The evaluation of plant-available sulphur in soils

Summary Examination of a range of naturally occurring calcium carbonates and calcareous soils has shown that insoluble sulphate associated with calcium carbonate may comprise an important fraction of soil sulphur. One soil contained as much as 93 per cent of its sulphur in this form. It seems likely that this sulphate occurs as a co-precipitated or co-crystallized impurity in the calcium carbonate.Most surface soils had only low capacity to adsorb sulphate and contained only small amounts of sulphur in this form. Two acid surface soils and many acid subsoils, however, adsorbed sulphate quite strongly and in some acid subsoil clays adsorbed sulphate made up an important fractions of the total sulphur.Sulphate adsorption was found to be negligible above pH 6.5 and adsorbed sulphate may be determined by aqueous extraction after increasing the pH above this value by addition of solid calcium carbonate. Adsorption of sulphate during acid extraction of soils can lead to low values in the determination of acid-soluble sulphates. Sulphate so adsorbed can be determined by a second extraction with water after the addition of solid calcium carbonate to increase the pH to a value greater than 6.5.     

Some factors affecting the mineralization of organic sulphur in soils

Summary Factors affecting the release of sulphate from a number of eastern Australian soils were studied.All of the soils released sulphate when dried. The amounts released were influenced by the manner in which the soil was dried. Air-drying in the laboratory at 20°C released least sulphate.Sulphate was mineralized in all soils by incubation at 30°C but the amounts mineralized could not be related to soil type or any single soil property. The ratio of nitrogen mineralized: sulphur mineralized varied widely between soils and was generally appreciably greater than the ratio of total nitrogen: organic sulphur in the soils.A rapid flush of mineralization of both sulphur and nitrogen took place when some of the soils were rewetted and incubated after they had been dried in the laboratory and stored for 4 to 5 months. Following this, the rate of mineralization was similar to that in the original undried soil. During this flush, the enhancement of sulphur mineralization was relatively greater than that of nitrogen so that the ratio of nitrogen mineralized: sulphur mineralized was considerably smaller than that during later phases of the incubation or that of the original moist soil. Soils collected after they had remained dry in the field for a similar period of time did not show this type of mineralization although they had initially done so when collected moist and air-dried in the laboratory.The effects of temperature, soil moisture, toluene and formaldehyde, and the addition of calcium carbonate to soils on the mineralization of sulphur were similar to their effects on the mineralization of nitrogen.     

Sulphur uptake by ryegrass and its relationship to inorganic and organic sulphur levels and sulphatase activity in soil

Summary Yield of perennial ryegrass and uptake of S at 10, 18 and 25°C were studied in a pot trial using 12 soils with different levels of adsorbed sulphate, total organic S, ester-S, C-bonded S and sulphatase activity.Adsorbed sulphate gave the best correlations with yield and S-uptake at each temperature. At the end of the trial appreciable levels of adsorbed sulphate remained in all the soils. For a given soil, the amount remaining was the same irrespective of the growth temperature employed. This adsorbed sulphate was considered to be unavailable for plant growth.Sulphur uptake was greater in all cases (except at 10°C for one soil) than the decrease in adsorbed sulphate during the trial. This additional sulphur was presumed to have come from organic sources and was significantly correlated (0.1%) with organic S, ester-S, C-bonded S and sulphatase activity at each temperature. These correlations were, however, markedly influenced by one organic S rich soil. When this was omitted the significance of the organic S, ester-S and sulphatase activity correlations fell to 5% and the C-bonded S correlations became non-significant. In this latter instance the absence of significant relationships with C-bonded S was taken to indicate that in this trial the fraction was of less importance in S mineralisation than ester-S.Although sulphatase activity is implicated in the mineralisation process by its significant correlations with the uptake of organic S, other factors are discussed which suggest that the correlations may not be conclusive.     

On the effect of ferrous sulphate on the available manganese in the soil and the uptake of manganese by the plant II

Summary Pot experiments with oats on manganese deficient sandy and moor soils, which are not deficient in iron, showed a steady increase in yield and manganese uptake by the plants with increasing additions of ferrous sulphate at four levels of manganese. Residual effects from ferrous sulphate application were not found either in the case of yields or manganese uptake by oats on a sandy soil.The effect observed following application of ferrous sulphate was due to a manganese effect. This is corroborated by the coincidence of the yield curves, showing the relationship between absorbed manganese and yield of dry matter, following the application of manganese sulphate and ferrous sulphate.Soils treated withM magnesium nitrate in the presence of equivalent quantities of either ferrous sulphate or hydroquinone yielded the same amounts of manganese. This result suggests that the manganese effect of ferrous sulphate is due to reduction of higher manganese oxides by ferrous sulphate.     

Sulphur and nitrogen changes in forest soils of East Africa

Summary Sulphur measurements made on soils in East Africa revealed the unexpected occurrence of sulphates in forest subsoils. Soil profiles from plantation and natural forests were analysed and a general pattern of sulphur distribution was found.It has been found that in addition to the sulphur cycle dependent upon microbiological oxidation of organic sulphur there exists a cycle whereby sulphate is circulated as such between trees and soil.The biological oxidation of organic sulphur in the soil is an extremely slow process compared with that of carbon and nitrogen. Treatment with calcium carbonate, nitrates and non-sulphur-containing amino acids had no effect upon the subsequent sulphate production. Incubation with either S-containing amino acids or with calcium sulphate however, resulted in an increased rate of soil-sulphur oxidation and this effect is being further investigated.An aqueous extract of wattle leaves proved to inhibit nitrification in the soil for short periods and evidence is presented which suggests that methionine may be involved in this effect.     

Sulphur mineralization and adsorption in soils

Summary Studies were conducted to determine the comparative sulphur mineralizing capacity of selected Malaysian and Iowan soils and to determine the amounts of available and adsorbed sulphate in a number of Malaysian soils. Results of the mineralization study indicated that more sulphur mineralised from Malaysian soils although their average contents of total sulphur were lower compared to Iowan Soils. For both sets of soils, significant correlations between contents of organic carbon and total sulphur existed indicating that most of the sulphur was in organic combination. Phosphate solution consistently extracted higher quantities of sulphate in comparison to chloride solution in the Malaysian surface soils implying that a portion of the sulphate existed in adsorbed form. Adsorption of sulphate in soils was found to be dependent on concentration of sulphate added and followed the Langmuir adsorption isotherm.     

Effects of adding calcium and sodium salts to field soils on the incidence of clubroot

The effect on clubroot of the addition to a mineral soil of calcium carbonate, calcium sulphate and sodium carbonate was studied in two field experiments during a 3-yr period. Applications of calcium carbonate (ground limestone) at 10 or 20 t/ha increased the soil pH from 6×7 to 7×9; sodium carbonate at 7×5 t/ha to pH 8×3 and calcium sulphate (gypsum) at 20 t/ha caused a slight depression in pH to 6×6. 98% of cabbage plants showed clubroot symptoms in the untreated plots after 3 yr and the percentages were 0×7, 1×6 and 66×6 for the calcium carbonate, sodium carbonate and calcium sulphate treatments respectively. Yields were significantly increased by all three materials. There were no significant differences in disease incidence or in yield when calcium carbonate was used at 10 or 20 t/ha either applied as a single application or as two half-rate applications. Soil from the calcium carbonate treated plots was used in a greenhouse experiment where the addition of inoculum of Plasmodiophora brassicae resulted in a large increase in clubroot incidence. There was no evidence that a biological suppressor was present in the high pH, low disease incidence soils.     

Effects of dry and moist incubation of soils on the extractability of native and applied soil copper

Summary The amount of copper extracted from soils by EDTA was found to increase with the temperature of extraction. Under controlled conditions of extraction, the amount of applied copper extracted from soils by EDTA decreased during incubation of soils under both moist and dry conditions. The recovery of applied copper by EDTA appeared to be related to the amounts of manganese and iron oxides in the soil. On the basis of comparisons with calcium chloride-extractable copper and isotopically-exchangeable copper it is suggested that EDTA extraction could over-estimate the availability of soil copper to plants.     

Studies of ‘Available’ and isotopically exchangeable sulphur in some North Queensland soils

Summary Using ³⁵S-sulphate, the specific activity of various sulphur fractions in some diverse North Queensland soils has been followed for up to 185 days in a glasshouse experiment. The sulphur extracted with 0.01 M calcium phosphate was from the same pool as that used by the test plants, and since near full recovery of added ³⁵SO₄ was obtained initially, this fraction is comparable to the L-value. On the other hand, 0.5 M NaHCO₃ removed some soil sulphur that was not available to the plants.Liming caused an initial increase in the phosphate extractable fraction, the sulphur seemingly being released from the NaHCO₃ extractable fraction, but decreased sulphate sorption also contributed to the increase in S uptake by the plants upon liming. re]19750507     

Nitrogen balance studies for different Egyptian soils cropped with corn

Summary The nitrogen balance for four different Egyptian soils cropped with corn and fertilized with increasing amounts of ammonium sulphate was studied in pot experiments. The tested soils were clay loam, calcareous sandy loam, sandy loam and sand. Nitrogen added as seeds, fertilizers, irrigation water and insecticides, nitrogen removed by the plants including thinned plants and fallen leaves and the nitrogen content of the soils before sowing and after the harvest were determined and used to draw the balance sheet.The obtained results showed that nitrogen loss ranged between 6.8 and 51.5 per cent depending mainly on soil type and to some extent on the rate of applied ammonium sulphate. Nitrogen uptake by the corn plants was lowest from the clay loam soil and highest from the sandy soil. In all cases it increased slightly with the addition of nitrogen fertilizer. re]19720918     

Metal uptake by iron-efficient and inefficient oats

Metal uptake by oats depending on plant responses to Fe-deficiency stress was investigated. Coker 227 oats classified as Fe-efficient and TAM 0–312 oats as Fe-inefficient cultivars (Hopkins et al., 1992) were grown either alone or in combination in three sandy soils using a pot experiment. These soils were from a field trial with sludge-borne metals applications leading to an increased metal content. Plant shoots were harvested one month after growth. Because soil pH increased from 5.4 to 6.8, shoot Fe level decreased in the Fe-inefficient TAM 0–312 oats compared to Coker 227 oats when plants were grown alone. In combination, TAM 0–312 oats had a negative impact on the availability of Fe in the Fe-efficient Coker 227 oats. Especially, Coker 227 and TAM 0–312 shoots showed chlorosis in mixed culture with high Zn and Mn content in the soil (soil B). However, Fe content in TAM 0–312 shoots in mixed culture did not increase compared to monoculture in all soils. In metal-contaminated soils, TAM 0–312 oats grown alone obtained less Zn and Cd than Coker 227 oats. Additionally at soil pH 6.8, shoot Ni and Mn levels were also lower in TAM 0–312 oats than in Coker 227 oats. Shoot Zn, Cd, and Ni levels decreased in Coker 227 oats from mixed cultures, and were not different compared to those in TAM 0–312 oats. Cu uptake was similar in all treatments except for the mixed culture in soil B. Coker 227 oats have been found to release a phytosiderophore whereas TAM 0–312 did not (Brown et al., 1991). Results indicated that phytosiderophores may lead to a higher Zn, Cd and Ni supply in the rhizosphere of Coker 227 oats and to higher metal contents in their shoots than in TAM 0–312 oats which did not activate such mechanisms.     

Effects of sulphate and pH on the plant-availability of phosphate adsorbed on goethite

The adsorption of phosphate on metal (hydr)oxides may be influenced by the pH and by the adsorption of other ions. In this study, the influence of sulphate and pH on phosphate adsorption on goethite and the availability to plants of adsorbed phosphate was examined. Maize plants were grown on suspensions of goethite with adsorbed phosphate, containing the same total amount of phosphate and either 0.11 mM or 2.01 mM sulphate at pH 3.7, 4.6 or 5.5. The uptake of phosphorus by the plants increased with the larger sulphate concentration and decreasing pH. Mean P uptake in the treatment with 2.01 mM sulphate and pH 3.7 was 55 µmol plant^-1, whereas in the treatment with 0.11 mM sulphate and pH 5.5 it was 2 µmol plant^-1. Batch adsorption experiments using³² P and speciation modelling of ion adsorption showed that in the presence of sulphate, the phosphate concentration in solution strongly increased with decreasing pH, due to competitive adsorption between sulphate and phosphate on goethite. Modelled phosphate concentrations in solution in the uptake experiment were all below 0.6 µM and correlated well with the observed P uptake. This correlation indicates that the strong influence of the sulphate concentration and pH on the plant-availability of adsorbed phosphate results from the competition between sulphate and phosphate for adsorption on goethite.     

Some factors influencing production of sulphate by oxidation of elemental sulphur and thiosulphate in upper horizons of spruce forest soils

Some factors influencing the oxidative activity of upper horizons of spruce forest soils (a mixture of fermentative and humus layers) toward intermediates of the oxidative part of the sulphur cycle were investigated. Preincubation of the soil with added cysteine, sulphide, elemental sulphur or thiosulphate was found to stimulate enzyme systems oxidating any of these compounds. Sulphite and sulphate were ineffective in this respect. The oxidation of elemental sulphur was stimulated by CaCO3, technical urea and high doses of superphosphate and potassium sulphate. It was inhibited by KH2PO4, pure urea, 40 % potassium salt, ammonium nitrate with calcium carbonate and the fertilizer NPK I. It proceeded at the highest rate at approximately 60 % capillary capacity (61 % of mass water content). Oxidation of thiosulphate was stimulated by KH2PO4, pure urea, superphosphate, potassium sulphate and only slightly by the fertilizer NPK I. It was inhibited by CaCO3, 40 % potassium salt and only slightly by ammonium nitrate with calcium carbonate. Potassium chloride, glucose and technical urea were without effect. The oxidation proceeded at the highest rate at 35 % maximal capillary capacity (48 % mass water content).     

The role of soil microbes in plant sulphur nutrition

Chemical and spectroscopic studies have shown that in agricultural soils most of the soil sulphur (>95%) is present as sulphate esters or as carbon-bonded sulphur (sulphonates or amino acid sulphur), rather than inorganic sulphate. Plant sulphur nutrition depends primarily on the uptake of inorganic sulphate. However, recent research has demonstrated that the sulphate ester and sulphonate-pools of soil sulphur are also plant-bioavailable, probably due to interconversion of carbon-bonded sulphur and sulphate ester-sulphur to inorganic sulphate by soil microbes. In addition to this mineralization of bound forms of sulphur, soil microbes are also responsible for the rapid immobilization of sulphate, first to sulphate esters and subsequently to carbon-bound sulphur. The rate of sulphur cycling depends on the microbial community present, and on its metabolic activity, though it is not yet known if specific microbial species or genera control this process. The genes involved in the mobilization of sulphonate- and sulphate ester-sulphur by one common rhizosphere bacterium, Pseudomonas putida, have been investigated. Mutants of this species that are unable to transform sulphate esters show reduced survival in the soil, indicating that sulphate esters are important for bacterial S-nutrition in this environment. P. putida S-313 mutants that cannot metabolize sulphonate-sulphur do not promote the growth of tomato plants as the wild-type strain does, suggesting that the ability to mobilize bound sulphur for plant nutrition is an important role of this species.     

Assessment of a heavy metals‐contaminated site using sequential extraction,TCLP, and risk assessment techniques

The mobility of selected heavy metals in contaminated soil at a previous industrial site in Brisbane, Australia, was assessed using a sequential extraction technique. Copper, Pb, Zn, Cr, Fe, and Mn were extracted from the soil solution/exchangeable, carbonate, Fe and Mn oxides, and organic matter fractions. The amounts of metals adsorbed by these fractions were used as an indicator of each metal's mobility in the soil. Copper and Pb were largely adsorbed by the organic and oxide fractions, while a significant amount of Zn was extracted from the carbonate fraction. The potential mobility and biological availability of the metals in these soils is Zn > Cr = Cu ≈ Pb. Soils were also analyzed using the toxicity characteristic leaching procedure (TCLP) to determine whether the contaminated soil could be disposed of by landfilling. The leachability of all metals from the soils was very low, with metal concentrations below the allowable limits. The TCLP also showed that Zn was the most mobile metal in these soils. An environmental and health risk assessment was undertaken, and it was concluded that the site did not represent a risk despite the “total”; concentrations of some metals being up to 40 times the investigation threshold value adopted in Australia.     

Uptake of radioactive calcium by groundnut (Arachis hypogaea L.) and efficiency of utilisation of applied calcium

Summary A pot experiment was conducted with groundnut applying labelled calcium as its sulphate and carbonate at two levels namely 75 and 150 kg Ca per ha with varying levels of P, K and Mg. Plant samples were taken at different stages of crop growth and analysed for the content of radioactive calcium. Calcium sulphate treatment has resulted in larger uptake of calcium compared to calcium carbonate. An application of 150 kg Ca per ha has caused significantly higher uptake by groundnut plant than 75 kg Ca per ha. The percentage of utilisation of added calcium ranged from 2.2 to 5.4. Recovery of calcium by plants was more in calcium sulphate treatment rather than in calcium carbonate. The plants showed preference to absorb applied calcium rather than native calcium.Forms a part of the Ph.D. thesis of the first author approved by Tamil Nadu Agricultural University, Coimbatore.     

Effects of organic matter on sulphur oxidation in soil and influence of sulphur oxidation on soil nitrification

Summary The effects of wheat straw and pressed sugar beet pulp on sulphur oxidation were determined in a loam soil amended with 1% (w/w) elemental sulphur. Wheat straw stimulated the oxidation of elemental sulphur over the first 2 to 3 weeks of the incubation period, resulting in an increase in LiCl-extractable sulphate. After 4 to 7 weeks incubation however, the only significant increase in soil sulphate followed the 1% straw addition, while at week 7 sulphate concentrations in the 0.25% and 5.0% straw amended soils were lower than the control. Pressed sugar beet pulp (1% w/w) initially stimulated the oxidation of elemental sulphur in the soil, but by weeks 3 to 7 of the incubation period rates of oxidation in pulp-amended soils were lower than the control. Towards the end of the incubation period however, sulphate concentrations in the amended soils exceeded the control values, significantly so by week 11. The concentration of thiosulphate and tetrathionate also increased in soils receiving sugar beet pulp. Nitrification was inhibited in soils in which sulphur oxidation was actively occurring. Although possible alternatives are mentioned, such inhibition appears to result from a decrease in soil pH brought about by the oxidation of elemental sulphur to sulphuric acid.     

Speciation analysis of nickel in soil solutions and availability to oat plants

Soil solutions were collected for speciation analysis of nickel from a pot experiment with oats. Oat plants (Avena sativa L.) were grown on 3 soils differing in total amount and origin of nickel (Ni) (Luvisol, LS with 28 mg kg^-1; sludge amended Luvisol, LS+SS with 32 mg kg^-1; Cambisol, CS with 95 mg kg^-1). Results were compared with those for soil solutions obtained from corresponding unplanted pots. Separation methods were used for characterization of size and charge distribution and stability of the Ni species. In addition, short-term experiments were performed on the uptake rates of Ni by oat plants from the different soil solutions as well as from nutrient solutions with increasing concentrations of a synthetic chelator.The Ni concentrations in the soil solutions of unplanted soils increased in the order: LS5000 g mol^-1) was the predominant form, whereas in the other soils the low-molecular-size cationic and chelated Ni species (500–1000 g mol^-1) dominated in the soil solution. In the short-term uptake studies, the uptake rates of Ni from the solutions decreased in the order: nutrient solution > soil solutions, and in the latter in the order: LS>LS+SS>CS, which was inversely related to the concentrations of dissolved organic carbon in the soil solutions.The results demonstrate that Ni availability to plants is not only affected by total concentration of Ni in the soil solution and the rate of replenishment from the solid phase, but also by Ni species, which can differ considerably between soil types.     

Lead uptake from soils by perennial ryegrass and its relation to the supply of an essential element (sulphur)

Summary The lead status of 16 soils of England and Wales was studied by pot-culture and soil chemical procedures. Perennial ryegrass was grown on the soils, with and without added sulphur, in a controlled environment cabinet with carbonfiltered air. Plant-available lead comprised uptake in 4 successive harvests of tops plus that in roots at the final harvest. The concentration of lead in the tops of healthy plants, i.e. those with adequate sulphur, was lower than in the roots, e.g. at harvest 4 the means were 5.0 and 12.9 ppm, respectively. However, with sulphur-deficient plants the concentration of lead in the tops was often higher than in the roots, the means at harvest 4 being 16.3 and 13.0 ppm, respectively. The marked increases in the concentration of lead in the tops of sulphur-deficient plants coincided with decreases in dry-matter yield, but for any one soil the tops of such plants contained similar amounts of lead to those of healthy plants. The lead content of the tops was poorly correlated with soil lead whereas that of the roots, in terms of both concentration and total amount, was highly correlated. The amount of lead extracted by 0.5 M BaCl₂ or 0.05 M EDTA provided a slightly better assessment of availability than ‘total’ content or the amount extracted by 2.5 per cent acetic acid. The solutions of acetic acid, BaCl₂ and EDTA extracted, on average, 1.0, 16.3 and 32.7 per cent respectively, of the total lead in the soils. The greater replacement of lead by the Ba ion than by the H ion (acetic acid) is ascribed to valence and the similar radii of Pb⁺ and Ba⁺. It is concluded that in soil-grown ryegrass the roots restrict the movement of lead into the tops of high-yielding plants, but when growth is limited by sulphur deficiency the concentration in the tops increases markedly. re]19720922