摘要:

SummaryA large proportion of the world tropical region is covered by savannas, which have been little used for agriculture. Locally known as Cerrado, this ecosystem occupies about 20% of Brazilian territory. Preliminary surveys indicate that at least 50 × 106ha of the Cerrado are of potential use for intensive mechanized agriculture, but development has been limited by soil‐related constraints. Most soils are acid and with very low native fertility, especially in terms of phosphorus. Chemical barriers to root growth may cause severe yield losses during dry periods. Field crop responses to lime and fertilizers, obtained during the last 5–8 years, show that once soil‐related constraints are removed, high yields can be achieved. This review gives an account of the main lines of research which are leading to economic methods of developing the agricultural use of the Cerrado, and gives future research

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1983.tb01045.x

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY

摘要:

SummaryCricital state soil mechanics, which expresses relationships between deformation and effective stresses, has not yet been established for unsaturated soils. Effective stresses in unsaturated soils are not only complicated functions of the applied stress and the pore‐water pressure, but are also hysteretic. Furthermore, they cannot yet be determined independently, because present theories are inadequate, and they must be obtained by the less satisfying method of comparing the deformation of unsaturated and saturated soils subjected to similar stress paths. The paper discusses the further measurements that need to be made to validate the extension of critical state soil mechanics to unsaturated soils, and concludes that unless the effect of hysteresis proves to be small, the theory may be too difficult, if not impossible, to apply quantitativel

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1983.tb01046.x

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY

摘要:

SummaryPore size distributions in the 10/104nm e.c.d. range in aggregates from three New Zealand soils with largely monomineralic clay fractions, were determined by mercury porosimetry after oven drying and also after critical point drying following methanol and then CO2exchange from a range of water contents.A soil containing halloysite showed considerable porosity in the fine pores (10–30 nm) regardless of the method of drying. A smectitic soil showed virtually no porosity in the 102–104nm range when oven dry. A soil containing allophane was dominated by large pores (>103nm). The change from a fine (clayey) to a coarser (clay loam) texture within the profile of one soil was reflected in an increase in large po

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1983.tb01047.x

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY

摘要:

SummaryThe calibration of a high resolution gamma‐ray density probe and a simple experiment comparing the probe with an earlier, lower resolution version are described. An assessment is made of the performance of the probe in three experiments investigating compaction by tractor wheels, two of which were in the field and the third in an indoor soil tank. A linear calibration relationship was obtained, although the addition of a quadratic term improved the fit of the curve slightly. The probe was found to be much more accurate than the earlier version within 100 mm of the soil surface where treatment effects were largest and most numerous in the field experiments, and thin layers of high density could be detected. Over the 15 month period of the experiments, the stability of the system was found to be satisfactor

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1983.tb01048.x

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY

摘要:

SummaryThe water and oxygen status of four upland soils under Sitka spruce plantations was studied for 2 years. In a brown earth, waterlogging only occurred ephemerally in the subsoil and oxgen concentrations were generally high. In contrast, waterlogged and near‐anaerobic conditions persisted for much of the year in stagnogley and stagnohumic gley soils. In a peaty stagnopodzol moist conditions occurred above and below the thin ironpan, but while high oxygen concentrations generally persisted in the subsoil, low concentrations were common in the soil above. In the brown earth, healthy roots of Sitka spruce were present at a depth of 85 cm, but in the gley soils rooting was mainly limited to 25 cm and many roots were dead. In the peaty stagnopodzol, roots penetrated the ironpan and grew in the subsoil to 75 cm dept

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1983.tb01049.x

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY

摘要:

SummaryResults are presented from a 3 year investigation into nitrate leaching from isolated 0.4 ha grassland plots fertilized with 250, 500 and 900 kg N ha−1a−1. Cumulative nitrate leaching over the 3 years was equivalent to 1.5%, 5.4% and 16.7% of the fertilizer applied at 250, 500 and 900 kg N ha−1rates respectively. Over a whole drainage season, mean nitrate leachate concentrations at 250 kg N ha−1did not exceed 4 mgl−1, although maximum values of 13.3 mgl−1were observed. In contrast, at 900 kg N ha−1, the mean nitrate leachate concentration in two of the years exceeded 90 mgl−1. Mineral nitrogen balances constructed for the 1979 growing season indicated that leaching at 250 kg N ha−1was low because net mineralization of soil organic nitrogen was small, and crop nitrogen uptake almost balanced fertilizer application. Although the pattern of nitrate leaching suggested that by‐passing occurred in the movement of water down the soil profile, it was not possible to confirm this using simulation models of leaching. Possible reasons for this, including the occurrence of rapid water flow down gravitationally drained macrop

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1983.tb01050.x

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY

摘要:

SummaryIn a well‐structured soil at or close to pedal saturation, rainfall intensity in excess of pedalKsatis predicted to result in by‐passing flow through well connected structural voids.The depth of solute penetration (tracer dye and nitrate) under by‐passing flow has been predicted byDf=(P‐Cp)/θf, whereDfis the depth of penetration of the leading edge of solute in by‐passing flow,Pis gross precipitation,Cpis the input volume transmitted through the soil peds and θfis the operational volumetric moisture content in by‐passing. The timing of surface applied nitrate arrival at a channel bank is demonstrated to be related to by‐passing flow rather than uniform displacement.The relationship between nitrate load in soil drainage waters (Y, mg s‐1) and water discharge (X, dm3s−1) wasY∼6Xafter nitrate fertilizer application but subsequently declined toY0.5XIt is deduced that the decline resulted from inefficient leaching under by‐passing conditions once surface applied nitrate had become relocat

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1983.tb01051.x

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY

摘要:

SummaryWe have investigated a model of fulvic acid based on randomized positioning of functional groups and aromatic rings which can be used to deduce the types of metal chelating sites and provide statistical estimates of their concentrations. A FORTRAN program ‘RANDOM’ has been written which generates random molecules of fulvic acid. For each molecule generated the program also searches for and counts 14 types of binding site and calculates average concentrations over one thousand such molecules. A formula relating percentage aromatic carbon to the molar hydrogen/carbon ratio is also derived. The results show that the predominant bidentate sites are likely to be phthalate‐ and salicylate‐type sites with a significant proportion of aromatic carboxyl and phenolic hydroxyl not participating in chelatin

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1983.tb01052.x

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY

摘要:

SummaryThe purpose of this investigation was to compare structural characteristics of PMA with those of FA in order to assess the suitability of PMA to serve as a model for FA. Chemical (elemental analyses, CO2H group determinations, increases in OCH3after methylation, oxidative degradation) and spectroscopic (visible, UV, fluorescence, IR,13CNMR, and ESR) methods were used for this purpose. While PMA was found to resemble FA in a number of characteristics (water solubility, rich in CO2H groups, dark brown colour), it significantly differed from FA in a number of respects (UV, fluorescence,13CNMR spectra as well in oxidation products). Compared to FA, PMA appeared to be better organized and more homogeneous at the molecular level, richer in olefinic structures and less aromatic. But the chemical structures of both PMA and FA remain largely unknown.

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1983.tb01053.x

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY

摘要:

SummaryA soil of the Countesswells series was repeatedly methylated by the Hakomori procedure and a chloroform‐soluble product isolated after each methylation. Ninety‐two per cent of the material engendered by seven methylations was released during the first four methylations. This had a methoxyl content of about 20% and contained 2% N. Later fractions had lower methoxyl and N contents.Residual carbohydrate in the soil had reducing sugar content on hydrolysis, equivalent to about 3% of the original value.The extracted material had the infrared spectrum of a methylated soil polysaccharide and, on hydrolysis by 2mtrifluoracetic acid, released methylated sugar derivatives of which 68 were characterised by GC‐MS.Derivatives corresponding to (1→4) linked sugars predominated for both hexose and pentose sugars but there was also a large proportion of (1→3) linkages, (1→3) linkages predominated for the deoxyhexose sugars. There were more sugars with only one or two methylated hydroxyl groups than could be accounted for as branching points because of the relatively small numbers of end groups.Prior reduction of the soil with sodium borohydride had no measurable effect on the nature or yield of the methylated product. This indicates that amino acid sugar linkages susceptible to β‐elimination reactions can have only a very small influence on the reaction.The isolated sugar derivatives accounted for 70% of the total soil sugars. The methylated material before hydrolysis had a low nominal molecular weight on diafiltration, with 68%<10000. Some of the sugars unaccounted for were probably lost during the dialysis stage necessary to remove dimet

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1983.tb01054.x

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY