Showing papers on "Soil organic matter published in 1985"

Interactions of bacteria, protozoa and plants leading to mineralization of soil nitrogen.

Abstract: Summary-The capacity of bacteria and protozoa to mineralize soil nitrogen was studied in microcosms with sterilized soil with or without wheat plants. The effect of small additions of glucose or ammonium nitrate or both, twice a week was also tested. Plant dry weight and N-content, number of microorganisms and biomass plus inorganic N were determined after 6 weeks. The introduction of plants profoundly influenced the N tr~sfo~ations. In the presence of root-derived carbon, much more N was mineralized from the organic matter and immobilized mainly in plant biomass. “Total observable change in biomass N plus inorganic N” was negative in the unvegetated soils without additions, while a mmeralization of 1.7 mg N microcosm-’ was observed in microcosms with wheat plants grown with bacteria only. When protozoa were included, the N taken up by plants increased by 75%. Sugar additions resulted in an 18% increase of total N in the shoots when protozoa were present, but had no significant effect in the absence of grazers. Plants with the same root weight were more efficient in their uptake of inorganic N when protozoa were present. Plants grown with protozoa also had a lower R/S ratio, indicating a less stressed N availabiIitv situatron. The lowest ratio was found with N additions m the presence of protozoa. The results indicate that, with energy supplied by plant roots or with external glucose additions, soil bacteria can mineralize N from the soil organic matter to support their own growth. Grazing of the bacteria is necessary to make bacterial biomass N available for plant uptake.

Global patterns of soil nitrogen storage

Abstract: The amount of nitrogen stored in soil is related to climate through biotic processes associated with productivity of vegetation and decomposition of organic matter. Other factors, particularly rainfall input, dry deposition input, nitrogen fixation and losses of inorganic nitrogen due to leaching contribute to the variability of nitrogen storage. Here we report that soil nitrogen storage ranges from 0.2 kg m−3 in warm deserts to 2 kg m−3 in rain tundra, with a peak of 1.6 kg m−3 in subtropical wet forests. Soil carbon storage shows a similar pattern. The global nitrogen pool in the surface metre of soil comprises an estimated 9.5 × 1013 kg. Each soil profile examined was classified according to the Holdridge life zone1 in which it was found. Soil carbon/nitrogen ratios range from 20 in cool, wet forests or rain forests. We determined C/N ratios of 15–20 in cool life zones and 10–15 in warm life zones. These results indicate that: (1) relatively large amounts of soil nitrogen in wet tropical regions are associated with recalcitrant humic materials in an advanced state of decay, with low C/N ratios; (2) the seasonal climate contrast in temperate regions, combined with variable litter quality due to the mix of coniferous and deciduous species, results in moderate carbon and nitrogen storage in soil and variable C/N ratios; and (3) slow decomposition in wet tundra regions results in high carbon and nitrogen storage, with high C/N ratios.

Microbial biomass and activity in an agricultural soil with different organic matter contents

Abstract: Changes in soil fertility caused by various organic and N-fertilizer amendments were studied in a long-term field trial mostly cropped with cereals. Five treatments were included: (I) fallow, (II) cropping with no C or N addition, (III) cropping with N-fertilization (80 kg ha −1 yr−1), (IV) cropping with straw incorporation (1800kg Cha−1 yr−1) and N-fertilization (80 kg ha−1yr−1), and (V) cropping with addition of farmyard manure (80 kg N + 1800kg Cha−1yr−1). The treatments resulted in soil organic matter contents ranging from 4.3% (I) to 5.8% (V). Microbial biomass and activity were determined by chloroform fumigation, direct counting of fungi (fluorescein diacetate (FDA)-staining and Jones-Mollison agar-film technique) and bacteria (acridine orange staining), most probable number determinations of protozoa, esterase activity (total FDA hydrolysis) and respiration. Both biomass estimates and activity measurements showed a highly significant correlation with soil organic matter. Microbial biomass C ranged from 230 to 600 μg C g−1 dry wt soil, as determined by the fumigation technique, while conversions from direct counts gave a range from 380 to 2260 μg C. Mean hyphal diameters and mean bacterial cell volumes decreased with decreasing soil organic matter content.

Soil organic matter and biological activity

Abstract: List of Contents- Soil Organic Matter - A Perspective on its Nature, Extraction, Turnover and Role in Soil Fertility- 1 Soil organic matter- 2 Biological changes in soil organic matter- 3 Role of soil organic matter in fertility- 4 Conclusions- 5 References- 1 Influence of Humic Substances on Growth and Physiological Processes- 1 Introduction- 2 Influence on growth- 3 Uptake of humic substances- 4 Nutrient content of plants- 5 Humic substances and membranes- 6 Conclusions- 7 References- 2 Influence of Humic Substances on Biochemical Processes in Plants- 1 Introduction- 2 Energy metabolism- 3 Nucleic acids- 4 Protein metabolism- 5 Biological activity and structure of humic substances- 6 Conclusions- 7 References- 3 Phenolic Acids in Soils and their Influence on Plant Growth and Soil Microbial Processes- 1 Introduction- 2 Detection and determination- 3 Phenolic acids in plant tissues- 4 Phenolic acids in soils- 5 Influence of phenolic acids on plant growth- 6 Influence of phenolic acids on soil micro organisms- 7 Conclusion- 8 Acknowledgements- 9 References- 4 Origin, Nature and Biological Activity of Aliphatic Substances and Growth Hormones Found in Soil- 1 Introduction- 2 Ethylene- 3 Gibberellins, auxins, cytokinins and abscisic acid- 4 Aliphatic organic acids- 5 Hydrogen sulphide- 6 Alcohols- 7 Antibiotics- 8 Conclusion- 9 Summary- 10 References- 5 Soil Enzymes- 1 Introduction- 2 Range and assay of soil enzymes- 3 Activity of soil enzymes- 4 Activity and properties of extracted soil enzymes- 5 Conclusions- 6 References- 6 The Soil Biomass- 1 Introduction- 2 Methods- 3 Soil organic matter and the biomass- 4 Growth and activity of the biomass- 5 The soil biomass and plant nutrition- 6 The soil biomass as a source of plant nutrients- 7 Summary and Conclusions- 8 References- 7 Carbohydrates in Relation to Soil Fertility- 1 Introduction- 2 Biological nutrition- 3 The formation of stable soil structure- 4 Influence on the behaviour of water in soil- 5 Influence on ion exchange and complexing properties- 6 Influence on plant root growth- 7 Protection of soil enzymes- 8 Influence on the growth of soil organisms- 9 Conclusion- 10 References- 8 Soil Nitrogen: Its Extraction, Distribution and Dynamics- 1 Introduction- 2 The extraction of soil nitrogen- 3 Distribution of soil nitrogen- 4 Soil nitrogen dynamics- 5 Agriculture and nitrogen- 6 Modelling- 7 Conclusions- 8 References- 9 Soil Phosphorus- 1 Introduction- 2 Global cycle- 3 P and pedogenesis- 4 Forms of soil P and their characterisation- 5 Soil P cycle- 6 Mineralization and immobilisation- 7 Factors influencing P transformation- 8 Availability of Po to plants- 9 Current perspective- 10 Acknowledgements- 11 References- 10 Sulphur in Soils and Plants- 1 Introduction- 2 Soil sulphur- 3 Sulphur transformations- 4 Sulphur deficiency in plants and soils- 5 Anthropogenic sulphur inputs- 6 References- 11 Organic Matter and Trace Elements in Soils- 1 Introduction- 2 Solution chemistry of trace elements- 3 Humic substances- 4 Biochemical substances in the rhizosphere- 5 Adsorption of trace elements on solid surfaces- 6 Distribution of soil organic matter in different soils- 7 Trace element uptake by plants- 8 A perspective- 9 References- 12 Organic Farming- 1 Introduction- 2 Historical perspective- 3 The present situation- 4 Soil structure- 5 Comparison of levels of production- 6 The future- 7 References- List of Abbreviations

The Effect of Green Manuring on the Physical Properties of Temperate-Area Soils

Abstract: Green manuring has received little attention by the research community in the past few decades. But with renewed concern about our soil resources, particularly soil degradation, green manuring is increasingly appealing as a means to reduce losses of soil organic matter, compaction, and soil erosion and still maintain economic returns. Recent attempts to reevaluate the usefulness of green manures have been hindered by a lack of readily available and current information. This paper attempts to compile and synthesize what information is presently available.

Decomposition of plant material in Australian soils. III. Residual organic and microbial biomass C and N from isotope-labelled legume material and soil organic matter, decomposing under field conditions

Abstract: After eight years decomposition of 14C, 15N-labelled legume (Medicago littoralis) material previously mixed into topsoils (0-10 cm) at four field sites in South Australia, residual organic 14C and 15N to 30 cm depth accounted for respectively 11-13% of input 14C, and 31-38% of input 15N. About 90% of the residual organic 14C and 70-80% of the residual l15N was recovered in topsoils. For sites in similar rainfall areas, soils of heavier texture retained slightly greater amounts of 14C and15N-labelled residues. Throughout the eight-year experimental period, the rates of decline of residual organic 14C and 15N exceeded those of native soil organic C and N. A comparison of the decline of organic 14C in topsoils, averaged for the four South Australian sites, with the average decline reported for 14C-labelled plant residues in soils at English and Nigerian field sites, suggests that net decomposition rates doubled approximately for an 8-9°C rise in mean annual air temperatures. Microbial biomass 14C and 15N of topsoils with time accounted for decreasing proportions of total biomass C and N, and of residual organic I4C and I5N. The relatively greater retention after eight years of biomass 14C and 15N in soils of heavier texture is consistent with the concept that the net decay of C and N in soils is dependent upon the turnover of biomass C and N, and that decay rates are decreased in soils which have the greater capacity to protect decomposer populations.

Influence of Humic Substances on Growth and Physiological Processes

Abstract: It is now generally accepted that soil organic matter (humus) plays a major role in maintaining or improving soil fertility2,104. Because of the complexity of soil organic matter, the precise nature of that role has been the subject of a considerable and long lasting debate. The present, and all too often inadequate, approaches to elucidating the mode of action of soil organic matter on plant growth are still partially influenced by historical concepts. An appraisal of the current ideas can only be undertaken by reference, albeit briefly, to the past. More detailed historical accounts are to be found elsewhere2,104,171,225.

Soil science and management

Abstract: The importance of soil. Soil origin and development. Physical properties of soil. Soil water. Water conservation. Irrigation and drainage. Life in the soil. Organic matter. Soil fertility. Soil pH and salinity. Plant nutrition. Soil sampling. Fertilizers. Organic amendments. Tilage and cropping systems. Horticultural uses of soils. Soil classification and survey. Soil conservation. Urban soil. Where to go for help.

Quantifying the contribution from the soil microbail biomass to the extractable P levels of fresh and air-dried soils

Abstract: Levels of P extractable in 0.5 M NaHCO3 and the microbial biomass C were measured on a range of 15 New Zealand pasture soils, with and without an air-drying treatment. The microbial biomass C, estimated using a modified substrate-induced-respiration method, was decreased 11-68% by air-drying. Two soils showed no increase in extractable inorganic phosphorus (P) levels after air-drying, but increases for the other 13 soils ranged over 14-184%. In general, the biomass C was not related to the overall P, levels of moist or air-dried soils. However, the increase in P, after air-drying was related to the microbial P content of the moist soil, estimated by CHCl3 fumigation, and the proportion of the biomass C killed by air-drying. A comparison of the actual measured Pi levels of dried soils and those estimated allowing for the microbial contribution, showed excellent agreement, strongly suggesting that the whole of the Pi increase on air-drying of soils was derived from killed microbial cells. The microbial contribution to Pi levels in NaHCO3 extracts of dried soils ranged over 4-76% and was therefore a significant large proportion in some soils. A large microbial contribution to Pi in extracts is most likely on air-dried soils from under pasture with >2% organic matter and NaHCO3-extractable P values of <20 µg g-1.

Origin and assessment of water repellency of a sandy South Australian soil

Abstract: The water repellency of a sandy lucerne pasture soil is critically dependent on aqueous extraction and subsequent drying procedures. Freeze-drying converts a very severely water-repellent soil into a readily wettable soil, but subsequent rewetting and oven-drying regenerates water repellency. These changes are ascribed to changes in the molecular conformation of the organic matter. Prolonged shaking detaches organic matter coatings from sand particles and so reduces repellency. Allowing for these effects, it is shown that a sequence of alcohol-benzene extraction, acid leaching, and NaOH extraction removes the major part of the water-repellent substances present.

The Soil Biomass

Abstract: The role of the microbial biomass in the transformation of organic matter in soil is a crucial one and the rates of turnover and mineralization of organic substrates are largely governed by the activity of the soil biomass. Inhibition of microbial activity by a low or high temperature, drought, waterlogging, extremes of pH or xenobiotic substances may result in the persistence in soil of potentially decomposable and mineralizable compounds with subsequent effects on fertility, nutrient cycling and soil structure. The microbial biomass is itself part of the soil organic matter, typically about two percent of the total organic C110 and is defined as the living microbial component of the soil and includes bacteria, actinomycetes, fungi, protozoa, algae and microfauna. Usually, plant roots and fauna larger than 5x103 μm3, such as earthworms, are not included. For the purpose of this chapter, the biomass will be considered as an undifferentiated whole, usually expressed quantitatively as biomass-C., cell biomass or biovolume (μm3). However, the activities of various components of the biomass — protozoa, nematodes, etc. — have been included where this seems appropriate.

Carbon isotope ratios of soil organic matter and their use in assessing community composition changes in Curlew Valley, Utah

Abstract: Stable carbon isotope ratios of roots and soil organic matter were measured in Curlew Valley, Utah to determine if changes in the relative dominance of two shrub species had occurred in this salt-desert community. Measurements were made on soil cores along transects stretching from monospecific stands of Ceratoides lanata, a C3 shrub, to monospecific stands of Atriplex confertifolia, a C4 shrub. δ13C values of roots and soil organic matter under Ceratoides cover appeared to be in equilibrium with the current plant community. By contrast, δ13C values of roots and soils under Atriplex portions of the transects were more negative than would be expected for a C4-dominated community. These results indicate that a change in relative C3/C4 dominance has occurred, and suggest that the C4 shrub Atriplex confertifolia is increasing in importance in this salt-desert community.

The role of organic matter in soil acidification

Abstract: The pH and buffer capacity of two soils increased or remained constant after incubation with different amounts of plant material (lucerne cham at field capacity and when air dry. For both soils, the pH changes were greater at field capacity, whereas the buffer capacities were independent of the water treatments. The pH changes observed could be explained in terms of the organic anion concentration of the plant material. The results indicate that the initial soil pH and the anion concentration (i.e. the per cent dissociation of soluble organic acids when released into the soil) determine the acidifying effect of organic matter.

The chemical composition and ionic strength of soil solutions from New Zealand topsoils

Abstract: In preliminary experiments a centrifuge method for extracting soil solutions was examined. Neither the time nor speed of centrifuging had any effect on the concentrations of cations in soil solution. The concentration of cations increased with decreasing soil moisture content, and NO3, Ca, Mg, and Na concentrations increased with increasing time of storage of freshly collected moist soils. It was concluded that to obtain soil solutions, which accurately reflect the soil solution composition and ionic strength (I) in situ, requires that soil samples are extracted immediately (<24 h) following sampling from the field. Prior equilibration of soil samples, to adjust soil moisture contents, is therefore not valid. The effect of time of sampling and soil type, and the effects of fertilizer and lime applications, on soil solution composition and ionic strength, were measured on freshly collected field moist topsoils. Concentrations of Ca, Mg, K, Na, NH, and NO, were lowest in the winter and highest in the summer. Consequently, there was a marked seasonal variation in ionic strength which ranged from 0.003 to 0.016 mol L-1 (mean, 0.005 s.d. 0.003) over time and soil type. Withholding fertilizer (P, K, S, Ca) for two years had only a minor effect on ionic composition and strength, and liming increased solution Ca, Mg and HCO3, but decreased Al, resulting in a twofold increase in ionic strength. These results suggest that the ionic strength of temperate grassland topsoils in New Zealand lie within the range 0.003-0.016 and are typically 0.005.

Sulfate Adsorption Relationships in Forested Spodosols of the Northeastern USA

Abstract: Soil SO²⁻₄ adsorption has important implications in the study of acidic deposition effects on terrestrial and aquatic systems, because this process results in the release of alkalinity or prevents leaching of H⁺ and cations. Forested Spodosols of the northeastern USA have generally been considered poor adsorbers of SO²⁻₄, in part due to the interfering effects of soil organic matter. Soils from both the Adirondack Mountains, NY and the Hubbard Brook Experimental Forest, NH (Typic and Lithic Haplorthods and Aquic Fragiorthods, respectively), however, contained SO²⁻₄ in Bs horizons predominately in an insoluble (nonwater-extractable) form and were capable of adsorbing additional SO²⁻₄ given higher SO²⁻₄ concentrations than currently found in situ. Concentrations of insoluble SO²⁻₄ in Bs horizons ranged from 0.3 to 2.1 × 10⁻³ mol SO²⁻₄ Kg⁻¹ by H₂PO⁻₄ extraction. Fractionation of free Fe and Al suggest that both elements were present largely in organically complexed forms. Insoluble SO²⁻₄ was highly correlated with several forms of free Fe and Al, but crystalline Fe and organic Al content appeared to explain most of the variability in SO²⁻₄ adsorption capability. These soils may not presently be accumulating SO²⁻₄ through the adsorption process on an annual basis. Their SO²⁻₄ adsorption capability, however, indicates that the adsorption process may well buffer seasonal variations in SO²⁻₄ concentration or serve as a net source or sink of SO²⁻₄ subsequent to conditions which either influence concentrations of SO²⁻₄ in the soil solution (eg. atmospheric inputs) or affect the adsorbent phase.

Phytotoxicity and Adsorption of Chlorsulfuron as Affected by Soil Properties

Abstract: The phytotoxicity of chlorsulfuron {2-chloro- N- (((4-methoxy -6 -methyl - 1,3,5 -triazin-2-yl)amino)car- bonyl)benzenesulfonamide} was compared in six soils, and the relationship of activity to soil physical and chemical properties was evaluated. The influence of soil pH (4.2 to 7.8) on phytotoxicity and adsorption of chlorsulfuron in- corporated into high-organic-matter soil was also studied. For the phytotoxicity studies, corn (Zea mays L. 'Pioneer 3320') was used as the bioassay plant. Organic matter was the soil variable most highly correlated with chlorsulfuron phytotoxicity. There was an inverse relationship between phytotoxicity and organic matter. No significant relation- ship between clay content and chlorsulfuron toxicity was observed. The adsorption of chlorsulfuron decreased with increasing soil pH while desorption was greater at alkaline pH. Phytotoxicity of chlorsulfuron increased with increasing soil pH and reached a maximum at pH 6.9. Additional index words. Soil organic matter, soil pH, soil clay content, bioassay.

The dynamics of organic matter in grassland soils

Abstract: MAINTENANCE of soil organic matter in semiarid agroecosystems is important. Organic matter is a source of, and temporary sink for, several plant nutrients. It also is important in maintaining soil tilth, abetting infiltration, promoting water retention, and reducing wind and water erosion. Reductions in organic matter following cultivation of grassland soils are reasonably well documented ( 32, 34, 51 ). That is important because the organic matter portion of the global carbon pool, estimated at four times that present in biota or the atmosphere, makes carbon from organic matter significant to global carbon flows ( 16, 62, 81 ). We reviewed the research pertaining to organic matter in semiarid grassland soils with emphasis on studies relating organic matter characteristics to soil environment. These studies are descriptive in nature and help us understand the behavior of organic matter in soil (39). Transformations of the organic fraction are considerably influenced by inorganic components (30), and soil is a three-dimensional body in equilibrium with its environment; therefore, studies of the whole soil system are emphasized. Less emphasis is given ...

Effects of Soil Type, Liming, and Sludge Application on Zinc and Cadmium Availability to Swiss CHARD1

Abstract: An investigation was made of the effects of sludge application, liming, and soil type of Zn and Cd availability to Swiss chard (Beta vulgaris L.) in pot culture under field environmental conditions. Increasing sludge application reduced soil pH, increased the amounts of Zn and Cd extractable by MgCl/sub 2/ and DTPA, and increased metal accumulation in the plants. Total metal levels in soil did not predict the accumulation in plants as well as did levels of extractable metals, even though the coefficient of determination (R/sup 2/) between extractable and total levels was high. Soils containing high amorphous Fe oxide levels generally had lower metal extractability at a constant rate of sludge application than those with low amorphous Fe oxide levels. Although pH was important in influencing metal availability for any particular soil, its influence was often less than that of amorphous Fe oxide content when availability was compared among soils. Inclusions of both pH and amorphous Fe oxide content, along with total metal content, gave the best prediction of metal availability. Soil organic matter had little relation to metal availability. Results suggest that soils high in Fe oxide content, in conjunction with soil liming, may give the best control of more » metal availability for accumulator species, such as Swiss chard. « less

The effects of plants on soil sulfur transformations

Abstract: Changes in the mineralization of S and in the redistribution of 35S-labelled sulfate among organic matter fractions were followed during incubation of cropped and uncropped soils. Net mineralization was significantly greater in cropped soils compared with uncropped soils. The distribution of 35S was significantly influenced by the addition of sulfate or cellulose or a combination of both and by the presence of plants. When the solution sulfate level was high the majority of 35S incorporated into soil organic matter was found in the Hi-reducible S fraction. When soil solution sulfate concentrations were lowered by plant uptake or through addition of cellulose there was a reduction in the 35S incorporated into the HI-reducible S fraction.

Effects of phosphate buffer capacity on yield response curvature and fertilizer requirements of wheat in relation to soil phosphate tests

Abstract: Data from 39 fertilizer field experiments in north-western New South Wales were used to examine the effects of phosphate buffer capacity on yield response curvature and fertilizer requirements of wheat in relation to six soil phosphate tests (Bray1, Bray2, BSES, Truog, lactate, and bicarbonate). The soil tests were also evaluated for their accuracy in predicting yield responsiveness in a total of 48 experiments. There was a highly significant negative correlation between buffer capacity and response curvature, accounting for nearly 50% of the variance in curvature. The accuracy of the relationship was highest for moderately and strongly buffered soils. When used to predict curvature and hence fertilizer requirements, buffer capacity increased the variance accounted for by the most effective soil test (lactate) from 32% to 75%, compared with 93% using actual response curvatures. Whether used to predict responsiveness or fertilizer requirement, the lactate test was superior and the bicarbonate test was inferior to other soil tests. The bicarbonate test accounted for only half as much variance in responsiveness as the lactate test, and it accounted for none of the variance in fertilizer requirement. The results confirmed earlier studies showing that the bicarbonate test has several intrinsic properties which make it inferior to other soil tests on moderately acid to alkaline soils.

The effect of soil pH on the formation of VA mycorrhizas by two species of Glomus

Abstract: Two species of vesicular-arbuscular (VA) mycorrhizal fungi differed in their ability to infect subterranean clover roots when soil pH was changed by liming. In a glasshouse experiment, Glomus fasciculatum infected extensively at each of four levels of soil pH (range 5.3-7.5). Glomus sp. (WUM 16) only infected extensively at the highest pH level. Liming the soil depressed plant growth, but this effect was almost entirely overcome by inoculation with G. fasciculatum. In the second experiment, Glomus sp. (WUM 16) failed to spread from existing infection within roots of subterranean clover when soil pH was 5.3 or lower. The lack of spread of infection was associated with an inability of hyphae of this fungus to grow in the soil used unless it was limed to give a pH at least greater than 5.3.

Soil degradation and management under intensive sugarcane cultivation in North Queensland

Abstract: . Sugarcane yields in the Herbert Valley in North Queensland have been declining over the past 15 years. Better yields are obtained where crops are grown on previously unused land. Soils under cane are more compacted, more acid, contain less organic matter and are lower in cation exchange capacity and exchangeable cations. These differences reflect soil degradation caused by intensive cultivation. Contributing factors to the degradation of soils include soil compaction and structural breakdown occurring during harvest and cultivation operations, losses of organic matter due to burning of crop residues and acidification of soils due to large applications of nitrogen fertilizers. Soil management practices should aim to increase soil organic matter levels, provide a more favourable biological environment, reduce physical damage to soils during harvesting and cultivation, reduce soil acidity and improve the effectiveness of fertilizing practices.

Mechanized tillage systems effects on properties of a tropical alfisol in watersheds cropped to maize

Abstract: The effects of 6 years of continuous cultivation in 12 consecutive crops of maize ( Zea mays ) on changes in the physical and chemical properties of an Alfisol were investigated for two mechanized tillage systems imposed on twin watersheds of about 5 ha each. The tillage methods were no-tillage, with herbicides for weed control, and conventional plowing and harrowing. The soil organic matter content of the 0- to 10-cm layer of the no-tillage watershed increased by 7%, and that of the plowed watershed decreased by 72%. Soil pH declined by 0.3 and 1.7 units in no-tillage and plowed watersheds, respectively. As a consequence of pH changes, exchangeable cations declined with continuous cultivation, regardless of the tillage method, although the rate of decline was more drastic in the plowed than in the no-tillage watershed. Continuous mechanized cultivation substantially decreased infiltration rate. Soil compaction was more severe at the tractor's turning points along the plot's border. The results are discussed in terms of tillage needs and implications for mechanized farming in the tropics.

A basis for soil profile modelling in a geomorphic context

Abstract: SUMMARY A mathematical model for soil profile evolution is developed in three parts; respectively for the weathering profile, the organic profile and for the inorganic profile associated with nutrient cycling. Processes modelled are percolation, equilibrium solution, leaching, ionic diffusion, organic mixing, leaf fall, organic decomposition and mechanical denudation. In each sub-model some of these processes may be neglected, and time-scales differ, so that they are best separated. Each leads to a second-order linear partial differential equation with non-constant coefficients. Equilibrium profiles are derived for the simplest cases, and numerical models exemplify profile evolution in more realistic cases. Modelled weathering profiles generally show a zone of near-constant composition at the surface, with saprolite developing in deep profiles. Nutrient-cycling profiles show the variable development of B-horizons, and may be classified according to the relative depths of rooting, organic soil, and organic mixing processes.

Uptake of six heavy metals by oat as influenced by soil type and additions of cadmium, lead, zinc and copper

Abstract: The influence of heavy metal additions on availability and uptake of cadmium, lead, zinc, copper, manganese and iron by oat was studied. The experiments were carried out as pot experiments using sandy loam, sandy soil and organic soil. Selective extractants were used to remove metals held in different soil fractions. Lead and copper were preferently bound by organics and oxides, zinc by oxides and inorganics, and cadmium by inorganics and organics. Addition of cadmium to the soils resulted in higher cadmium concentrations in all plant parts but lower concentrations of lead, zinc, copper, manganese and iron, and the accumulation indexes of these metals were also lower when cadmium was added to the soil. Addition of cadmium plus lead, zinc and copper resulted in higher cadmium concentrations in leaves and straw of plants grown in sandy loam and sandy soil, but lower concentrations when plants were grown in organic soil as compared with the results when cadmium was added separately. The transfer of cadmium, lead, zinc and copper from soil to plant was greatest from sandy soil, and zinc and cadmium were more mobile in the plant than were lead and copper. Cadmium concentrations in leaves correlated significantly with CaCl2 and CH3COOH extractions in sandy loam and sandy soil and with CH3COOH extractions in organic soil. Generally, the total metal uptake was lowest from organic soil.

Acidification of soils by trees and forests

Abstract: The impact of isolated trees and natural forest vegetation on soil acidity is discussed. There is a considerable variation in impact between species on similar soils and between sites for any given species. The effect of coniferous plantations on soil acidity is reviewed and the causes of any increased acidity discussed. Crop species, initial soil conditions, silvicultural practices and the proportion of the tree removed at felling are all important factors influencing the long-term impact of plantations on soil acidity.

Effect of acid rain on soils

Abstract: Soil formation is an acidifying process that involves dynamic chemical and biochemical equilibria. Chemical weathering involves hydrolysis, hydration, oxidation‐reduction reactions, carbonation, and solution of compounds and elements found in the parent materials. The sources of acidity in soils include H2CO3 formed from high CO2 concentrations in the soil atmosphere produced from root and microbial respiration, acidity produced in mineralization of organic N and S and from NH4 +‐forming fertilizers, organic acids produced from litter decomposition, and addition in precipitation. Additions of acidity in precipitation to soils are insignificant in comparison with the acidity produced in natural processes and by agricultural soil amendments. The pronounced intrinsic buffering capacity provided by soil minerals and organic matter tends to minimize changes in soil pH when acid precipitation is added. The amounts of N and S added by precipitation are beneficial to agricultural, forested, and unmanaged soils. T...