Factors affecting the oxidation of elemental sulfur in soils

The use of elemental sulfur (S⁰) to alleviate widespread S deficiencies in agricultural soils is limited by the unpredictability of its oxidation to plant available sulfate. Here we review the biological, fertilizer and soil-related factors that control S⁰ oxidation. Sulfur oxidation in soil is mediated primarily by microorganisms, and thus it is the size, composition and activity of the microbial community which dictate oxidation rates. Because S⁰ oxidation is a biological process, it is strongly influenced by factors directly affecting microbial activity including soil temperature, water potential, and aeration. In many soils these factors represent the primary constraints to S⁰ oxidation. Oxidation is also influenced by the effective surface area of the S exposed to microbial activity. Thus oxidation is favored by reducing the particle size and abundant populations of heterotrophic bacteria and fungi capable of oxidizing S⁰, thus the availability of organic substrates from residue additions or root exudates may also affect S oxidation. Previous application of S⁰ may increase oxidation rates in many soils, presumably by stimulating S⁰ oxidizing populations. The large number of factors that govern S⁰ oxidation account for the variability in oxidation rates among soils, climatic regions, and agronomic practices. Many of these factors are subject to agronomic control, however, and it should be possible to devise S fertilizer strategies that exploit the slow release characteristics of S⁰ to meet crop demands efficiently in a variety of conditions.     

Fate and efficiency of sulfur fertilizer applied to food crops in West Africa

Increased food production in West Africa must be linked to increased fertilizer use. However, the increased use of the high analysis sulfur-free materials currently available in the region will lead to increased incidence of sulfur (S) deficiency. In order to determine the S fertilizer requirements of major cereal crops, and compare the fate and efficiency of alternative S sources, experiments were conducted at six sites in semiarid and subhumid West Africa from 1985–1988. Sulfur fertilizers increased grain yields from 10% to 65% (200 to 2000 kg/ha) in 14 out of 20 site-years (at 5 out of 6 sites). Maximum response was generally obtained with only 5–10 kg S/ha. Both powdered (–60 mesh S⁰) and granular (S⁰-fortified TSP) elemental S sources were usually as effective as sulfate sources (gypsum or SSP). The residual effectiveness of S⁰, however, was superior to that of sulfate at most sites. Sulfur-35 balances demonstrated substantial leaching and low crop recovery (5%–10% of S applied) of fertilizer S. Up to 50% of the applied S was lost from sulfate sources, whereas <20% was lost from S⁰ sources. Although S⁰ sources were at least as effective as sulfate sources, the low S rates required suggest that S deficiencies in the region can be corrected at relatively low cost with sulfate-containing fertilizers, provided they can be supplied at more appropriate nutrient ratios.     

Response of irrigated maize to urea-ammonium nitrate and ammonium thiosulphate solutions on a sulphur deficient soil

Ammonium thiosulphate solution, (ATS, (NH₄)₂S₂O₃, 12% NH₄-N and 26% S), is a nitrogen-sulphur fertiliser which can also inhibit nitrification, inhibit area hydrolysis and also solubilize micronutrients in alkaline soils. A three year field study was conducted in northeastern Italy to compare the growth, yield, and nutrient uptake of irrigated maize (Zea mays L.) fertilised with 250 kg N ha^-1 urea-ammonium nitrate solution (UAN, 30-0-0) or UAN plus ATS. Dry matter (DM) yield, sulphur (S) and nitrogen (N) uptake were measured at several growth stages. Grain was measured and analyzed at maturity. Maize grain yield and N uptake were increased respectively 30.6% and 42.2% in the first year by adding ATS to UAN. Adding 10% by weight ATS to UAN (22.8 kg S ha^-1) increased grain yields by 1.9, 1.7 and 1.6t ha^-1 for the three years of the study. To distinguish whether the response was due to S or other ATS attributes, ATS was compared to an equivalent amount of S from single superphosphate (SSP). Plots fertilised with ATS gave grain yields 0.5 and 1.2 t ha^-1 greater than plots fertilised with equal rates of S from SSP in the last two years of the study. This added yield from ATS over SSP may have been due to beneficial effects of ATS on N or micronutrient availability or to the split application of the S from ATS.     

Nitrogen-15 and sulfur-35 balances for fertilizers applied to transplanted rainfed lowland rice

A field experiment was conducted on a poorly-drained Aeric Paleaquult in northeastern Thailand to determine the effect of N and S fertilizers on yield of rainfed lowland rice (Oryza sativa L.) and to determine the fate of applied¹⁵N- and³⁵S-labeled fertilizers. Rice yield and N uptake increased with applied N but not with applied S in either sulfate or elemental S (ES) form. Rice yield was statistically greater for deep placement of urea as urea supergranules (USG) than for all other N fertilizer treatments that included prilled urea (PU), urea amended with a urease inhibitor (phenyl phosphorodiamidate), and ammonium phosphate sulfate (16% N, 8.6% P).The applied¹⁵N-labeled urea (37 kg N ha^–1) not recovered in the soil/plant system at crop maturity was 85% for basal incorporation, 53% for broadcast at 12 days after transplanting (DT), 27% for broadcast at 5–7 days before panicle initiation (DBPI), and 49% for broadcast at panicle initiation (PI). The basal incorporated S (30 kg ha^–1) not recovered in the soil/plant system at crop maturity was 37% for sulfate applied as single superphosphate (SSP) and 34% for ES applied as granulated triple superphosphate fortified with S (S/GTSP). Some basal incorporated¹⁵N and³⁵S and some broadcast¹⁵N at PI was lost by runoff. Heavy rainfall at 3–4 days after basal N incorporation and at 1 day after PI resulted in water flow from rice fields at higher elevation and total inundation of the 0.15-m-high¹⁵N and³⁵S microplot borders. Unrecovered¹⁵N was only 14% for 75 kg urea-N ha^–1 deep placed as USG at transplanting. This low N loss from USG indicated that leaching was not a major N loss mechanism and that deep placement was relatively effective in preventing runoff loss.In order to assess the susceptibility of fertilizer-S to runoff loss, a subsequent field experiment was conducted to monitor³⁵S activity in floodwater for 42 days after basal incorporation of SSP and S/GTSP. Maximum³⁵S recoveries in the floodwater were 19% for SSP after 7 days and 7% for S/GTSP after 1 day. Recovery of³⁵S in floodwater after 14 days was 12% for SSP and 3% for S/GTSP.This research suggests that on poorly drained soils with a low sorption capacity, a sizeable fraction of the fertilizer S and N remains in the floodwater following application. Runoff could then be an important mechanism of nutrient loss in areas with high probability for inundation following intense rainfall.     

Single superphosphate depresses molybdenum uptake and limits yield response to phosphorus in groundnut (Arachis hypogaea L.) grown on an acid sandy soil in Niger,West Africa

The effect of phosphorus (P) fertilization on dry matter production and nitrogen (N) uptake of groundnut (Arachis hypogaea L.) was studied during the growing seasons of 1989, 1990 and 1991 under rainfed conditions on an acid sandy soil in Niger, West Africa. Annual application of 16 kg P ha^–1 as single superphosphate (SSP) failed to increase the total dry matter production significantly in all three years.Fertilization with SSP increased the concentrations of P and sulfur (S) in shoots from deficiency to sufficiency levels. It decreased the already very low concentrations of molybdenum (Mo), especially in the nodules, and also the N concentration in the shoot dry matter.With SSP application, total N uptake declined over three years. Foliar application of P and soil application of triple superphosphate (TSP) enhanced dry matter production, N and Mo uptake.Although these acid sandy soils are known to be deficient in P and S, care must be taken in using SSP in groundnuts as it may induce Mo deficiency, unless supplementary Mo is applied.ICRISAT Journal Article No. 1230     

Modifying sandy soils with the fine residue from bauxite refining to retain phosphorus and increase plant yield

Production of alumina from bauxite in Western Australia results in large quantities of processing residue. The fine portion of the residue (red mud) has a high phosphorus (P) absorption capacity compared with the native sandy soils of the coastal plain. When neutralised with gypsum or acidic materials, the residue can be incorporated into, or spread on, the surface of sandy soils for horticulture using simple agricultural equipment. Neutralisation with gypsum is unnecessary for application to pasture at less than 100 t ha^–1. Field and laboratory experiments show that 10-80 t ha^–1 of bauxite residue, spread evenly over the surface of the soil, significantly reduced P leaching from coastal plain sands fertilized with superphosphate. Rates of 500 t ha^–1, or more, significantly increased the yield of pastures on well drained sandy soils, primarily due to the increased water holding capacity of the amended soils, while rates of 10-80 t ha^–1 significantly increased the yield of pastures primarily due to increased pH.Analysis of leachate from bauxite residue indicates that it is unlikely to cause adverse environmental impacts as a result of agricultural-scale amendment of sandy soils. Amendment with bauxite residue offers potential as a practical component of an integrated strategy to reduce P losses from sandy soils. Economic and logistic considerations indicate soil amendment may be most applicable to intensive land uses such as horticulture and for land treatment of wastewaters from animal industries and urban areas. However, economical methods are being developed to spread low rates of bauxite residue on land used for more extensive agriculture.     

Enhancement of mass transfer from particles by local shear-rate and correlations with application to drug dissolution

We analyze hydrodynamic enhancement of mass (or heat) release rate from small spherical particles within fluid flows from local flow shear-rate, with application to drug dissolution. Combining asymptotic theories in the high/low shear Peclet number limits in Stokes flow with 205 carefully-developed computational experiments, we develop accurate correlations for shear enhancement of Sherwood/Nusselt number (Sh/Nu) as a function of shear Peclet and Reynolds number (S^*, Re _S). The data spanned S^* from 0 to 500 and Re _S from 0 to 10. In Stokes flow our correlations are highly accurate over the entire S^* range, whereas for finite Re _S < 1 accuracy is good for S^* up to a few thousand. Shear enhancement results from highly three-dimensional spiraling flow created by particle spin. We develop a model for particle slip velocity that is inserted into the Ranz/Marshall correlation to show that shear-rate enhancement strongly dominates convection, a result important to drug dissolution.     

Evaluation of partially acidulated phosphate fertilisers and reactive phosphate rock for hill pastures

The initial and residual effectiveness of two partially acidulated fertilisers, a single superphosphate (SSP): reactive phosphate rock (RPR) physical mix (SSP:RPR) and a partially acidulated phosphate rock (PAPR), and a RPR, North Carolina, were compared with SSP at two phosphate (P)-responsive sites in hill country. One site had received small annual inputs of SSP (125 kg ha^–1 y^–1) fertiliser for 10 years (LF) and the other site no SSP in the past 5 years (NF). The SSP, PAPR and RPR were applied at 3 rates (20, 40 and 60 kg P ha^–1) and SSP:RPR at one rate (40 kg P ha^–1) once only in the first year. Fertiliser treatments were applied with or without Grasslands Huia white clover (Trifolium repens L.) seed. Initial and residual pasture and legume responses were measured over two years.In the first year, pasture and legume response to applied P was much greater at the LF than NF site. A deterioration in legume content and vigour brought about by withholding fertiliser, rather than a difference in soil-fertiliser reactions, appears to be the main reason for the different response at the two sites in the first year. At the LF site the fully (SSP) and partially (SSP:RPR and PAPR) acidulated fertilisers were far more effective in stimulating legume growth than the RPR, while at the NF site no differences in pasture or legume production were found between fertilisers in the first year. Where fertiliser has been withheld for a number of years the use of SSP appears to be a wasteful and inefficient use of a processed fertiliser.Residual effects of RPR were greater than those of SSP, as shown by the greater yield of legume at both sites in the second year. The residual effectiveness of both the partially acidulated materials was much less than that of the RPR. Mixing and sowing white clover with the fertilisers had some beneficial effects on legume content at the NF site in both years and improved legume production at this site in the second year.     

The agronomic value of co-granulated Christmas Island Grade C phosphate rock and elemental sulphur

Certain low grade phosphate rocks have low agronomic value as direct applied fertilizers and make poor quality superphosphates. With some rocks there is potential to increase their solubility in soils and hence the fertilizer value by mixing with finely divided elemental sulphur (S°). The agronomic value of a prototype low cost, granular P and S fertilizer, Christmas Island Grade C phosphate rock/elemental sulphur (Xmas C/S°), was compared against single superphosphate (SSP) and reactive phosphate rock (RPR)/S° dry blends, North Carolina phosphate rock/S° (NCPR/S°) and NCPR/50% S-super (granular SSP + 44% S°). An eight- month glasshouse trial using ryegrass (Lolium perenne L. cv. Nui), grown in a central yellow-grey earth (Aeric Fragiaqualf), indicated that fertilization with Xmas C/S° produced dry matter yields, between 54 to 73% and 10 to 40% lower than SSP and NCPR/S° blends respectively, and lower plant P and S uptake. Fertilization with Xmas C/S° however produced significantly higher yield and P and S uptake than unfertilized pots and pots receiving Xmas C PR and S° alone. The two RPR/S° dry blends, namely NCPR/S° and NCPR/50% S-super, produced significantly lower yield and P and S uptake than the same two fertilizers granulated with water. The difference in yields and P and S uptake between blends and granulated forms increased with time. The recovery of fertilizer P and S by plants ranged from 3 to 35% and 2 to 45% respectively with the lowest recoveries for Xmas C PR alone and S° alone and the highest for SSP and SSP + S°. During the period of plant growth the percentage of S° oxidised from the S°, Xmas C/S° co-granule, NCPR/S° physical blend and NCPR/S° granular treatments were 37, 32, 32 and 45% respectively. Field evaluations, or use, of the co-granule should consider it's slow P and S release rates.     

Potential losses of fertilizer phosphorus by animal transfer

Pasture production, phosphorus (P) concentration, and P uptake by mixed pasture following addition in the autumn of 50 and 100 kgP ha^–1 as single superphosphate (SSP), triple superphosphate (TSP) and Sechura phosphate rock (SPR), and of 50 kgP ha^–1 of Chatham Rise phosphorite (CRP) were measured for one year on a Wainui silt loam (Typic Dystrochrept) and Tokomaru silt loam (Typic Fragiaqualf). A sharp increase was measured in the P concentration of mixed pasture immediately following the application of 50 and 100 kg P ha^–1 as either SSP or TSP at both sites. However, this increase was not accompanied by an increase in pasture production. In contrast, the application of 50 kgP ha^–1 as either SPR or CRP resulted in only small initial increases in the P concentration of mixed pasture, as did the addition of 100 kgP ha^–1 as SPR at both sites. The potential P losses by animal transfer in dung, which could result from the use of these four P fertilisers, were calculated using a P cycle constructed for intensively grazed, steep hill country pasture. Potential losses of fertilizer P, calculated as a percentage of fertilizer P added, were 7–14% for SSP and TSP, and 4–5% for SPR and CRP in the first year at the two sites. The implications of these results to the efficiency of P fertilizer use are discussed.     

Influence of co-granulated nutrients and granule size on plant responses to elemental sulfur in compound fertilizers

Experiments were undertaken to determine the effect of granule size and nutrients in granulated compound fertilizers fortified with finely divided elemental sulfur (S^o) on the rate of S^o oxidation. In one experiment, S^o was banded together with or apart from triple superphosphate (TSP) while in two others, S^o was granulated with nutrient and inert carriers. A fourth experiment examined response to S in an S^o-fortified TSP from a range of granule sizes. Response and, in some cases, S^o recovery (using ³⁵S labels) by test crops (maize, wheat, upland rice) was measured. In all experiments, P mixed with S^o increased plant growth and S recovery above treatments in which P and S^o were physically separated. There was however, no effect of distance of separation on S recovery. In one experiment, N as urea and N and P as diammonium phosphate (DAP) were also found to enhance response to S^o although to a lesser degree than P alone. These observations were attributed to a nutritional requirement of S^o-oxidizing microorganisms for P and N. Granulation of S^o with carriers also influenced oxidation rate, as inferred from the fertilizer S recovery. For a given S^o concentration, the effect was inversely proportional to the mean diameter of granules. It is shown that this relationship can be explained if one assumes that S^o particles in granules collapse into a fixed number of aggregates per granule irrespective of granule size when the soluble nutrient carrier dissolves and diffuses away from the point of application.     

Comparison of efficiency of Mussoorie partially acidulated phosphate rock and single superphosphate in a shallow Alfisol of the Indian semiarid tropics

The agronomic effectiveness of a partially acidulated phosphate rock (PAPR) was measured in a field experiment with sorghum (Sorghum bicolor cv. CSH-6) in a shallow Alfisol at the ICRISAT farm, Patancheru (Hyderabad), India. The experiment compared PAPR with single superphosphate. The PAPR was made by acidulating an indigenous Indian phosphate rock (Mussoorie) with H₂SO₄ at 50% acidulation level. P response was evaluated at a single relatively high N rate (120 kg ha^–1) with five rates of P (0, 2.2, 4.4, 8.8, and 17.6 kg P ha^–1). A significant response to P was obtained at rates up to 17.6kg P ha^–1.There was no significant difference due to source of P in terms of sorghum grain yield or total P uptake. Both Olsen and Bray 1 soil tests underestimated P availability from PAPR with respect to that from SSP.A rapid rate of P uptake was observed during grain filling to maturity (81–102 days), when 40% of the total P was taken by the plant. The internal efficiency of both P sources was the same.     

Gypsum usage in irrigated agriculture: A review

Gypsum is the source of calcium most commonly used to reclaim sodic soils and to improve soil water infiltration when it has been decreased by low electrolyte concentration. The electrolyte concentration of the soil water and exchangeable sodium fraction,E _Na, are the two predominant chemical factors that influence soil hydraulic properties. In sodic soil reclamation with gypsum, the maximum electrolyte concentration increases with increasingE _Na: for soil solutions in equilibrium with both gypsum and anE _Na of 0 and 0.4, the concentrations are 15 and 133 mol m^–3. These concentrations are generally adequate to maintain the existing hydraulic conductivity. Higher concentrations can increase the soil hydraulic conductivity and the rate of reclamation. Thus, combining calcium chloride or sulphuric acid with gypsum, can reduce both the time and amount of water required to achieve reclamation.Infiltration is especially sensitive to low electrolyte concentration. Thus, surface application of gypsum or its direct addition to irrigation water, can potentially increase water infiltration rates of soils susceptible to aggregate dispersion and crusting. This is particularly true in areas where rainfall is interspersed with the application of irrigation water — as in the Midwest and Plains States of the US — or where mediterranean or monsoon climates prevail and wet and dry (irrigation) seasons alternate. As supplemental irrigation and the use of poor quality irrigation waters (sewage effluents, brackish well waters, irrigation drainage waters, etc.) intensify the need for gypsum will increase and phosphogypsum may help meet this need.Recent data indicate phosphogypsum dissolves faster than mined gypsum, and hence it develops a higher electrolyte concentration during an infiltration event. This attribute of phosphogypsum, in conjunction with its acid content, would increase its effectiveness as compared to mined gypsum for both sodic soil reclamation and maintenance of infiltration rates. The acid content of phosphogypsum is of direct benefit for reducing aggregate dispersion through promotion of soil particle flocculation and bonding by calcium, iron and aluminium released by dissolution of calcite and other soil minerals. It would decrease soil pH, thereby increasing the availability of trace metal nutrients and of phosphate which are typically deficient in sodic soils (E _Na > 0.15) because of high pH. Also, the phosphate content of phosphogypsum has value as a phosphate fertilizer.Significant expansion of the agricultural use of gypsum would depend on its application in both irrigated and dryland agriculture to increase water infiltration. An annual production rate of phosphogypsum of 30 × 10⁶ Mg (1 Mg = 1 tonne) is sufficient to treat 73 000 km² (29 000 mi²) at a rate of 4 Mg per ha, or nearly half the total area irrigated in the USA. Although extensive farmland areas with limited rainfall exist along the Gulf of Mexico within the North American continent, market development within this area would require extensive field evaluation by local agricultural research personnel in cooperation with the phosphate fertilizer industry to determine if the economic benefits exceed the cost of phosphogypsum.     

Comparison of Olsen and Pi soil tests for evaluating phosphorus bioavailability in a calcareous soil treated with single superphosphate and partially acidulated phosphate rock

The Pi test for phosphorus (P) is a new method in which strips of iron oxide impregnated filter paper are used as a sink to sorb and extract P from a soil solution. In a greenhouse experiment, the Olsen and Pi tests were compared for their effectiveness in evaluating P availability to maize on calcareous soils. Phosphate rock from Togo, partially acidulated with H₂SO₄ at 50% acidulation level (PAPR 50% H₂SO₄) and single superphosphate (SSP) were applied at different rates to a calcareous soil (Vernon Clay, pH 8.2, CaCO₃ 18.9%) which was preincubated with KH₂PO₄ to raise plant-available P to different levels. In soils treated with SSP, dry-matter yield of maize correlated equally well with Pi-P and with Olsen-P (r = 0.96^***). P uptake correlated significantly with P_i-P (r = 0.94^***) as well as Olsen-P (r = 0.97^***). Likewise, in soils fertilized with PAPR, significant correlations were found between dry-matter yield and Pi-P (r = 0.97^***) and between dry-matter yield and Olsen-P (r = 0.94^***). When all the data were pooled, Pi-P and Olsen-P correlated equally well with both dry-matter weight (r = 0.97^***) and P uptake (r = 0.94^***). Phosphorus extracted by the Pi test correlated significantly with P extracted by the Olsen test (r = 0.99^***).     

Environmental chamber measurements of mercury flux from coal utilization by-products

An environmental chamber was constructed to measure the mercury flux from coal utilization by-product (CUB) samples. Samples of fly ash, FGD gypsum, and wallboard made from FGD gypsum were tested under both dark and illuminated conditions with or without the addition of water to the sample. Mercury releases varied widely, with 7-day experiment averages ranging from −6.8 to 73 ng/m² h for the fly ash samples and −5.2 to 335 ng/m² h for the FGD/wallboard samples. Initial mercury content, fly ash type, and light exposure had no observable consistent effects on the mercury flux. For the fly ash samples, the effect of a mercury control technology was to decrease the emission. For three of the four pairs of FGD gypsum and wallboard samples, the wallboard sample released less (or absorbed more) mercury than the gypsum.     

Use of low grade phosphate rocks as biosuper fertilizer

A pot trial was conducted for 10 months to evaluate the fertilizer value of two low grade phosphate rocks applied either as biosuper (phosphate rock/sulphur granules, PR/S^*) or as untreated granulated rocks. The phosphate rocks were Chatham Rise nodules (CR), a marine deposit containing calcium phosphate and calcium carbonate, and the C grade ore from Christmas Island (CC) containing predominantly aluminiumiron phosphate and free oxides of iron and aluminium. Perennial ryegrass was grown as the test crop in a highly phosphate retentive allophanic soil, limed to pH 6.2. Single superphosphate was used as the standard fertilizer.Phosphate uptake and dry matter yields showed that biosuper prepared from CR was agronomically as effective as superphosphate whereas that prepared from CC was less effective. At the highest rate of application CR increased the yield of ryegrass by 80% over that of control but granulating it with sulphur increased the yield by 143%. The corresponding values for CC and CC/S were 39% and 50%. The time lags from the addition of CR/S and CC/S to apparent maximum phosphate availability were less than 18 and 49 days respectively.Olen bicarbonate extractable phosphate of soils increased by 100% and 33% respectively when CR and CC were added as biosuper in comparison with addition as phosphate rock granules.Liming the soil to raise its pH from 5.1 to 6.2 lowered the quantity of superphosphate needed to reach 90% of maximum yield by 23%.     

Study on physiochemical structure and in vitro release behaviors of doxycycline‐loaded PCL microspheres

This study aimed to develop drug delivery system of doxycycline‐loaded polycaprolactone (PCL) microspheres. The investigated microsphere formulation can be considered for local application in bone infections and degenerative joint diseases, which generally require long‐term treatments via systemic drugs. PCL‐14 kDa and 65 kDa were used in microsphere preparation. Before release, the microspheres were characterized by scanning electron microscopy, differential scanning calorimetry, and X‐ray photoelectron spectroscopy. The mean particle size of microspheres was in the range of 74–122 µm and their drug loadings ranged between 10 and 30%. In vitro release profiles were described using the Higuchi and the Korsmeyer–Peppas equations. Diffusion model was applied to experimental data for estimating diffusion coefficients of microspheres; calculated as between 4.5 × 10^?10 and 9.5 × 10^?10 cm²/s. Although long‐term release from microspheres of PCL‐14 kDa obeyed diffusion model, PCL‐65 kDa microspheres showed this tendency only for some period. Modeling studies showed that the drug release mechanism was mainly dependent on loading and molecular weight differences. Release behavior of PCL‐65 kDa microspheres, however, might be better represented by derivation of a different equation to model for the total release period. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41768     

Temperature effects on soil organic sulphur mineralization and elemental sulphur oxidation in subtropical soils of varying pH

The increasing sulphur (S) deficiency in soils of several parts of world has led to the use of fertilizer S, an important factor in enhancing the production and quality of crops. Very limited information is available on the use of elemental sulphur (S⁰) as a fertilizer, its oxidation into SO4^2- and transformation into organic S in semiarid subtropical soils. We studied the impact of three temperature regimes on the mineralization of soil organic S, and the oxidation and immobilization of S⁰ in acidic (pH 4.9), neutral (pH 7.1) and alkaline (pH 10.2) subtropical soils of north-western India. Repacked soil cores were incubated under aerobic conditions (60% water-filled pore space) for 0, 14, 28 and 42 d with and without incorporated S⁰ (500 g g-1 soil). Temperature had profound effects on all three soils processes, the rates of mineralization of native soil organic S, oxidation of applied S⁰ and transformation of S⁰ into soil organic S being greatest at 36 °C, irrespective of soil pH. Mineralization of native soil organic S (without added S⁰) resulted in the accumulation of 39, 66 and 47 g SO4^2-–S g-1 soil in acidic, neutral and alkaline soil in 42 d period at 36 °C. Of the total mineralization, the majority (62 – 74%) occurred during the first 14 d period. Oxidation rate of added S⁰ during initial 14 d period at 36 °C was highest in alkaline soil (292 g S cm-2 d-1), followed by neutral soil ((180 g S cm-2 d-1) and lowest in acidic soil (125 g S cm-2 d-1). Of the applied 500 g S⁰ g-1 soil, 3.2 – 10.0%, 6.8 – 15.4% and 10.0 – 23.0% oxidized to SO4^2-, and 13.4 – 28.6%, 16.0 – 29.0% and 14.6 – 29.0% were transformed into organic S in 42 d period in acidic, neutral and alkaline soil, respectively. The results of our study suggest that in order to synchronize the availability of S with plant need, elemental S may be applied well before the seeding of crops, especially in acidic soil and in regions where temperature remains low. Substantial mineralization of native soil organic S in the absence of applied S⁰ and immobilization of applied S⁰ into organic S suggest that the role of soil biomass as source and sink could be exploited in long term S management.     

Application strategies for Sechura phosphate rock use on permanent pasture

At two phosphate (P) responsive sites in hill country the effectiveness of Sechura phosphate rock (SPR) as a direct application P fertilizer for permanent pasture was evaluated. Sechura was applied at two rates, in three different application strategies. The treatments were 16.7 and 50 kgP ha^–1 annually, 25 and 75 kgP ha^–1 biennially, and 50 and 150 kgP ha^–1 triennially giving a total of 50 and 150 kgP ha^–1, respectively, over three years. Single superphosphate (SSP) served as the standard P fertilizer. A comparison was also made between SPR and Chatham Rise phosphorite (CRP), another reactive PR. Total pasture and legume production and P uptake by pasture was measured with all fertilizer treatments over a three year period.In the year of application, SPR was as effective as SSP in stimulating total pasture and legume production and P uptake by pasture. This reflects the very reactive nature of this PR. In the second and third years of measurement, SPR did not show superior residual efffects to SSP. The ability of CRP to stimulate legume growth more than SPR in the second year following application demonstrates the danger of generalizing about the residual effects of reactive PR materials. Of the application strategies evaluated, a biennial appplication of 25 kgP ha^–1 as SPR maintained legume growth at a higher level than a smaller (16.7 kgP ha^–1) annual dressing. The biennial strategy also increased total pasture yield, in addition to legume production to a greater extent in the second and third years than a single (50 kgP ha^–1) triennial application.