A DUNE CATENA ON THE CLAY PLAINS OF THE WEST CENTRAL GEZIRA, REPUBLIC OF THE SUDAN

Fixed dunes occupy 400 000 acres in the west central Gezira. Isolated dunes occur sporadically through the central Gezira, and trend NW from Sennar on the Blue Nile to El Geteina on the White Nile. They probably originated as channel deposits laid down by former distributaries of the Blue Nile. Minor aeolian re-sorting took place, and was followed by plant colonization and dune stabilization. Erosion of the dunes is locally active, resulting in exposure of the underlying clays. The catena comprises leached sands (Psammustents) on the dune crests and illuvial loams and clays (Orthustents) in the swales. Swale soils contain pedogenetic and relict lacustrine carbonate. The catena is often superimposed upon former soils, resulting in polygenetic profiles.     

我国主要变性土的微形态研究

本文研究了变性土三大土类黑粘土、浊粘土,艳粘土的微形态特征。通过大形态、微形态、超微形态观察和膨胀收缩试验揭示了变性土的孔隙、结构、粘粒的就地重新排列以及土壤垒结与土壤粘粒含量、粘粒种类以及土壤膨胀势之间的关系,其中福建浊粘土的微形态特征发育最明显。     

EXCHANGEABLE SODIUM AND YIELDS OF COTTON ON CERTAIN CLAY SOILS OF SUDAN

Within Sudan there are extensive areas of Vertisols in the Nile River valley south of Khartoum. These soils have clay contents ranging from 40 to 60 per cent, very slow permeability, exchangeable sodium percentages up to 35 or more in the upper 60 cm, and pH values between 8.0 and 9.6. Under present management, satisfactory yields of long staple cotton are obtained under irrigation on soils having exchangeable sodium percentage (ESP) of up to 35 in the upper 60 cm. It is proposed that soil characteristics such as type of clay, cracking and exchangeable sodium in the hot, dry climate of Sudan influence soil structure and water availability and thus the yields of cotton.     

A COMPARISON OF FIELD ESTIMATES AND LABORATORY ANALYSES OF THE SILT AND CLAY CONTENTS OF SOME WEST MIDLAND SOILS

Field estimates of silt and clay contents were compared statistically with the silt (2-60 μm) and clay (<2 μm) contents determined by the pipette method of 184 soil horizons from a large range of west Midland soils (clay contents 1–81 per cent and silt contents 3–71 per cent). Regression equations were calculated (a) for each of four surveyors using combined topsoil and subsoil data and (b) for surface and subsoils separately using combined data for the four surveyors. There is slight evidence of ‘operator bias’ and a tendency for all four surveyors to underestimate clay in surface horizons though these conclusions have limited significance because of the small number and the distribution of some of the sample populations. Single equations for the combined data from all operators and all horizons explained 75.5 per cent of the variation in field estimates of silt content and 85.4 per cent of the variation of clay estimates. For silt, the regression line almost passes through the origin with a slope not significantly different from unity, whilst for clay, the line intercepts the y-axis close to zero and has a slope of 0.904. The results show that, with experience and adequate reference samples, surveyors can confidently estimate the particle-size distribution of a wide range of soils. These studies were done during soil mapping in Staffordshire, Hereford and Worcester, and Salop using the newly introduced soil classification (Avery, 1973) and revised Handbook (Hodgson, 1974) of the Soil Survey of England and Wales.     

DETECTION OF CLAY IN SOIL SOLUTION WITH METHYLENE-BLUE PAPER AND THE ASSESSMENT OF CLAY MOBILITY IN SOILS

The adsorption of methylene blue and the accompanying colour change appears to be a fairly specific and sensitive reaction for colloidal soil clay, especially when used with chromatography paper as a supporting medium. Thus minute amounts of clay in migrating soil solutions can be detected, and stability and electro-phoretic mobility of very dilute suspensions evaluated.     

滇桂地区变性土的发生特性和系统分类研究

本文研究了我国西南地区变性土的形成条件和发育程度，并结合世界主要分类系统进行了分类研究。结果表明，相对于地带性土壤而言，变性土风化成土作用较弱，发育程度较低，土壤中元素的淋溶迁移及相对富集程度也远低于地带性土壤。由于粘粒含量尤其是粘粒矿物组成的差异，变性土表现出极高的膨胀潜势。通过研究，本文还对我国土壤系统分类中变性土土钢、亚纲、土类等的划分标准和依据及变性土性土的确立等提出了见解。     

PROCESSES OF HUMUS FORMATION AND TRANSFORMATION IN SOILS OF THE CANADIAN GREAT PLAINS

A study of the A horizons of cultivated soils from Saskatchewan, Canada showed that the organic carbon contents were in the sequence semi-arid Dark Brown soils (1.7 per cent), < Black soils (2.55–2.8 per cent), < sub-humid Dark Gray soils (4.5 per cent). The relative proportions of humic acids and the ratios of humic acid: fulvic acid increased in a similar sequence. There were accompanying increases in extinction values at 280 nm, and in levels of nonhydrolyzable nitrogen suggesting more extensive polycondensation of humic materials. These observations are discussed in terms of a working hypothesis for humus formation and transformations, that gives particular emphasis to the soil environment and to interactions between organic and inorganic components.     

THE MODIFICATION AND APPLICATION OF THE QUANTITATIVE X-RAY DIFFRACTION METHOD OF SCHULTZ (1964) TO THE MINERALOGICAL STUDY OF LONDON CLAY SAMPLES

Over 300 samples of London Clay from twenty-one widely scattered sample locations in the London and Hampshire Basins were to be examined in detail by X-ray diffraction techniques. The quantitative method of Schultz (1964) was chosen as being most suited for the analysis of the samples. Series of tests designed to investigate the reproducibility characteristics of the method were conducted. These showed that the sampling, sample preparation, and interpretation procedures used all introduced cumulative reproducibility errors of roughly equal magnitude which completely overshadowed errors introduced by machine-response variation. It was also established that minerals whose contents were below 10 percent of whole sample failed to be consistently quantitatively estimated. The accuracy of this method of analysis depended heavily on Schultz's quoted ‘intensity factors’ which were involved in the calculation of individual mineral contents. The authenticity of these factors with regard to the London Clay suite of minerals was checked by comparing accurately determined mineral percentages of six standard London Clay samples with the results obtained by use of the Schultz procedure and factors. The method was found to be basically sound although the intensity factors for quartz, total clay minerals, carbonates, and illite had all to be slightly adjusted. The accurate quartz percentages were established by three different X-ray diffraction techniques and the carbonate percentage checks were accomplished by specially designed wet chemical methods. The total clay mineral and illite contents checks were carried out by back-calculation of the Al₂O₃, and K₂O percentages as determined from full wet chemical analysis. Using the adjusted factors, quartz percentages are estimated accurate to ±6 percent, total clay minerals to ±7 percent, carbonate percentages to ±1.5 percent, and illite contents to ±4 percent of the whole rock.     

中国变性土的分布、特性和分类

本文研究了我国主要变性土的区域分布、某些发生学特性和分类问题。研究结果表明:我国变性土及变性土型土壤有广泛分布,从热带、亚热带到暖温带。其共同的成土条件是:有一定干湿交替的气候、低平的地势、富含盐基的母质和粘重的质地。据估计,此类土壤约占全国总土地面积的1%左右。变性土是在特殊的地球化学环境和干湿交替的条件下形成的,深受地域性因素的影响,其成土年龄古老而发育程度较弱。一般变性土带有某些水成土的特征,而钙质变性土兼有水成土和钙层土但又不完全与它们相同的成土地球化学特点。所研究的11个剖面中,有8个属于中国土壤系统分类(首次方案)中的变性土土纲,其余3个则属于潮湿土土纲。     

THE CHARACTERIZATION OF SOME SCOTTISH ARABLE TOPSOILS BY AGRICULTURAL AND ENGINEERING METHODS

Samples from fifty-eight arable topsoils in south-eastern Scotland were analyzed for compactability, plasticity, organic matter content, particle density, and particle-size distribution. The samples were classified by the methods employed for engineering soils and the groupings so obtained were compared with the textural classes. The criteria for the classification of engineering soils provide useful information on the likely mechanical behaviour of arable soils in the field. Tests were made for correlation between fifteen soil properties. The organic matter content, which in the samples studied ranged from 2 to 10 per cent, was shown to influence several mechanical properties, including optimum moisture content, maximum dry bulk density, liquid limit, and plastic limit. Maximum bulk density and optimum moisture content were strongly correlated with Atterberg limits and organic matter content. The liquid limit was more closely correlated with particle-size distribution than was the plastic limit. Unlike soils encountered in engineering, the particle density of these soils was not correlated with particle-size distribution but, like the Atterberg limits, was strongly correlated with organic matter content.     

EFFECT OF SOIL CLAY TYPE ON THE 'AVAILABILITY' OF ZINC IN SOME MEDITERRANEAN RENDZINA SOILS

The influence of soil clay type on the ‘availability’ of zinc was studied in six rendzina soils from Israel. The quantities of zinc extracted by EDTA—(NH₄)₂CO₃, were higher in the soils with a lower total of zinc content. Values of extracted zinc calculated on the basis of soil minus CaCO₃ were higher in soils with attapulgite than in soils with montmorillonite as the predominant mineral. Thus, it appears that zinc is more strongly bound to montmorillonite. This was confirmed by a greenhouse experiment in which total zinc uptake after zinc fertilization was higher from rendzinas containing attapulgite than from montmorillonitic rendzinas.