The largest adsorption (up to 30% of the initial soluble C) occurred with the extracts recovered at high temperature, in relation to the more hydrophobic properties of the colloids extracted with hot water maintained in subcritical conditions. After adsorption, the particle size distribution in the colloidal fraction extracted at 20°C moved towards finer fractions; by contrast, the colloidal fraction extracted at 175°C moved towards coarser fractions. The coarsest colloids coming from the soil disappeared during the adsorption experiment, probably because of the coprecipitation with the finest colloids coming from compost.     

Losses of carbon and nitrogen with prolonged arable cropping from sandy soils of the South African Highveld

A knowledge of the kinetics of organic matter transformations in arable soils is important for managing them sustainably. Our aim in this study was to elucidate the effects of cropping period on pools of C and N in coarse‐textured savanna soils of the South African Highveld. Composite samples were taken from the top 20 cm of soils (Plinthustalfs) that have been cropped for lengths of time varying from 0 to 98 years in each of three different agro‐ecosystems in the Free State Province of South Africa. Thereafter, soil organic C and N concentrations were determined in the bulk soil (< 2 mm) as well as in the clay (< 2 μm), silt (2–20 μm), fine sand (20–250 μm), and coarse sand (250–2000 μm) separates. Long‐term cultivation of native grassland reduced soil C and N concentrations by 65 and 55%, respectively. Losses of soil organic matter occurred from all particle‐size separates, although rate loss constants increased as particle size increased. The concentrations of organic C reached equilibrium after 34 years for the bulk soil and after 55 years for clay‐size separates. Nevertheless, organic matter attached to silt continued to be lost as the cropping continued, probably due to wind erosion. Changes in soil properties thereby continued even after almost 100 years of cultivation.     

酰胺类除草剂在土壤上吸附的位置能量分布分析

根据异丙甲草胺、乙草胺、丙草胺和丁草胺在六种土壤上的等温吸附结果 ,计算了它们在六种土壤上的位置能量分布和有机质标化的平均分配常数。结果表明 :四种除草剂在六种土壤上的吸附等温线为Freundlich型 ;低浓度范围内 ,农药在土壤上的吸附首先发生在土壤表面的高能吸附位置上 ;土壤表面位置能量分布的具体情况与被吸附农药性质有关 ;土壤上的吸附位置数或吸附容量主要与土壤有机质含量有关 ,粘土对吸附也有一定的贡献 ;疏水键合机理在四种酰胺类除草剂吸附过程中起着重要的作用     

Incorporation of S into soil organic matter in the field as determined by the natural abundance of stable S isotopes

In agricultural systems with low S inputs, soil organic matter is a major source of S and the transformations between organic and inorganic S pools are important for the supply of S to plants. This study was conducted to determine the effect of S fertilizer on the size and activity of organic S pools. For 5 years S fertilizer with a known composition of stable S isotopes was applied to a rotation on a loamy soil and a coarse sandy soil at rates higher than the plant demand. Total organic S in soil organic matter was not affected by sulphur application, but a small increase occurred in the sulphate ester fractions (P<0.05). Inorganic sulphate concentrations in the soil reflected the S application in the year of sampling, whereas S applied in earlier years was not recognized. Organic matter below the plough layer in both soils was enriched with S, possibly as a result or organic matter leaching or an increased clay content in the subsoils. At 0–20 cm, the C:S ratio in organic matter was ca. 100 for both soils, decreasing to 73 and 46 at 60–80 cm for the coarse sandy soil and the loamy soils, respectively. In both soils, isotope data showed that ca. 30% of organic-bonded S at 0–20 cm originated from fertilizer S applied during the last 5 years, irrespective of the S application rate. At 20–40 cm the rate of incorporations was lower and at 40–60 cm no incorporation of fertilizer S into organic matter was recognized. The fertilizer application did not induce net changes in the total organic S fraction, but isotope data indicated that a considerable part of the organic S pool was involved in S cycling in the field.     

Aggregate-occluded black carbon in soil

The great stability of black carbon (BC) in soils may not be solely attributable to its refractory structure but also to poor accessibility when physically enveloped by soil particles. Our aim was to elucidate the intensity of physical entrapment of BC within soil aggregates. For this purpose, the A horizon of a forest, and of a grassland soil, and of three soils under tillage, were sampled at the experimental station Rotthalmünster, Germany. Black carbon was assessed in water‐stable aggregates and aggregate‐density fractions using benzene polycarboxylic acids as specific markers. The greatest BC concentrations made up 7.2% of organic carbon and were found in the < 53 μm fraction. The smallest BC concentrations occurred in the large macroaggregate fractions (> 2 mm). This pattern has been sustained even after tillage. The C‐normalized BC concentrations were significantly greater (P < 0.05) in the occluded particulate organic matter (OPOM) fractions than in the free particulate organic matter (FPOM) and the mineral fractions. This enrichment of BC compared with organic carbon in the OPOM fractions amounted to factors of 1.5–2.7. Hence, BC was embedded within microaggregates in preference to other organic carbon compounds. Only 2.5–3.5% of BC was located in the OPOM fraction < 1.6 g cm^?3, but 22–24% in the OPOM fraction with a density of 1.6–2.0 g cm^?3. This suggests that BC possibly acted as a binding agent or was selectively enriched during decomposition of protected SOM, or both. Physical inclusion, particularly within microaggregates, could therefore contribute to the long mean‐residence times of soil‐inherent BC.     

栗钙土的年龄

在阐明粟钙土发生特性的基础上,本文运用放射性碳方法断定了玄武岩熔岩台地上栗钙土有机质和腐殖质组成中胡敏酸、胡敏素的年龄.结果表明,我国栗钙土与其它国家的软土系同一时期的产物,它们形成于大约6000年以前.栗钙土有机质诸部分的年龄序列是:胡敏酸>胡敏素>土壤有机质.