摘要:

SummaryCultivating grassland changes the physical state of the soil. The paper describes observations and measurements made in the field and the laboratory on a wide range of soils (with series names) in England over a period of 25 years. Dry sieving showed that natural weathering of grassland aggregates after three months'exposure was very considerable. The drawbar‐pull on ploughing soil recently out of grass, as compared with old arable, increased with the number of years out of grass.The number of earthworms per hectare on old grassland was 6 to 9 times that on old arable of the same soil series: after three years'cultivation it was reduced by about a half.The possibility of erosion by wind increased with the number of years out of grass. Old arable land on gentle slopes had its resistance to flowing water (sheet erosion) much increased by as little as 2 years under grass. The apparent densities of soils were at a minimum under old grass and increased under cultivation. On some soils arable for many decades, the apparent densities were near those considered limiting for root penetration. The volurne of water per cent in the soil at the sticky point decreased as the years out of grass increased, suggesting a decrease in the number of days the soil could be cultivated without smearing.The water‐stability of soil aggregates decreased as the number of years after ploughing old grassland increased. Whatever the texture, on ploughing old grassland, the water stability of air‐dry aggregates feil sharply in the first – years and then approached the value corresponding to that of old arable soils more slowly, apart from soils of low clay content in which it differed little from old arable land after 2 years. Measurement of changes in water stability and field observations in a 6‐course rotation (3 years ley, 3 years cereal) on a loam which had been in a similar rotation for about a decade after old grassland, suggested that this balance was probably right for maintaining such a soil at a good cropping level. Puddling (poaching) by animals of wet arable loam soils overlying clay resulted in gleying to the surface: ferrous iron was detected chemically. On putting down to grass there was little increase in water stability of the aggregates after 4 years.The reduction in the total pore space caused by harvesting machinery and also as a result of some years'cultivation of grassland was considerable. The ease of penetration of soils when taking cores was compared by counting the number of blows to drive in a corer: the number was much less in a soil recently out of old grass compared with old arable.The root development of cereals was poor in old arable silt soils very unstable to wetting and drying and with few visible biopores (>10–00 μm). On soils with many visible pores, root development was much greater and yields with similar rates of nitrogen were double.The total N in the soils feil when old grassland was ploughed, sometimes by as much as 75 per cent in about 20 years. There was sometimes a significant corre‐lation between total N and the reduction in water‐stable aggregation during the years following ploughing grassland: the correlation coefficient was much higher on ploughing old grassland rather than leys. The N level dropped much more quickly in the first few years after ploughing old grassland than later.All single‐property measurements give only a limited assessment of the physical state of a soil. Soils can be assessed satisfactorily only by making a range of measurements appropriate to

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1972.tb01668.x

出版商:Blackwell Publishing Ltd    

年代:1972

数据来源: WILEY

摘要:

SummaryHumic acid was treated for 30 days with normal NaOH under either N2or O2. In both cases some acid‐soluble material resulted which was enriched with free and peptide α‐amino N and also with total N. The molecular weight of the humic acid fell under alkaline conditions, while there was a decrease in total UV and visible absorption due to the production of less‐coloured material which was largely present in the acid soluble fraction. Under O2these effects were considerably enhanced and simultaneously a significant arnount of O2was consumed. Other changes produced by O2were an increase in the cation‐exchange capacity, particularly of the acid‐soluble fraction, the production of carbonate, an increase in the Oxidation state of the acid, and a lessening of the resemblance of the acid to lignin as shown by the infra‐red and UV spectra.In the main the changes produced by O2are similar to those observed for humic acids of decreasing molecular weight that can be extracted from a soil, either by differential extraction or by extraction followed by fractionation into different molecular‐

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1972.tb01669.x

出版商:Blackwell Publishing Ltd    

年代:1972

数据来源: WILEY

摘要:

SummaryWhole humic acid extracts are usually too polydisperse for reliable molecular‐weight measurement to be made in the ultracentrifuge by the sedimentation velocity technique. Consequently, the humic acid used in this study was fractionated with respect to molecular weight into fractions of low polydispersity by extensive use of gel‐permeation chromatography and other fractionation techniques.The sedimentation and diffusion coefficients of the fractions were determined and molecular‐weight values calculated. These values ranged approximately from 2 × 103to 1.5 × 106, the higher figure not necessarily representing the upper limit for these substances.On the basis of the frictional parameters calculated from the experimental data, it is proposed that the molecule adopts the solution conformation of a randomly coiled polymer in which branching may be significant, particularly at higher molecular

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1972.tb01670.x

出版商:Blackwell Publishing Ltd    

年代:1972

数据来源: WILEY

摘要:

SummaryFresh Fe(ClO4)3solutions, 0.01 M or 0.001 M in iron (III), were seeded with iron (III) hydroxide brown sols and the reaction processes were investigated. In two experiments, HC1O4was added to fresh Fe(ClO4)3solutions to lower their degrees of super saturation with respect to iron (III) hydroxide. Depending upon the quantity of seeding solution added, it was found that samples differed greatly in appearance and in the stability during ageing. With a small amount of seeding solution added and following a relatively long induction period, the sample rapidly developed to a dense, cloudy yellow suspension with FeOOH precipitate settled shortly afterwards. With an increased amount of seeding solution added, the sample gradually became more brownish and less turbid and eventually a clear brown sol was observed. Although α‐FeOOH was the major hydrolysed species in all samples, a yellow precipitate which settled under gravity was found only in those containing a small quantity of seeding solution. The above results provide experimental evidence supporting the hypothesis (Hsu and Ragone, 1972) that the initial number of nuclei relative to the concentration of mono‐meric species is the key factor governing the appearance and the stability of an hydrolysed iron (III) solution.In another experiment, varying amounts of a seeding solution were added to a fresh, pure 0.001 M Fe(ClO4)3solution to which no HC1O4was added. All samples rapidly hydrolysed to clear brown sols in less than 6 hours and no noticeable seeding effect was observed. It is suggested that a pure 0.001 M Fe(ClO4)3solution is highly supersaturated with respect to iron (III) hydroxide and a large number of nuclei spontaneously formin situshortly after prepara

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1972.tb01671.x

出版商:Blackwell Publishing Ltd    

年代:1972

数据来源: WILEY

摘要:

SummaryRadioactivity in soils labelled with14C was determined by suspending the finely ground soil in a thixotropic gel, dispersing it by ultrasonics and counting the dispersion in a liquid scintillation spectrometer. The relationship between the weight of sample and counting efficiency was found to be almost linear. Using 30 mg soil, the counting efficiency is 50 per cent and14C may be determined with an average coefficient of variation of 1.5 per cent to within 5 per cent of the value determined by oxidative procedures.

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1972.tb01672.x

出版商:Blackwell Publishing Ltd    

年代:1972

数据来源: WILEY

摘要:

SummaryAn examination has been made of the electron‐spin resonance spectra of humic acids from (i) two groups of cultivated soils from the north‐east of Scotland, and the related natural soils from which the cultivated soils are considered to be derived, and (ii) some Rothamsted soils.With the Scottish soils the humic acids from the more acid natural soils (pH4.6) almost structureless spectra (class II).The cultivated soils contain humic acids with the same class of spectrum as that of the soils from which they are derived, even though they are now much less acid (PH 5.9 –6.5).Humic acids from Rothamsted soils, sampled recently, were compared with those from the same site sampled 80 years previously. Where the soil has been allowed to revert towards a natural state but still contains free calcium carbonate the pH remained high (7.8–7.9) and the spectrum of the humic acid remains class II. Where the soil contained no free calcium carbonate the pH has fallen from 7.1 to 4.5 on reversion and the spectrum class of the humic acid has changed from II to I.The results support the hypothesis that the class of spectrum of the humic acid depends on the pH at which it

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1972.tb01673.x

出版商:Blackwell Publishing Ltd    

年代:1972

数据来源: WILEY

摘要:

SummaryField capacity was measured directly, and soil moisture characteristics were determined on undisturbed cores, for a wide textural range of ferrallitic soils in Uganda. The initial moisture conditions of the soils were shown to affect results and thus standardized procedures were adopted for field and laboratory determinations. Laboratory estimates of field capacity for undisturbed and disturbed samples were shown to be unreliable, but a correction factor was found which improved them. There was no single moisture tension for undisturbed core samples that corresponded to field capacity.Particle‐size composition could be related to field capacity, permanent wilting‐point, and available water capacity by multiple regression equations having correlation coefficients of 0.96, 0.98, and 0.88 respectively. The relationships predicting available water capacity were different from those found for soils in England, but those for field capacity and permanent wilting‐point were similar. Particle‐size analyses were carried out using three methods of dispersion of different efficiencies. Particle‐size composition following gentle dispersion by shaking soil with distilled water was poorly correlated with moisture‐holding properties. There was little advantage in using vigorous ultrasonic dispersion compared with overnight shaking with sodium hexametaphosphate.Two soils formed on alluvial deposits with kaolinite the only clay mineral, and one soil with montmorillonite the dominant clay mineral, showed markedly different relationships between moisture‐holding properties and particle‐s

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1972.tb01674.x

出版商:Blackwell Publishing Ltd    

年代:1972

数据来源: WILEY

摘要:

SummaryThe ability of clay to move ahead of an ice‐liquid interface depends on the rate of freezing, the particle size, the release of dissolved air from the water through its probable effect on local freezing rates, and on the electrolyte and its concentration through their effects on interparticle forces and particle mobility. A freezing‐rate of 1 cm per hour did not move flocculated clay in most cases, but moved dispersed clay up to 0.7 cm before being trapped in the ice with released air. Aggregates of clay (0.1 to 1 mm) were produced by freezing. After thawing they swelled and dispersed depending on the electrolyte and its concentration in a similar manner to aggregates formed by dry

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1972.tb01675.x

出版商:Blackwell Publishing Ltd    

年代:1972

数据来源: WILEY

摘要:

SummaryIt was required to be able to measure the shapes of soil particles in order that the influence of shape on soil mechanical properties could be investigated. Two methods were developed, both of which can be applied to particle cross‐sections or outlines. The first is a shape function which compares chord lengths with arc lengths around the particle outlines. The second method involves taking discrete Fourier transforms of the normalized curvatures of the outlines, and the production of curvature spectra. Both methods are illustrated with the use of the same data. The data were for three sizes of particle from a sandy soil. Both methods illustrate the greater asphericity or angularity of the smaller particles, which is consistent with the Griffith theory of comminution. Change in shape with particle size may be useful for classifying soils according to their methods of formatio

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1972.tb01676.x

出版商:Blackwell Publishing Ltd    

年代:1972

数据来源: WILEY

摘要:

SummaryA sample of Harwell soil containing 36 percent fine clay (<0.3μm) and 14 per cent coarse clay plus fine silt (0.3–5μm) was separated into fractions, and the K‐supplying power of soil and fractions measured by cropping with ryegrass, exchange with Ca resin and double‐label isotopic exchange with42K and45Ca ions.Mineralogical examination of the fractions coupled with the cropping experiments showed that the K‐supplying power of the soil to ryegrass can be explained by the presence of a zeolite, clinoptilolite‐heulandite, in addition to the clay minerals, mica, and interstratified illitic smectite, commonly found in a glauconitic clay‐rich soil. The 0.3–5 μm fraction, containing much zeolite, has an exchange diffusion coefficient for K ions to Ca resin of 1.8 × 10−16cm2sec−1compared with a value of 5.7 × 10−20for the<0.3μm fractions in which interstratified illitic smectite is the dominant mineral. Isotopic exchange shows that all Ca ions in fractions<50μm are isotopically exchangeable. In fractions coarser than 20μm, some of the K ions in felspar and mica were not exchangeable within the

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1972.tb01677.x

出版商:Blackwell Publishing Ltd    

年代:1972

数据来源: WILEY