Showing papers in "European Journal of Soil Science in 1979"

Cadmium accumulation in soils from long‐continued applications of superphosphate

Abstract: SUMMARY A simple, sensitive method developed for the analysis of geostandards was used to measure the accumulation of Cd in soils from superphosphate applied annually to grass-land and arable soils for many years. Rates of application were equivalent to 33 kg P and 5 g Cd ha−1 yr−1 for 95 yr in three experiments in England and to 37 kg P and 20 g Cd ha−1 yr −1 for 30 yr in one experiment in New Zealand. Very little Cd accumulated in the surface horizons (0–22.5cm) of either of the arable soils from England; about one-quarter of the applied Cd was detected in the sub-soil (22.5–45.0 cm) of one experiment (Broadbalk) but none in the second (Barnfield). About one-half of the applied Cd was retained in the 0–22.5 cm horizon of grassland soils from both England and New Zealand. The light (<2.2 gcm−3) organic-rich fraction of Park Grass soil from England contained about three times as much Cd as the heavier, mineral-rich fraction. This suggests that when Cd is incorporated into organic matter its mobility is decreased and soil pH then has smaller effects on its mobility. Uptake of Cd by grass-clover pasture in New Zealand averaged only 0.4 g Cd ha−1 yr−1 or 2% of the amount applied.

Synthetic allophane and imogolite

Abstract: Summary In order to prepare allophane and imogolite in the laboratory, solutions containing l–2× 10–3 M orthosilicic acid and 4–0.5 × 10–3 M A1C13 (SiO2/Al2O3 molar ratio; 0.5, 1.0, 2.0, 4.0 and 8.0) were heated at 95–100°C for 113 hours after addition of NaOH (NaOH/Al molar ratio; 1.0, 2.0, 2.8 and 3.0). Boehmite was found in the precipitates from all solutions with initial SiO2/Al2O3 ratios of 0.5. Imogolite was found with allophane II in the products from solutions with SiO2/Al2O3 ratios of 1.0 or greater and with NaOH/Al ratios of 2.8 or less (final pH 5.0), whereas allophane I was found in the precipitates from solutions with the same SiO2/Al2O3 ratios but with the NaOH/Al ratio of 3.0 (final pH = 5.0–6.3). The mode of formation, chemical composition, infrared spectra, electron micrographs, electron diffraction patterns and differential thermal analysis curves of synthetic imogolite and allophanes (I and II) were compared with those of their natural counterparts.

Brittle fracture of soil aggregates and the compaction of aggregate beds.

Abstract: Summary A statistical theory is developed for the brittle fracture of soil aggregates. Fracture is assumed to depend on the flaws, such as cracks and holes, within the aggregates, the weakest flaw limiting strength. An equation known as the third asymptote distribution of smallest values gives the distribution of these weakest flaws. The crushing strength of aggregates of five soils from South Australia and Queens land is compared with theory and agreement is good. Larger aggregates have smaller mean values of tensile strength than smaller aggregates at both -100 kPa and -153 MPa water potential. A sudden increase in strength occurs for aggregates larger than about 6 mm in the Queens land soils which is more marked with the drier samples. This is consistent with the idea that these larger aggregates are produced by the compacting effects of machinery, whereas the smaller aggregates are produced by the self-mulching effect. Changes in the internal structure of beds of aggregates are described as a function of amount of uniaxial compaction. A statistical method gives information about the size distributions and relative positions of aggregates and voids within the compacted beds. Compaction occurred almost exclusively at the expense of inter-aggregate porosity, and the largest voids were excluded first by compaction.

Effects of different methods of cultivation and direct drilling, and disposal of straw residues, on populations of earthworms

Abstract: Summary Numbers of earthworms collected from direct-drilled or ploughed land were studied during successive years on contrasting soil types sown with cereals. Samples were taken in autumn using a combination of chemical expulsion and hand sorting of soil. The population was consistently larger on direct-drilled than on ploughed land, and the difference became greater in each successive year. Deep burrowing species were affected similarly to the population as a whole. Population differences on the two treatments were greater with spring — than autumn - sown crops. After tine cultivation numbers were usually similar to those after ploughing. On a direct-drilled clay soil on which straw had been either burned, or chopped and spread, no differences were recorded in the overall population. However, significantly larger numbers of deep burrowing species were found on the chopped and spread areas, whereas on the burned areas numbers of Allolobophora chlorotica were significantly greater.

A crystallinity index for soil kaolins and its relation to parent rock, climate and soil maturity

Abstract: Summary To sub-divide the large group of kaolinitic soil clays a new empirical index of crystallinity was obtained from the X-ray diffraction patterns of eight standard kaolins. The index used is the ratio of the intensity above background of the peak at 4.46A to the intensity above background at about 2.43A (43.2° 2θ CoKα) and ranges from 4.8 for halloysite to 83 for a well-ordered triclinic kaolinite. For the kaolins from a group of soils from S.W. Nigeria, the ratio lies between 4.3 and 13.7 and can be tentatively related to the presence of weatherable minerals in the soil and/or the length of the dry season.

Processes of humus formation and transformation in soils of the canadian great plains

Abstract: Summary 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 description of desorption of phosphate from soil

Abstract: Summary After incubation with soil, phosphate was desorbed in dilute calcium chloride solutions. After short periods of incubation, the amount of phosphate desorbed increased rapidly at first but then net re-adsorption occurred. After long periods of incubation, desorption was slower and there was no net re-adsorption with prolonged desorption. Desorption was described in terms of two limiting values: the amount of phosphate (K) which would be desorbed as the concentration of phosphate approached zero, and the solution concentration of phosphate (Co) at which no desorption occurred. Both values decreased as the period of incubation increased. The value of K increased during the desorption phase, but the value of Co decreased. The decrease in Co during desorption was most marked after short periods of incubation and was important in describing the trend for re-adsorption of phosphate.

Effects of solution: soil ratio and vigour of shaking on the rate of phosphate adsorption by soil.

Abstract: Summary The effects were measured of varying the solution: soil ratio, and the vigour of shaking on the rate of adsorption of phosphate. One soil appeared to break down with vigorous shaking, prior to adding phosphate, especially at low solution: soil ratio. As a result, subsequent phosphate adsorption was faster. With gentle shaking, there was no effect of solution: soil ratio with the soil. Another soil was more stable and there were only small differences due to vigour of shaking and no effect of solution: soil ratio with any method of shaking. The change in phosphate concentration was not proportional to the reciprocal of time as reported by Ryden and Syers (1975) and Hope and Syers (1976). Hence the validity of extrapolating from short sections of reciprocal-time graphs in order to estimate equilibrium is questioned.

Rothamsted studies of soil structure II. Measurement and characterisation of macroporosity by image analysis and comparison with data from water retention measurements

Abstract: Summary Macroporosity (>60 μm) in the three pairs of soils described in Part I was measured and characterized by image analyses. There are significant differences in macroporosity between the soils in each pair, particularly in the subsoil horizons. The results show that the Ragdale, Denchworth and Salop soils are more impervious than those of the Hanslope, Evesham and Flint soils. Comparison of the image analysis results with those derived from air content at 0.05 bar suction shows that in most cases macroporosity by the former is larger. In image analysis the maximum diameter of each pore is measured whereas the water retention method measures the diameter of the smallest exit point of each pore. Differences between the two types of measurement will thus be at a maximum in soils containing many large pores with narrow exit necks.

Mössbauer effect and magnetic studies of secondary iron oxides in soils

Abstract: Summary Simple, rapid and non-destructive measurements of magnetic properties (magnetic susceptibility, saturation isothermal remanent magnetization and coercivity of isothermal remanence) coupled with more time consuming and sophisticated analyses such as thermomagnetic and Mossbauer effect studies, provide a basis for identifying forms of iron oxide present in the soil. At two of the sites studied, Caldy Hill, Merseyside, and Llyn Bychan, N. Wales, the secondary ferrimagnetic oxide formed in surface soil as a result of recent forest fires is shown to be non-stoichiometric magnetite approximating to the formula Fe2. 904. No evidence for the presence of maghemite was found in any of the soil samples from these sites or in soils from the Annecy region of S.E. France.

Uv and visible absorption spectroscopic investigations in relation to macromolecular characteristics of humic substances

Abstract: Summary Humic substances were investigated by both UV and visible absorption spectroscopic methods with special emphasis on their macromolecular characteristics. The spectral ranges examined were 200–350 nm and 350–700 nm. Conclusions were drawn on the basis of absorption intensities in optical density (O.D.) vs. wavelength (λ) curves, slopes of log O.D. vs. log λ plots, nature of shoulders in the 250–270 nm region, and E4/E6 ratios. As macromolecular characteristics are expected to be dependent on the medium, studies were carried out at different pH values as well as at different neutral salt concentrations, the two major parameters influencing the soil environment. It was noted that with increasing neutral electrolyte concentrations, the particle size decreased, probably due to the coiling of the macromolecule. On increasing the pH from 6.5 to 9.5, the particle size increased slightly, possibly because of partial association through homolytic bonding. At pH 3.5 (with FA only), the degree of association was greater than that at pH 9.5. However, here association occurred by a different mechanism, that of intermolecular hydrogen bonding. At pH 2.0 (with FA only), association was markedly pronounced because of the very dominant role played by intermolecular hydrogen bonding.

Collection and evaluation of large soil monoliths for soil and crop studies

Abstract: Summary A technique is described which allows collection and transportation of undisturbed soil monoliths in glass fibre casings 80 cm in diameter and 135 cm deep. The technique has been used to obtain 150 monoliths from a range of soil types in England and Wales, including soils with compact or chalky horizons. Measurements from lysimeters containing a non-swelling sandy loam and a swelling clay showed that the hydraulic properties of both soils were not affected by encasement of the profiles, provided that supplementary drainage outlets at the depth of field mole drains were provided in the lysimeters containing the clay soil. Aeration of the clay monoliths was comparable with that of the same soil in the field. When winter wheat plants growing on the lysimeters were surrounded by a similar guard crop, yields were equivalent to those obtained in the field. Edge effects were not significant; plants grown adjacent to the lysimeter wall yielded the same weight of grain per unit soil area as those in the central area of the monolith.

Selective extraction of amorphous iron oxides by edta from a danish sandy loam

Abstract: Summary EDTA extracts (pH 7.5–10.5) from a Danish sandy loam showed no significant (95% level) increase in amounts of aluminium, calcium, iron, and magnesium between three and nine months' shaking. The amounts were independent of the soil: solution ratio (1:10–1:50), the EDTA concentration (0.01 M–0.2 M), and crushing the soil ( 7.5 seems to extract selectively iron oxides with solubility products > ∼ 10−41.

Three effects of temperature on the reactions between inorganic phoshate and soil

Abstract: Summary Phosphate was added to soil, incubated at a range of periods and of temperatures, and the rate of desorption was measured at several solution: soil ratios at 25°C. One sample was incubated at a high temperature for several days and the rate of desorption was measured at 5, 15, 25 and 38 °C. A regression approach was used to describe the results which were interpreted in terms of a model in which phosphate was present (a) in solution, (b) adsorbed in equilibrium with the solution, and (c) firmly held. When neither adsorption nor desorption occurred, the phosphate concentration in solution increased with temperature. This was interpreted as an effect of temperature on the position of the equilibrium between solution phosphate and adsorbed phosphate. The direction of the effect indicates that adsorption was exothermic. When either adsorption or desorption occurred the rate increased with temperature and to a similar extent. For both reactions, the change in heat content was small. Thus, while temperature affected the rate of interchange between adsorbed and firmly-held forms it should not affect the position of the equilibrium between these forms.

Mineralisable organic nitrogen in soil fractionated according to particle size

Abstract: Summary A sandy-loam under wheat-pasture-pasture rotation was sampled both after legumeenriched and legumedepleted pasture leys and then fractionated according to particlesize using mainly sedimentation in water. The C/N ratios decreased from 18 to 7 with decreasing particle-size and were similar for both pasture types, despite ratios of 13 and 31 respectively for organic material collected at the surface of the soil before sampling. Organic matter was most concentrated in fractions having mineral particle-sizes between 0.3 and 4.0 μm e.s.d. Mineralisation of the N of high C/N fractions was much greater in incubations under waterlogged anaerobic conditions than under aerobic conditions, yet for low C/N fractions, there was little difference. Most of the readily mineralisable soil N resided in fractions of mineral e.s.d. less than 4 μm and much of this was probably recently formed microbial tissue or metabolites. An increasingly greater proportion of the N within size-fractions was mineralised as the particle size decreased, the increase being greater for aerobic incubations. The proportion of N mineralised in the surface organic material was much greater for legume-rich than for legume-poor pasture, but in the soil fractions, a corresponding difference was established only for sand-sized organic matter.

Effects of ionic strength and nature of the cation on desorption of phosphate from soil

Abstract: Summary The effects of cations on desorption of phosphate previously added to soil were studied by mixing phosphated soil with solutions of chloride salts at a range of solutionitoil ratios and for periods which ranged from 1 to 96 h. Phosphate desorbed was then related to the experimental variables by a pair of simultaneous equations. In calcium chloride, the rate of desorption of phosphate was inversely proportional to the calcium concentration. Desorption was faster in 0.01 M magnesium chloride than in 0:01 M calcium chloride, and faster in 0.03 M sodium chloride than in either magnesium or calcium chloride. Addition of a further supply of the cation on an exchange resin increased the rate for both sodium and magnesium but decreased it for calcium. A range of monovalent cations formed a sequence from fastest to slowest of: Li+ > Na+ > NH4−> K +, Rb + > Cs +. The identity and concentration of the cations had a large effect on the concentration of phosphate when the solution: soil ratio was small. There were also large effects in the amount of phosphate desorbed when the solution: soil ratio was large and the concentration of phosphate approached zero. This suggested that the escaping tendency of the phosphate was decreased when the cations which balanced the negative charge on the adsorbed phosphate were close to the surface.

Adsorption of selenite, phosphate and sulphate on hydrous alumina

Abstract: Summary Selenite was adsorbed on a positively charged hydrous alumina at a solution pH of 5.0, and the OH − released and changes in the surface charge were measured. The adsorption isotherm levelled off at high concentrations suggesting a definite adsorption maximum. The OH− released yielded a curvilinear relationship of increasing slope with the selenite adsorbed. The positive charge on the alumina surface was neutralized by the adsorbed selenite and the net charge became close to zero as the adsorption of selenite was near maximum. The molar ratio of the ligands (OH−+ OH2) displaced over selenite adsorbed was more than one at surface saturations of <0.5, but it was approaching one with increasing adsorption of selenite. The results are explained in terms of preferential adsorption of divalent (SeO23−) and monovalent (HSeO−3 selenite on a highly positively and less positively charged surface respectively, in agreement with the VSC− VSP model of Bowden et al. (1973; 1977). The supply of SeO23− for adsorption is considered to be favoured by an expected lower H activity in solution adjacent to the positively charged surface than that in the bulk solution. A general discussion is given on the ligand exchange adsorption of selenite, phosphate and sulphate on hydrous alumina.

The influence of magnesium in saline and sodic soils: a specific effect or a problem of cation exchange?

Abstract: Summary Na-Mg and Na-Ca exchange isotherms were determined at electrolyte concentrations of 500, 50, 5 and 1 meq per litre for illite, vermiculite and montmorillonite and for three soils containing illite and montmorillonite. This enabled comparisons to be made of clay swelling, dispersion and soil hydraulic conductivity changes between the Na-Mg and Na-Ca systems at known SAR, ESP and electrolyte concentration. Na-Mg montmoriUonite and a montmorillonitic soil behaved identically to the Na-Ca systems at the same ESP and electrolyte concentration: there was no specific effect. At the same SAR, the higher ESP in the Na-Mg system caused greater changes than in the Na-Ca systems. The Na-Mg vermiculite, illite, illitic soil and mixed illite-montmorillonitic soil showed greater changes than the Na-Ca systems at the same ESP, and there was a specific effect. At the same SAR, the higher ESP in all the Na-Mg systems apart from vermiculite increased the differences, but for vermiculite with a lower ESP, the differences were reduced. The lack of a specific effect for montmorillonite is probably related to the demixing of cations with the divalent ions concentrated on the non-swelling (internal) surfaces of the crystals.

Soil deformation and shear strength characteristics of some clay soils at different moisture contents

Abstract: Summary The soil deformation and shear strength characteristics of three clay soils were determined at different moisture contents and spherical pressures, using a quick un-drained triaxial compression test. The soils were found to deform either in a compressive way or by brittle fissuring, depending upon the relative values of moisture content, dry density and spherical pressure. The critical state concept of soil deformation can explain qualitatively the behaviour of these spils at high moisture contents but not at low. The Bridgman concept for fracture in brittle materials describes the soil behaviour at low moisture contents. The shear strength of the clay soils tested was more closely related to the soil moisture suction and to the amount of shrinkage which occurred on drying than to the absolute dry bulk density. The influence of moisture content and spherical pressure on the effectiveness of certain cultivation operations are considered.

Calcium deficiency in some Solonetzic soils of Alberta

Abstract: Summary Analysis of displaced soil solutions, at half available moisture percentage, indicated that Mg to Ca (Mg/Ca) ratios and Ca to total cation (Ca/TC) ratios ranged from 0.35 to 3.64 and 0.74 to 0.04, respectively, in the A and B horizons of the solonetzic soils under study. The growth of barley in simulated solution cultures commenced to decline when the Mg/Ca ratio in solution exceeded 1.0, or when the Ca/TC ratio was lower than 0.15, regardless of levels of salinity, concentrations of Ca or Mg, or differences in ionic strength. Reduced growth was followed by symptoms of Ca deficiency. Growth of barley in soil from various horizons of solonetzic soils supported the results found in solution culture. Calcium amendments to soils with low Ca/TC ratios or high Mg/Ca ratios in the soil solution, gave increases in growth and prevented Ca deficiency.

Rothamsted studies of soil structure. III. Pore size distributions and shrinkage processes.

Abstract: Summary The response of three pairs of soils having contrasting management behaviour to changes in water content was investigated by measuring moisture characteristics, shrinkage, and pore size distributions by mercury porosimetry. Sample preparation for mercury porosimetry was by direct drying, or water replacement by methanol and liquid CO2 followed by evaporation above the critical temperature. In heavy textured horizons, water release on shrinkage between -0.05 and -15 bar matric potentials occurs when pores of 10–200 nm equivalent plate separation contract. Structure development is dependent on the stability and regeneration of pores in the size range 200 nm-30 μm during cycles of wetting and drying. Soils with little stable porosity in this size range show nearly normal shrinkage and have coarse structural units, whereas soils with stable or regenerating porosity depart more from normal shrinkage and structure in the profile tends towards a finer per unit. The application of mercury porosimetry shows promise for determining possible porosity limitations in soil profiles.

Pore size distribution in critical point and freeze dried aggregates from clay subsoils

Abstract: Summary Using aggregates from 2 clay soils over a range of water contents from pF 1 to oven dry, shrinkage and water release curves were measured and pore size distributions found by mercury porosimetry after critical point and after freeze drying. Freeze drying caused less shrinkage, the maximum being 6 cm3 100 g−1 from pF 1, but gave a large increase, up to 10 cm3 100 g−1, of pores in the 0.1–10 μm size range. Critical point drying produced more shrinkage from pF 1, more than half of which was attributed to loss of interlamellar water and the rest to the collapse of pores larger than 10 μm. In these clays the volume of pores of over 3 μm diameter was very small (<3 cm3 100 g−1) and most of the plant available water was released by collapse of narrower pores and not by pore emptying.

Some priorities for british soil science1

Abstract: A Presidential Address provides an opportunity for an account of the state of the subject and a reading of the signposts which seem to point to future progress. The broad philosophical aspects of soil science were discussed by E. M. Crowther (1953) a quarter of a century ago in ‘The Sceptical Soil Chemist’, a paper which is worth reading again. This year our Vice-President, D. J. Greenland (1978) discussed ‘The Responsibilities of Soil Science’. I shall concentrate on discussing some ways in which soil scientists may contribute further to agricultural development.

Desorption of phosphate from goethite

Abstract: Summary The reversibility of the adsorption of phosphate on goethite was measured by diluting suspensions of goethite on which phosphate was adsorbed with large volumes of phosphate-free solution at the same ionic strength. The effect on the reversibility of various adsorption and desorption conditions was studied, particularly pH, temperature, ionic strength and time of reaction. The apparent irreversibility of the adsorption reaction seems to be due to a slow adsorption reaction of part of the phosphate after a very rapid initial adsorption, with a similar behaviour when the phosphate is desorbed. When the total adsorption plus desorption time is long enough, both adsorption and desorption points tend to lie on a single curve which corresponds to the isotherm calculated according to the Stern double-layer theory.

Effects of benomyl on iron and manganese reduction and redox potential in flooded soil

Abstract: Summary Benlate (50 per cent wettable powder), the commercial formulation of the systemic fungicide benomyl, was applied to a flooded alluvial soil at concentrations of 5 to 100 μg/g active ingredient. At about the field application rate (S μg/g), benomyl retarded the decrease in the redox potentials of the flooded soil and curtailed the accumulation of iron and manganese in solution, both without and with added rice straw (0.5 per cent).

Soils and quaternary geology in britain

Abstract: Summary The succession of Quaternary deposits in Britain records only small, fragmentary parts of a very complex history of Quaternary events resulting from many climatic fluctuations. Deep sea sediments provide a more continuous record, but correlation between the oceanic and terrestrial successions is at present extremely difficult, mainly because of the lack of absolute dates for older parts of the latter. The often conflicting evidence provided by indirect methods of dating and long-distance correlation is illustrated by reference to the chalky tills of East Anglia and the English Midlands, and the importance of soil studies in elucidating regional Quaternary stratigraphy is emphasised. The scanty history of pedogenesis in various parts of Britain is tentatively related to the provisional succession of Quaternary stages. Important changes in soil development processes occurred in mid-Flandrian times as well as in earlier cold and temperate (inter-glacial) stages, Many features of unburied soils are probably early Flandrian or older relicts, and difficulties consequently arise over use of the word ‘palaeosol’. Interglacial relict features in soils on pre-Devensian deposits (paleoaigillic subgroups) are distinguished from Devensian and Flandrian soil characters, but it is difficult at present to differentiate pedogenesis of successive interglacials. More than two-thirds of England is covered by Quaternary deposits, mainly of cold periods, and these generally provide better agricultural soils than pre-Quaternary sediments. Some relict features of both cold and temperate Quaternary stages also seem to be significant in determining the agricultural capability of British soils, but their effects need evaluating more precisely.

Availability of potassium to ryegrass from scottish soils i.: effects of intensive cropping on potassium parameters

Abstract: Summary Scottish soils were cropped with two sowings of perennial ryegrass (Lolium perenne, cv, Dutch Barenza) in the glasshouse, without addition of potassium, until growth virtually ceased. Potassium uptake and the corresponding changes in soil K properties were examined. Uptake of K correlated well over the entire cropping period with the initially labile K predicted from the Q/I isotherms of the soils, although K uptake was at least a factor of 1.9 greater than the initially labile K. The Q/I isotherms were similar for soils from the same series, and the form of the lower part remained virtually unchanged during intensive cropping. The equilibrium activity ratio, AR0, of soil K measured in 0.01 M calcium chloride was reduced in all soils to a narrow range close to a mean value of 3 × 10 −4 M1/2. Drying-and-wetting fresh soil samples severely depleted of ryegrass-available K raised their K status. The increases were negatively correlated with the percentage K saturation of the fresh depleted soils, and a K saturation of 1.4 per cent must apparently be left in the fresh cropped soils for no K to be released. Freezing-and-thawing did not affect the K status of these soils.

A MÖSSBAUER SPECTROSCOPIC STUDY OF THE EFFECT OF pH ON THE REACTION BETWEEN IRON AND HUMIC ACID IN AQUEOUS MEDIA

Abstract: Summary Mossbauer specttoscopy has been used to provide information on the effect of pH on the nature of the complexes formed between iron and humic acid. At an initial suspension pH greater than 3 the iron occurs in the ferric form, although it is difficult to assess to what extent it is in combination with organic matter. On lowering the pH, iron is reduced with a considerable proportion of the ferrous iron entering solution, partly as a solvated ion and partly as complex forms. Raising the pH leads to re-oxidation and the precipitation of a considerable proportion of the iron in an inorganic form with Mossbauer parameters similar to those of β-FeOOH. No evidence was obtained for Fe(III) in solution or for Fe(II) in any form at pH values greater than 4.

The erodibility of two soils from Kenya.

Abstract: Summary A portable rainfall simulator was used to measure the runoff rates and soil losses from a freshly ploughed luvisol at Katumani, and a nitosol at Kabete, on a 6° slope at varying rainfall intensities. The soil losses from storms of 50 and 100 mm/h intensities, in which a total of SO mm rain was applied to plots in the wet moisture state, varied from 0.8 to 1.4 metric tons/ha at Kabete and 8.5 to 13.9 metric tons/ha at Katumani. The percentage runoff from these storms was 15 to 17 per cent at Kabete and 67 to 71 per cent at Katumani. The much greater runoff rates from the Katumani soils were shown to be due to surface capping caused by degradation of the surface structure. It has been proposed that soil credibility K factors of 0.4 and 0.04 could be used for the Katumani and Kabete soils respectively in the Universal Soil Loss Equation

Oxygen concentrations in a clay soil after ploughing or direct drilling

Abstract: Summary The concentration of oxygen at three depths (15, 30 and 60 cm) has been measured in a direct-drilled or ploughed clay soil of low hydraulic conductivity (Evesnam series) which was cropped with winter wheat in three consecutive growing seasons (1972/73 to 1974/75). The mean oxygen concentrations between January and March varied with the seasonal rainfall; in the relatively dry winter of 1972/73they were 20 per cent (v/v) at 15 cm and 12 per cent (v/v) at 60 cm, but in the wetter winters of 1973/74 and 1974/75 they were less than 11 and 6 per cent (v/v) at 15 and 60 cms respectively. Direct drilling resulted in higher oxygen concentrations at 15 cm than ploughing in both wetter winters, the mean concentration being 10.2 and 7.2 per cent (v/v) in direct-drilled and ploughed plots respectively. Frequency distributions of values obtained at individual sampling points showed that when the mean oxygen concentration of the soil at 15 cm decreased to its annual minimum (in January-March), a significantly higher proportion of the sampling points under direct drilling continued to show concentrations of oxygen greater than 10 per cent (v/v). The results are interpreted as indicating that the higher oxygen concentrations found in this soil after direct drilling are due to the development of a system of continuous large pores and channels which would otherwise be destroyed by annual ploughing.