The relationship between soil structure and microorganisms is a unique two way interaction. Whilst soil structure influences microbial transport, activity and predation, microorganisms affect soil structure by playing a major role in its genesis and stabilisation. Microorganisms are largely thought to influence structural genesis and stabilisation through the exudation of polysaccharides, which act to bind soil particles together as well as microorganisms to soil particles. Filamentous microorganisms are also thought to influence structural genesis and stabilisation by enmeshing soil particles together. A laboratory-based experimental system was developed whereby the structural heterogeneity of soil cracking patterns, generated from homogeneous soil slurries, was quantified using fractal concepts. A fractal approach was used because it provided a theoretical framework in which structural heterogeneity could be quantified and linked to soil processes such as gaseous diffusion, water movement and microbial population dynamics. This study demonstrated the potential of fractal geometry to functionally quantify soil structure. A greater understanding was gained of some of the processes acting to direct the formation of particular structural geometries. The latter is particularly important since it is the geometry of soil structure which will regulate processes such as gaseous diffusion and water movement. Microbial activity (mainly through the production of polysaccharides) was shown to have a significant effect on the generation of soil structure as well as on its water retention properties. The nutritional management of the microbial population was also shown to be a key factor regulating its contribution to structural genesis. Microorganisms can play a significant role in structural genesis ultimately impacting upon all aspects of the soil-plant-microbe ecosystem. It will be essential to understand how to effectively manage the indigenous microbial population if their potential as soil conditioners is to be fully reached.

The requirement of zinc (Zn) for normal plant growth was first recognized in the late nineteenth century, but acceptance of this element as an essential plant nutrient did not occur until the early 1930s. Since then, Zn deficiency has been identified throughout the world on numerous crops grown on widely varying soils under a variety of management practices. Many soils contain appreciable amounts of Zn that plants are unable to utilize. The addition of Zn compounds to soils can also prove to be of limited benefit, as most of the Zn becomes "fixed" by soil components. The mechanisms responsible for Zn "fixation" or "release" have not been fully elucidated. An understanding of the controls on the concentration of Zn in soil solutions is of great significance to the art of maintaining and improving the fertility of soils, and is of rapidly growing importance in understanding the development of Zn deficiency in plants. Certainly a better understanding of soil Zn chemistry and availability, and plant-soil relations will help improve agronomic practices for efficient crop production. With a better understanding of the Zn-deficiency problem in crops in mind, a project has been undertaken on 'zinc reactions and availability in soil'. The work in the project concentrates mainly on evaluation of zinc extractants, identification of soil variables responsible for Zn unavailability, mechanisms involved in Zn fixation at elevated pH and response of barley to Zn and Cu application.

Long-term changes in soil profile characteristics and important soil physical, chemical and biological properties were evaluated and compared at 4 paired sites in NE Scotland, 44-61 years after the afforestation of agricultural soils planted with either Norway spruce (Picea abies (L.)) or Scots pine (Pinus sylvestris (L.)). At each paired site five profiles were studied in the afforested and five in the control agricultural soils. Comparison made between various properties in the pairs of soils showed a number of significant changes, which are summarised in Tables 2.18-2.20. The surface (O) horizon of the afforested soil could be differentiated into L, F and H horizons. The O horizon was rich in organic matter, crumb structured, porous and well-drained and was differentiated from the A horizon below by an irregular boundary. The A horizon was moderately well to excessively well-drained with a subangular blocky to crumb structure. This horizon was differentiated from the B horizon below by an irregular boundary. The agricultural soil profile was characterised by compact O and A horizons with a subangular blocky structure; each horizon was separated from the one below by a gradual, smooth horizon boundary. Afforestation increased the combined thickness of the O and A horizon by 0.21 cm yr-1. It caused a significant decrease in bulk density and had no effect on particle size distribution. At most sites % organic matter, C, C/N ratio, CEC and NH4+ concentration were higher and extractable P concentration lower in the O horizon and most of the A horizons of forest soils, compared to the agricultural soil. The concentration of N increased significantly in the O horizon. Soil pH, exchangeable Ca, Mg and K and % base saturation decreased significantly in most forest soil horizons compared to the agriculated soils. Exchangeable Na concentration increased in the B horizon. Accumulation rates of each element since afforestation were also calculated.

The objective of this work was to study the decomposition of five species of blue-green algae under varying environmental conditions in soil and to examine the availability of nitrogen and other nutrients supplied through the growth of blue-green algae to rice growing in pots. In the initial phase attempts were made to grow the blue-green algae on nutrient solution and culture conditions were tested to provide optimum growth. The results of pot experiments showed that growth of all five species of blue-green algae produced an increase in dry matter yield of rice significantly higher than the controls but for four species the yield was less than the lowest addition of fertilizer nitrogen. The fifth species, Anabaena variabilies produced the highest yield in pots supporting blue-green algae but it was lower than the best yields obtained by addition of fertilizer. There was little difference in yield produced by the two fertilizers, ammonium sulphate and urea, and both increased yield with increasing addition up to the third rate (90 mg N pot-1). Nitrogen addition influenced the nutrient content of the dry matter but the growth of blue-green algae appeared to cause a small but not significant competition for nutrient uptake.

The impact of land use on the N and P status of three contrasting river catchments within the north east of Scotland was studied over time and along the length of each river. Annual mean NO<SUB>3</SUB>-N concentrations increased significantly in the Ythan, Don (P<0.001) and Dee (P<0.05), from 1958 to 1992. Concentrations in the predominantly agricultural Ythan catchment showed the largest increase from 1.9 mg l<SUP>-1</SUP> in 1958 to 7.0 mg l<SUP>-1</SUP> in 1992. A significant increase in mean NO<SUB>3</SUB>-N concentrations in the spring from 1980 to 1992 in the Ythan appeared to reflect the increased N fertiliser usage associated with a change from winter to spring cropping which was evident over this period. Annual mean NO<SUB>3</SUB>-N loads over the period 1980-1992 were 1305, 2010 and 623 t yr<SUP>-1</SUP> for the Ythan, Don and Dee, respectively. Annual PO<SUB>4</SUB>-P loads were 17.6, 40-1 and 16.8 t yr<SUP>-1</SUP> over the same time period for the Ythan, Don and Dee respectively. These loads were found to have associated errors in the range 16-24, 16-17 and 18-32% for NO<SUB>3</SUB>-N, PO<SUB>4</SUB>-P and total P respectively. Annual loss coefficients, calculated for NO<SUB>3</SUB>-N, PO<SUB>4</SUB>-P total P, Si and suspended sediment were 27.2, 0.180, 0.319, 26.1 and 73.5 kg ha<SUP>-1</SUP>, respectively. Annual losses of N and P from forestry were calculated in the Kirkhill catchment, a small subcatchment of the Don and ranged from 0.6 to 2.5 kg ha<SUP>-1</SUP> depending on flow. Annual total P losses were also related to flow and ranged from 0.029 to 0.06 kg ha<SUP>-1</SUP>. A nutrient budget calculated for the Kirkhill catchment found the annual loss of N from the catchment accounted for 44.5% of N inputs, in contrast, only 2.7% of P input to the catchment was detected in the output.

A multi-element soil extraction procedure (P, K, Mg, and Ca) using a mixed cation/anion exchange resin has been developed and its performance evaluated. Ion uptake to and recovery from the resin were highly reproducible over the concentration ranges typically experienced in soil. The effects on the overall reproducibility of changing various experimental parameters such as soil:water:resin ratios were quantified. Plant nutrient uptake and yield parameters obtained from either field (coconut) or pot (rye-grass) experiments using tropical or temperate soils have been used to evaluate the predictive capabilities of the proposed method. Comparisons with existing more widely acceptable soil extractants are made. The proposed method generally gave highly significant correlations with crop data which were independent of soil type. The possibility of including a simultaneous multi-element analytical step, which greatly increases the overall advantages of the resin procedure, has also been assessed.

No description available.

The availability and retention of zinc by soils and minerals of relevance to the crop, especially rice, growing areas of Bangladesh was investigated. The 32 days of continuous soil submergence caused a significant decrease in available zinc, sulphate and copper. Available iron increased significantly while manganese showed a sharp increase followed by a rapid decrease. As pH increased, adsorption and/or precipitation and co-precipitation of zinc with aged or fresh iron and aluminium hydrated oxides was partially responsible for the frequently reported fixation and unavailability of zinc added to the soil. The particle size of the products increased with increasing pH and initial concentration of the relevant elements. For synthetic oxides, the adsorption capacity of manganese appeared to be higher than that of iron or aluminium varieties. The amount of zinc adsorbed by fresh or aged iron or aluminium oxides was similar, but was much different between the fresh and aged forms. The presence of nitrate and sulphate anions made no difference to the shape of the isotherm for zinc adsorption on soil. Variations in adsorption between soils were attributed to the content and nature of the clay fraction and magnitude of CEC. Soils with the highest percentages of clay and, especially, smectites had the highest adsorptive capacities for zinc. Zinc adsorption by soils, clays and synthetic hydrous oxide minerals conformed to the Langmuir isotherm model within certain limits. It was concluded that a higher content of iron, aluminium and manganese oxide hydroxides, a higher CEC and a higher proportion of smectites in the clay fraction were the principal reasons for increased retention by soils and resultant decreased availability of zinc to plants, particularly at higher pH.

Al equilibria with soil organic matter (SOM) was studied in terms of two basic reactions involving exchangeable Al and chemically bound (complexed) Al. Ca ↔ Al exchange on eight soils of variable SOM content resulted in cation exchange capacities (CEC) which varied with the degree of Al saturation. This effect was attributed to the release of organically bound Ca by 1M KC1 as shown by the relative homogeneity of Al adsorption strength and heterogeneity of Ca adsorption revealed by Langmuir analysis. Acetylacetone extraction gave virtually no 'non-exchangeable' Ca but the exchangeable Ca following Ca saturation, was invariably greater than the corresponding Al fraction even on a wholly organic soil. A persistant pH drop, despite repeated equilibration was definitely attributed to continuing Al adsorption. The weak acid nature of the SOM polycarboxylates was analysed by slow auto-titration. Analysis of the apparent acidity constant variation with polymeric charge showed trends consistent with 'electrostatic principles' where weakening of carboxyl-acid strength, occurs as a result of polymeric charge build-up. The extent of this depended on the COOH content/density. A high COOH content also resulted in a low 'intrinsic' pKa (at zero charge) and this was assumed to be a product of chemical dipole effects. However, when the SOM fractions were compared with poly-maleic acid fractions of various molecular weights, low m.w. pure polycarboxylates and pure dicarboxylic acids some other influence on pKa values was in evidence. The exceptionally low intrinsic pKa values for fulvic acids and low m.w. water soluble soil acids were considered to be due not only to COOH clustering but also to carbonyl or hydroxyl substitution around COOH groups. A semi-empirical study of pH buffer capacity and coagulation of soluble SOM by Al was conducted on peat, and the relative complexing and coagulating powers of six metals were assessed. Complexing power depended on the individual metal, whereas coagulation was largely a function of the metal ion valence. This meant that for any degree of complex formation, the solubility of different metal complexes was variable. Transition metals apparently produced the most soluble complex species. An attempt was made to assess 'differential scanning calorimetry' as an analytical tool with respect to metal-SOM complexes. The degree of metal saturation of a SOM fraction gave regular changes in the main DeltaT peaks and different metals produced different patterns. However, it was clear from the study that the major influence on the temperatures of thermal decomposition was of a physical rather than a chemical nature. Accordingly, it is possible that the micro-configuration of coagulated SOM-metal species dictate the DSC pattern rather than the strength of metal-SOM bond as suggested by other workers. The reproducibility of the technique as well as its qualitative interpretation limit its use. In order to test the electrostatic treatment of SOM with respect to Al bonding, Cu chelation was studied so that free ionic Cu could be determined by the use of an ion selective electrode (I.S.E.) and so provide a check on the more theoretical 'pH method' with respect to species prediction. The method used required computer analysis using successive approximations to resolve a 'proton competition' constant which proved to be approximately constant over a wide range of metal:SOM ratios. Furthermore, the true co-ordination number was virtually invarient at &ap;2, indicating only a single bidentate chelate with respect to Cu bonding. The pH and ISE derived stability constants showed good agreement. From the stability constants for Cu-SOM chelation, a model of the soil solution was constructed where the proportion of Cu present as organically complexed metal was expressed exactly as a function of mg SOM l-1 and p(L-) and approximately as a function of mg SOM l-1 and pH. The techniques used to study Cu-polycarboxylate equilibria were applied to Al-SOM and complex formation with low m.w. aromatic and aliphatic carboxylic acids was studied also. The bidentate chelation constant of Al-SOM at a polymer charge of zero closely resembled Al-succinic/glutaric/adipic acid equilibria and a 1:2 Al-acetate complex. As the polycarboxylate charge increased, the Al bond strength increased. The lower m.w. soil acids bonded with Al more weakly because of the same inductive effects controlling proton association. A model of the soil solution with respect of Al chelation was constructed including allowance for hydrolysis species. The relevance of the model with respect to solubility of Al-SOM and soluble SOM in soil solution was confirmed by a coagulation study and by measuring soluble SOM concentration in agricultural topsoil solutions covering a pH range of 4.5--7.5.

An automated Gas Phase Molecular Absorption Spectrometry (GPMAS) method for the determination of sulphide in solution has been developed. Sulphide ions react with 3M hydrochloric acid and the released hydrogen sulphide is swept into a gas-liquid separator by an air-stream. The absorbance was measured at 200 nm using a deuterium hollow cathode lamp. The method is relatively free from interference with a detection limit of 0.06 ug ml-1 of sulphide and relative standard deviations of 1.4-3.3% for repeated analyses. The calibration graph is linear up to 100 g ml-1 of sulphide and twenty samples can be analysed in one hour. The method has been applied to the determination of sulphate sulphur in plants. An automated procedure for determination of sulphite in aqueous solution by GPMAS was also developed. Using the optimised conditions obtained in this investigation, the method has a detection limit of 0.20 g ml-1 and relative standard deviations of 2.3% and 1.8% for 20 g ml-1 and 10 g ml-1 of sulphite respectively. The calibration graph is linear up to 120 g ml-1 and samples can be analysed at the rate of 20 per hour. The method has been applied to the determination of sulphur dioxide in synthetic samples and in white wines. The gas phase sampling introduction technique was also utilized in emission spectrometry. A sensitive and selective cool flame emission spectrometric method for the determination of sulphide in environmental solution samples was developed. The technique is based on passing the hydrogen sulphide liberated on acidification of samples with 3M hydrochloric acid into a cool, nitrogen-hydrogen diffusion flame and the resulting S2 molecular emission is measured at 384 nm. The method, which is automated, has a detection limit of 0.026 ng ml-1 for sulphide and relative standard deviations of 0.8- 2.5% for repeated analyses. Under the optimised conditions, the proposed method is applicable over the concentration range 0.1 - 8.0 g ml-1 of sulphide. Twenty samples can be analysed in 1 hour and the method has been applied to the determination of sulphide in waste water samples. The conversion of sulphite and sulphur dioxide [fixed as disulphitomercurate (II)] to hydrogen sulphide by reduction with sodium tetrahydroborate (III) has been used to provide a much improved sensitive, cool flame emission spectrometric method for the determination of sulphur dioxide and sulphite. The sample is mixed with NaBH4, acidified with 6M hydrochloric acid, and carried by a continuous-flow stream into a gas-liquid separator where the evolved hydrogen sulphide is swept by nitrogen into a cool, hydrogen- nitrogen-entrained air flame. The intensity of the blue S2 emission generated is measured at 384 nm. The proposed method has a detection limit for sulphite of 0.029 g ml-1 and relative standard deviations of 1.2 and 1.5% for 1 and 5 g ml-1 respectively. The calibration graph is linear up to 24 g m-l of sulphite and samples can be analysed at a rate of about 40 per hour. The method has been applied to the determination of sulphur dioxide in air and sulphite in wines. The proposed method, which is more sensitive than many earlier methods, is simple, versatile and capable of good precision. It is also rapid and convenient. Interferences are known but their effects may be minimized. The method is relatively free from nitrogen dioxide interference. Forty samples can be analysed in 1 hour and the results obtained by the proposed method correlate well with those obtained using an official method. The contribution of dissolved sulphate to total dissolved sulphur in a range of environmental samples was also studied. The results confirmed the importance of non-sulphate sulphur in a range of environmental samples. The importance of sample preparation in soil analysis was also demonstrated because of the effect of drying on sulphur constituents of soils.