Inhibitory effects of acidic minesoil on the sericea lespedeza/Bradyrhizobium symbiotic relationship 1

Effects of acidic minesoil on sericea lespedeza [Lespedeza juncea (L.F.) var. sericea (Mig.)] and its nitrogen (N₂)‐fixing symbiotic relationship with Bradyrhizobium spp. were examined. Sericea lespedeza was grown in pots with N fertilization, without N fertilization, or with commercial Bradyrhizobium as a seed inoculant. Minesoil (pH 5.2) was fertilized with calcium (Ca), phosphorus (P), molybdenum (Mo), and potassium (K), and the pH level was adjusted to 4.8 or 4.5 with aluminum or iron sulfate [Al₂(SO₄)₃; Fe₂(SO₄)₃]. Minesoil was also limed to pH 6.1. Shoot dry weights, shoot N concentrations, nodule dry weights, and nodule numbers were significantly lower (P < 0.05) when inoculated plants were grown in soil at pH 4.5 and 4.8 compared to limed soil. Thus, the N₂ fixation process was adversely affected below pH 5.0. Nitrogen‐fertilized plants grew well in acidified soil, and there were no significant differences in shoot dry weights of such plants among the soil acidification treatments including limed soil. Thus, the N₂‐fixing symbiosis appeared to be more sensitive to acidified soil than the plant host. The effects of Al toxicity versus other factors could not be determined because Al₂(SO₄)₃‐ and Fe₂(SO₄)₃‐amended soils contained similar levels of toxic Al at the highest pH (4.8) that prevented N₂ fixation.

Time periods required for cells of Bradyrhizobium strains to multiply by a factor of 10⁴ were significantly longer (P ≤ 0.05) in extracts of Al₂(SO₄)₃‐amended soil (pH 4.8 and 4.5) than in extracts of calcium carbonate [CaCO₃]‐amended soil (pH 6.1). These increases suggested that reduced multiplication of Bradyrhizobium in acidified minesoils may have been at least partially responsible for the large decreases in nodulation and N₂ fixation observed in these soils. It was also reasoned that the inability of existing bacteria to infect and nodulate plant roots may also have been a factor, based on the high inoculation rates used and the abilities of Bradyrhizobium cells to survive and multiply (albeit at a reduced rate) in extracts of acidified soil. Sericea lespedeza is known to tolerate soils of pH 4.5. However, results of this study suggested sericea lespedeza may not fix appreciable N₂ in acidic soil below pH 5 when inoculated with commercial Bradyrhizobium, even after the establishment of lespedeza plants tolerant of such conditions.     

Effect of some herbicides on microbial activity in soil

The effect of 25 herbicides and herbicide combinations, in amounts comparable to those used in agriculture, on microbial activity in two soil types was determined in the laboratory. Herbicides did not affect respiration, assayed by CO₂ evolution and dehydrogenase activity, in either silty clay loam or loamy sand. Organic matter decomposition, determined by the amount of CO₂ evolved and inorganic N formed from decomposing alfalfa tissue, was also not affected. Alteration in soil pH or moisture content did not affect herbicide action. Addition of herbicides 3 weeks before amendment, or fertilizer application, also did not influence herbicide activity. Selected herbicides (trifluralin, linuron, dinoseb) at concentrations 100-fold higher than the recommended rates did not affect alfalfa decomposition. Solubilization of Ca₃(PO₄)₂ in soil was not affected by herbicides. S oxidation to SO^2?₄ in soil, however, was increased by most herbicides. In silty clay loam, 18 of the 25 herbicides and herbicide combinations increased S oxidation almost up to 3-fold. Results in loamy sand were similar. Dinoseb effectively reduced the algal population in loamy sand by more than 90%. Trifluralin, linuron, and metribuzin did not inhibit algal populations.     

Salts and acids as determinants of the solution composition of acidic forest soils — anion effects

The study aimed at evaluating whether salt-induced mobilization of acidity may be modified by the type of anion. For this purpose, the effects of different neutral salts on the solution composition of acid soils were investigated. The results were compared with those of the addition of acids. Two topsoil (E and A) and two subsoil horizons (Bs and Bw) were treated with NaCl, Na₂SO₄, MgCl₂, MgSO₄, HCl, and H₂SO₄ at concentrations ranging from 0 to 10 mmol dm^?3. With increasing inputs of Cl^? the pH of the equilibrium soil solution dropped, the concentrations of Al and Ca increased, and the molar Ca/(Al³⁺ + AlOH²⁺ + Al(OH)₂⁺) ratios decreased. These effects were the least pronounced when NaCl was added and the most at the HCl treatments. According to the release of acidity, the topsoils were more sensitive for salt-induced soil solution acidification whereas on base of the molar Ca/(Al³⁺ + AlOH²⁺ + Al(OH)₂⁺) ratios, the salt effect seems to be more important for the subsoils. Addition of S0₄^2? salts and H₂SO₄ induced higher pH and lower Al concentrations than the corresponding Cl^? treatments due to the SO₄^2? sorption, especially in the subsoils. The Ca/(Al³⁺ + AlOH²⁺ + Al(OH)₂⁺) ratios were higher than those of the corresponding Cl^? treatments. In subsoils even after H₂SO₄ additions these ratios were not higher than those of the NaCl treatments. The results indicate (I) that speculation about the effects of episodic salt concentrations enhancement on soil solution acidification not only need to consider the ionic strength and the cation type but also the anion type, (II) that salt-induced soil solution composition may be more crucial in subsoils than in topsoils, and (III) that in acid soils ongoing input of HNO₃ due to the precipitation load may induce an even more acidic soil solution than the inputs of H₂SO₄ of the last decade.     

Influence of soil pH on the toxicity of aluminium for Eisenia andrei (Oligochaeta: Lumbricidae) in an artificial soil substrate

Toxicity of aluminium for the earthworm Eisenia andrei was studied in artificial soil at different pH levels. In a range-finding test, effects of three different aluminium salts on earthworm survival were determined. AlCl₃ appeared to be most toxic, with LC₅₀ values of 316, 359 and >1000 mg Al/kg dry soil at pH_KCl of 3.5, 4.4 and 6.7, respectively in the control soils. Effects of this salt interfered with a strong decrease of soil pH with increasing aluminium concentration. Al₂(SO₄)₃ was less toxic with LC₅₀ values of 457, >4000 and >4000 mg Al/kg dry soil at pH 3.24, 4.86 and 7.22, respectively. Al₂O₃ did not affect earthworm survival at concentrations of 5000 mg Al/kg and pH levels between 2.4 and 7.1.In the main test, earthworms were exposed for 6 weeks to soils treated with Al₂(SO₄)₃. As in the range-finding test, aluminium sulfate was most toxic at a pH of 3.4 with an LC₅₀ of 589 mg Al/kg dry soil. At this pH, growth and cocoon production of earthworms were significantly reduced at 320 mg Al/kg dry soil, while at 1000 mg Al/kg dry soil all earthworms died. Survival was not affected by 1000 mg Al/kg dry soil at pH 4.3 and 7.3. At pH 4.3, growth was significantly reduced at 1000 mg Al/kg dry soil and cocoon production at 320 and 1000 mg Al/kg dry soil. At pH 7.3, aluminium only affected cocoon production at the two highest exposure levels. At the highest two exposure levels at pH 7.3, growth was significantly increased, suggesting a trade-off between growth and reproduction. These effects of aluminium at the highest soil pH could not be explained from the concentration of extractable, monomeric (labile) aluminium in soil, which decreased with increasing soil pH.     

Yield,nutrient, and soil sulfur response to ammonium sulfate fertilization of soybean cultivars 1

With the reduction of sulfur levels in high‐analysis nitrogen (N) and phosphorus (P) fertilizers and in atmospheric deposition, sulfur (S) fertilization may become more important, especially with intensive cropping systems. When high clay content is likely to limit root development into the subsoil, low extractable sulfate‐sulfur (SO₄‐S) levels in the topsoil may suggest possible plant response to S fertilization. Even though ammonium sulfate [(NH₄)₂SO₄] is widely used and readily available for plant uptake, field data are limited on the use of (NH₄)2SO₄ as an S source for soybeans [Glycine max (L.) Merr.]. A study was initiated to determine the effect of S fertilization as (NH₄)₂SO₄ on: (i) the yield, seed weight, grain quality, and leaf and whole‐plant nutrient concentrations of four soybean cultivars grown on soils with high clay content subsoils; and (ii) selected soil chemical characteristics. Sulfur rates were 0, 28, 56, and 84 kg/ha, and soybean cultivars were two Maturity Group IV beans, DeSoto and Douglas, and two Maturity Group V beans, Bay and Essex.

The study was conducted on a Parsons silt loam soil (fine, mixed, thermic, Mollic Albaqualf) in 1986 and 1987, and on a Cherokee silt loam (fine, mixed, thermic, Typic Albaqualf) in 1987. Sulfur application did not significantly affect soybean yield or seed protein or oil concentrations. For whole plants, S concentration increased and N:S ratios decreased with increasing S fertilization. Similar trends were found in soybean leaves. Although N:S ratios of both whole plant and leaf tissue were lowered with S fertilization, the values generally were not below 20:1 which is above cited critical levels. Fertilization with (NH₄)₂SO₄ increased the levels of extractable SO₄‐S in the soil, especially in the 15–30 cm depth. The first‐year accumulation of soil SO₄‐S with increasing S fertilization appeared to be more at a site that was lower in organic matter.     

伴随阴离子对马铃薯种植冲击土中钾素固持与淋溶的影响

A column study was carried out to assess the influence of accompanying anions on potassium (K) leaching at potato growing sites with different soil textures (sandy loam and clay loam) in northwestern India. Potassium was applied in the top 15 cm layer of soil column at 30 and 60 mg K kg^-1 through different sources having different accompanying anions (Cl^-, SO₄^2-, NO₃^- and H₂PO₄^-). Maximum K was retained in the top 0--15 cm layer with a sharp decrease in K content occurring in 15--30 cm layer of the soil column. The trend was similar for both levels of applied K as well as frequency of leaching and soil type. The decrease of K content in soil column after four leaching events was maximum in case of Khanaura sandy loam, while only minor decrease was observed in Hundowal clay loam when K was applied at 60 mg K kg^-1, indicating higher potential of clay rich soil to adsorb K. In general, the K leaching in presence of the accompanying anions followed the order of SO₄^2- ≤ H₂PO₄^2- < NO₃^- = Cl^-. Highest 1 mol L^-1 CH3COONH4-extractable K was retained when K was applied along with SO42- and H2PO4- anions, and the least was retained when accompanying anion was Cl^-1. The influence of anions was more pronounced in the light textured soil and at high amounts of K application. Higher levels of K application resulted in higher losses of K, especially in sandy loam soil as observed from the leachate concentration. Among the different K sources, the maximum amount of K leaching was noticed in the soil column amended with KCl. After four leachings, the maximum amount of K leached out was 6.40 mg L^-1 in Hundowal clay loam and 9.29 mg L^-1 in Khanaura sandy loam at 60 mg K kg^-1 of soil application through KCl. These concentrations were lower than the recommended guideline of the World Health Organisation (12.00 mg L^-1).     

Simulated acid rain (H2SO4) and microbial activity in soil

Soil mixtures containing 9% kaolinite, 9% montmorillonite, or no clay supplements were amended with 1% glucose and treated with H₂SO₄ to lower their bulk pH to levels ranging from 5.4 to 0.8. Acidification had little effect on soil respiration (CO₂ evolution) until the pH was lowered below 3. Glucose was not degraded at approximately pH 2 but was degraded once the soil pH was raised to non-inhibitory levels, i.e. pH 4.1–4.3. When the soil pH was reduced to 1.4 or below, it was necessary to reinoculate the soil and raise the pH to a non-inhibitory level to obtain CO₂ evolution. The addition of clay minerals, particularly montmorillonite, mitigated the toxic effect of H₂SO₄, especially at pH values below 3. The growth of Aspergillus niger, A. flavipes, Trichoderma viride and Penicillium brefeldianum was reduced or completely inhibited in soils acidified below pH 3.5. The addition of montmorillonite enhanced fungal growth under these acidic conditions, but kaolinite had no effect.     

Influence of Sulfur Fertility on Wheat Yield Performance on Alluvial and Upland Soils

Abstract: In recent years, sulfur (S) deficiencies in winter wheat (Triticum aestivum L.) have become more common, particularly on coarse‐textured soils. In Study I, field experiments were conducted in 2001/2002 through 2003/2004 on Mississippi River alluvial soils (Experiment I) and an upland, loessial silt loam (Experiment II) to evaluate the influence of spring S rates of 0, 5.6, 11.2, and 22.4 kg ha^?1 and a fall rate of 22.4 kg sulfate (SO₄)‐S ha^?1 on grain yield of three varieties. In Study II, field experiments were conducted in 2001/2002 and 2004/2005 on alluvial soils to evaluate the influence of spring S rates of 0, 5.6, 11.2, and 22.4 kg SO₄‐S ha^?1 in fields where S‐deficiency symptoms were present. Grain yield response to applied S occurred only on alluvial, coarse‐textured, very fine sandy loam soils (Study II) that had soil SO₄‐S levels less than the critical level of 8 mg kg^?1 and organic‐matter contents less than 1 g kg^?1 in the 0‐ to 15‐, 15‐ to 30‐, and 30‐ to 45‐cm depths. Soil pH increased with soil depth. Optimum S rate was 11.2 kg SO₄‐S ha^?1 in 2001/2002 and 5.6 kg SO₄‐S ha^?1 in 2004/2005. On the upland, loessial silt loam soil, soil SO₄‐S levels accumulated with depth, whereas organic‐matter content and pH decreased. In the loessial soils, average soil SO₄‐S levels in the 15‐ to 30‐ and 30‐ to 45‐cm soil depths were 370% greater than SO₄‐S in the surface horizon (0 to 15 cm).     

Extraction of potassium from some kaolinitic soils of the tropics

Abstract

Three methods for soil potassium extraction (M NH₄OAc pH 7, 0.01 M AgTU and 30 % hot H₂SO₄) were compared for a variety of kaolinitic soils of the tropics. The AgTU‐extractable K was much higher than the M NH₄OAc‐extractable K when vermiculite clay was present in the soil. The correlation between both was given by an R value of 0.937. The amounts of K extracted by 0.01 M AgTU and by hot H₂SO₄ were approximately the same. The R value for these two methods was 0.843.

It is suggested that the AgTU extractant could be used for determination of plant‐available K in soil and for testing for the presence or absence of vermiculite clay in soils.     

Chemical effects of pH 3 sulphuric acid on a soil profile

Cores of podzolic soil (monolith lysimeters) were treated for 4.8 yr with 1500 mm yr^?1 of either 0.5 mM H₂SO₄ at pH 3, equivalent to 24 g S m^?2 yr^?1 (acid treated) or distilled water (controls). The acid treatment was about 37 times greater than the average annual input of H₃O⁺ from rain at the site from which the monoliths were taken. Acid treatment acidified the litter (from pH(CaCl₂)3.4 to pH(CaCl₂)2.6) and the mineral soil to a depth of 80 cm (mean pH(CaCl₂) decrease of 0.2 unit). In the litter and upper A horizon, ion-exchange reactions provided the main neutralizing mechanism, resulting in a decrease in the reserves of extractable (in 2.5 % acetic acid) Ca, Mg, and Mn of about 70 to 80 %. Dissolution of solid phase Al from hydrous oxides provided most neutralization below this depth. Al3⁺ was the principal soluble Al species throughout the profile. In the litter and upper A horizon, some of the mobilized Al³⁺ was retained on cation exchange sites resulting in an increase in exchangeable Al. Deeper in the profile, where the exchange sites were effectively saturated with Al³⁺, no increase in exchangeable Al occurred, and Al³⁺ was, therefore, available for leaching. Some reversible adsorption of SO₄ ^2?, associated with hydrous Al oxides, occurred in the Bs and C horizons. The results are discussed in relation to possible effects of acid deposition over regions of Europe and N. America.     

Aluminium sulphate solubility in acid forest soils in Denmark

Ion leaching in 3 sandy spruce forest soils of different origin and pH was investigated in the laboratory. Zero-tension lysimeters containing undisturbed soil columns of varying soil depth were subjected to H₂SO₄ loadings for a period of 9 weeks. The analysis of the resulting leachate supports the hypothesis that Al-sulphate minerals may form in acidic soils when exposed to acid (H₂SO₄) deposition. In the B horizon of a glaciofluvial sandy soil (pH 4.2), both H⁺ and sulphate ions were retained to maintain 2pH + PSO₄ = 11.9 in the leachate solutions. This relation between H⁺ and sulphate activity may be due to an adsorption mechanism or a precipitation mechanism. The precipitation mechanism is favored by the good fit of leachate composition to the conditions for jurbanite [AlOHSO₄] formation from gibbsite [Al(OH)₃]. In the B horizon of a sandy till at pH 3.7, the Al in soil solution (0.5 mmol L^?1) was leached with sulphate. As the sulphate load was increased, some sulphate was retained. This may also be due to the dissolution and precipitation of an Al-sulphate mineral. The ion activity products of leachate solutions from the B horizon of this soil were close to the pK_s reported for jurbanite. The conditions for the possible existence and/or formation of Al-sulphate minerals in acidic soils are discussed.     

The Contribution of Various Components to pH Buffering Capacity of Acrisols in Southeastern Vietnam

This paper examined the contribution of various soil components to pH buffering capacity (pH_BC) of haplic Acrisols in an upland area of Southeastern Vietnam. Sampling was conducted in 2016 in Tan Bien district, Tay Ninh province at seven sites from the surface to 60-cm depth. Soils were very acidic (pH_H2O 4.53 ± 0.05). The pH_BC were very low, 0.46 ± 0.04 and 0.44 ± 0.05 cmol H⁺ kg^?1 pH^?1, respectively, for original samples and those from which soil organic carbon (SOC, 0.52 ± 0.09%) were removed. The contribution of Al³⁺ to pH_BC was remarkable while that of SOC was of little significance. The contribution of clay minerals to pH_BC was unclear due to the low (9.37 ± 0.76%) and kaolinite-dominated clay content. The current soil conditions indicated a potential for further soil acidification. Liming would be one of the measures to remediate soil acidity in the research area.     

Suitability of Extractants for Predicting Available Sulfur in Natural Grassland in the Inner Mongolia Steppe of China

Abstract

It was the objective of this study to compare the suitability of different extractants for predicting the availability of sulfur (S) in natural grassland in a sulfur response trial on three different soil types in the Inner Mongolia steppe of China. For soil analysis, seven different extractants have been employed. The inorganic SO₄–S concentration was determined by ion chromatography. Additionally, in the Ca(H₂PO₄)₂ extract the total soluble S was determined employing turbidimetry. Weak salt solutions (0.15% CaCl₂, Ca(H₂PO₄)₂, and KH₂PO₄) extracted similar amounts of SO₄–S. Extraction with 0.025 M KCl provided the lowest SO₄–S values. Deionized water dissolved significantly more SO₄–S in the control plots than most weak salt extractants. The concentration of soluble organic S decreased in the control plots after 100 days of plant growth, indicating that the organic S pool contributed significantly to the S nutrition of the forage crops. Significant relationships among the SO₄–S in the soil determined in different extracts and crop yield, sulfur content in the forage, and total sulfur uptake were only found for the Ca(H₂PO₄)₂ extract. In general, the correlation coefficients proved to be unsatisfactory for field experimentation.     

Evaluation of calcium dihydrogen phosphate solution as an extractant for inorganic sulfate applied to soils

Abstract

To evaluate conventional calcium dihydrogen phospahte [Ca(H₂PO₄)₂] solution containing 500 mg P/L as an extractant for soluble plus adsorbed sulfate (SO₄), we added known amounts of SO₄ to 10 soil samples differed in clay mineral composition and extracted with Ca(H₂PO₄)₂ solution. The experimental results showed that the five successive extractions at a soihsolution ratio of 1:10 could quantitatively recover the added SO₄, and there was little effect of air‐drying the soils after addition of SO₄. Based upon these experimental results, we concluded that the Ca(H₂PO₄)₂ extraction is an excellent method for determining soluble plus adsorbed inorganic SO₄ in soils.     

Correlations between different acidity forms in amorphous loamy soils of the tundra and taiga zones

Pair correlation coefficients (r) between the acidity parameters for the main genetic horizons of soddy-podzolic soils (SPSs), typical podzolic soils (TPSs), gley-podzolic soils (GPSs), and tundra surfacegley soils (TSGSs) have been calculated on the basis of a previously developed database. A significant direct linear correlation has been revealed between the pH_water and pH_KCl values in the organic and eluvial horizons of each soil, but the degree of correlation decreased when going from the less acidic SPSs to the more acidic soils of other taxons. This could be related to the fact that, under strongly acid conditions, extra Al³⁺ was dissolved in the KCl solutions from complex compounds in the organic horizons and from Al hydroxide interlayers in the soil chlorites. No significant linear correlation has been found between the exchangeable acidity (H _exch) and the activity of the [H]⁺ ions in the KCl extract (a(H⁺)_KCl) calculated per unit of mass in the organic horizons of the SPSs, but it has been revealed in the organic horizons of the other soils because of the presence of the strongest organic acids in their KCl extracts. The high r values between the H _exch and a(H⁺)_KCl in all the soils of the taiga zones have been related to the common source and composition of the acidic components. The correlation between the exchangeable and total (H _tot) acidities in the organic horizons of the podzolic soils has been characterized by high r values because of the common source of the acidity: H⁺ and probably Al³⁺ ions located on the functional groups of organic acids. High r values between the H _exch and a(H⁺)_KCl have been observed in the mineral horizons of all the soils, because the Al³⁺ hydroxo complexes occurring on the surface and in the interlayer spaces of the clay minerals were sources of both acidity forms.     

成县土壤数值分类的初步研究

本文应用标准化数据—斜交距离—误差平方和聚类分析法对甘肃成县34个土壤剖面进行了数值分析研究。研究结果表明,供试34个剖面可分为五个土壤类型。对五类土壤的pH值、有机质、代换量、SiO₂、Al₂O₃、Fe₂O₃、CaCO₃及粘粒含量八项主要指标进行方差分析的结果表明,除pH、代换量和SiO₂含量未达类型间F测验显著水平外,其他各项指标均达显著或极显著水平。各类土壤在地理分布、植被类型、田间石灰反应等特性上也有明显的差异,并且每一类土壤都具有一定的发生学意义。并分别对应于棕壤、淋溶褐土、褐土、碳酸盐褐土和潮土五种发生学类型。     

Fractionation and distribution of selenium in soils

Abstract

A modified selenium (Se) fractionation procedure was used to study Se distribution in three soils (two silt loams and one silty clay). This sequential procedure consisted of: i) 0.2 M potassium sulfate (K₂SO₄)‐soluble fraction, ii) 0.1 M potassium dihydrogen phosphate (KH₂PO₄)‐exchangeable fraction, iii) 0.5 M ammonium hydroxide (NH₃H₂O)‐soluble fraction, iv) 6 M hydrochloric acid (HCl)‐extractable fraction, and v) residual fraction digested with perchloric (HClO₄) and sulfuric (H₂SO₄) acids. The fractionation procedure had high recovery rates (92.5 to 106%). The Se distribution in soil was controlled by soil properties, such as pH, oxide, clay, and calcium carbonate (CaCO₃) contents. In the untreated soil samples, residual Se fraction was dominant. In the Se‐enriched soils, the silty clay had significantly more Se in the NH₃H₂O and residual fractions while in the two silt loams the largest were KH₂PO₄ and residual fractions. The Se availability in the two silt loams was higher than in the silty clay. The Se availability pattern in the untreated soils was: unavailable (HCl + residual fractions) >> potentially available (KH₂PO₄ + NH₃H₂O fractions) > available (K₂SO₄ fraction), while in the Se‐enriched soils it was potentially available > unavailable > available.     

SULPHATE REDUCTION IN WATERLOGGED SOILS

To study the factors that influence the release of free H₂S, flooded soils were incubated with plant material and either sodium sulphate or elemental sulphur. Residual sulphate, FeS and H₂S were measured throughout the reaction. Less free H₂S was evolved from a periodically waterlogged soil than from a well-drained soil of comparable HC1- and dithionite-soluble Fe content. With a readily soluble synthetic Fe₂O₃ as the source of Fe, a slight excess of Fe over the SO₄^2– equivalent sufficed to prevent loss of H₂S. The native organic matter in the well-drained Rothamsted top-soil seemed to have no effect in promoting loss of H₂S, but the FeS:H₂S ratio was strongly influenced by the nature of the plant material used as the source of energy. With either Na₂SO₄ or S, considerably more free H₂S was evolved with lucerne than with rice blade. With S, the pH of the incubated mixtures was close to that of the nominally S-free system, whereas with Na₂SO₄ the mixture became quite alkaline. More free H₂S was evolved under the more alkaline conditions obtained with Na₂SO₄, which suggests that it is the initial mobilization of Fe that governs the distribution of the reduced S, rather than the pH-solubility relations of FeS. An oxidized surface layer is very effective in retaining sulphur in the soil.     

Effect of Fe,Mn or Al Compounds on Humification of Three Types of Plant Residues during Thermal Incubation

The effect of three inorganic minerals on the humification of three types of plant residues was determined by employing a model thermal incubation experiment. The plant residues consisiting of rice (Oryza sativa) straw, broadleaf tree (a mixture of oak/beech, Quercus serrata, Q. dentata, Q. acutissima etc.) sawdust and Japanese cedar (Cryptomeria japonica) sawdust were each mixed with Fe, Mn and Al in the form of hydroxides, oxides and sulfates. Humic materials were extracted after incubation and their composition was analyzed using a mixed solution of 0.02 M Na₄P₂O₇ and 0.1 M NaOH. The pH values of the samples after a longer duration of the incubation period were all less than 5.0, with the lowest value of 2.16 for a sample incubated with Al₂(SO₄)₃, except for the values of the samples incubated with MnO₂, which ranged from 4.75 to 6.0. The ΔlogK values decreased with the increase of the duration of the incubation period, whereas the RF values increased, as well as the amount of humus extracted (HE) and percentage of humic acid (PQ). Whereas most of the samples were identified as Type B and Type Rp humic acids, Type A humic acid was formed in all the plant residues incubated with Al₂(SO₄)₃, FeO(OH) and MnO₂ after ?180 d of incubation period. Moreover, the degree of humification of the plant residues was observed in the order of broadleaf tree > rice straw > Japanese cedar. It can be concluded that the inorganic compounds Al₂(SO₄)₃, FeO(OH) and MnO₂ contributed to the acceleration of the humification process of plant residues during the thermal incubation. The effect of Al₂(SO₄)₃ may be associated with the increase in the reactivity with other components in the system due to its high solubility, whereas FeO(OH) and MnO₂ may be involved in a reduction-oxidation reaction during the incubation. The browning and/or blackening of the plant residues were similar to the production of melanoidin which led us to consider that the mechanism involved in the study was similar to that of the Maillard reaction.     

Adsorption and translocation of sulfur in some tropical acid soils

In view of growing concern about sulfur (S) deficiency, we attempted to study the effect of soil characteristics on the adsorption and translocation of S in soils. Laboratory experiments were conducted with five surface soils collected from three regions in the state of Orissa (Eastern India). In an adsorption study, all the soils were equilibrated with graded doses of potassium sulfate (K₂SO₄). Freundlich adsorption isotherms provided good fit to S adsorption data. Free Fe₂O₃ and Al₂O₃ in the soils were primarily responsible for retaining added S in soils. Further, studies on the movement of sulfate‐S in 30‐cm plexiglass columns, where radio‐labeled S along with water (5 cm) was applied as gypsum and K₂SO₄, showed that K₂SO₄‐S migrated deeper than gypsum‐S. Sulfur moved deeper in case of initially water‐saturated soils than in initially air‐dry soils.