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.     

A turbidimetric method for determining phosphate‐extractable sulfates in tropical soils

Abstract

Turbidimetric methods, using Ba ions to precipitate SO₄, are frequently used to determine soil sulfates extracted with phosphate solutions. These methods, as routinely performed, seriously underestimate SO₄ in some soils of the tropics because phosphate is removed from the extractant by soil adsorption and because many extracts fail to yield satisfactory precipitate even if the extracting procedure is adequate. Decolorizing the extracts with carbon black, treating extracts with strong oxidizing agents, adding SO₄ spikes, and seeding the extracts with BaCl₂ seed‐crystals improve precision, but some extracts, especially those from soils derived from volcanic ash, do not yield reliable precipitates even though these procedures are employed. This paper presents a method that consistanlty yielded more SO₄ than other turbidimetric procedures with which it was compared. The proposed method was further validated against an ion‐chromatographic method for SO₄ determination. The two methods yielded virtually identical results.

The proposed method consists of extracting SO₄ with 0.04 M Ca(H₂PO₄)₂ pH 4, at a soil‐to‐solution ratio of 1: 10. Repeated extraction is necessary for phosphate‐retentive soils. (A. single extraction was approximately 40% effective for removing indigenous SO₄ from a Hydric Dystrandept subsoil, approximately 78% effective for an Eutrustox.) Organic materials are removed from the extracts by adsorption on charcoal; SO₄ is concentrated in the extract by volume reduction; a SO₄ spike is added; BaCl₂‐seed crystal is added, after which volume is increased by adding BaCl₂ solution. Optical density is read at 600 nm.     

Arsenate Displacement from Fly Ash in Amended Soils

Arsenic (As) is the biggest environment contaminant in most of the soils where fly ash is applied. Usually, it is not mobile and strongly adsorbed on to soil particles. However, in gypsum and phosphorus amended soils As may be much more mobile. A study in repacked columns was conducted to determine whether or not As becomes mobile when Ca(H₂PO₄)₂and CaSO₄are used as leaching solutions, and to compare the competitive interactions between PO₄-AsO₄and SO₄-AsO₄. Arsenic concentration in leachate was found to be approximately ten times greater when Ca(H₂PO₄)₂was used to leach the columns as compared to CaSO₄. A maximum concentration of 800 μg As L^-1was found in the leachate in this case, which is much higher than the groundwater limit of 50 μg L^-1for drinking water established by the United States Environmental Protection Agency. In fly ash, the portion of arsenate non-specifically adsorbed is believed to be much lower than that of specifically adsorbed. Sulfate anions were able to displace only non-specifically adsorbed arsenate. In this case the concentration of As in leachate was found to be within acceptable limits. On the other hand, phosphate can compete with arsenate for all available adsorption sites, non-specific and specific. Phosphate displacement of both forms of arsenates increases As mobility in both control and fly ash treatments.     

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.     

Sulfur status of volcanic ash‐derived soils in Hawaii

Abstract

Several rainwater samples and 14 profiles of Hawaii's volcanic ash‐derived soils were analyzed for sulfur (S). Atmospheric deposition was an important S source at the coast (24 kg S/ha), but its contribution decreased with increasing distance from the sea (1 kg S/ha at 24‐km inland). The S concentration of rainwaters also decreased linearly with increasing rainfall.

Several thousand mg SO₄‐S/kg can be extracted from many volcanic ash‐derived soils of Hawaii, and it was often required at least four extractions [0.04 M Ca(H₂PO₄)₂, 1:10 soil to solution ratio] to completely desorb this SO₄. There was a close association of high SO₄ retention with high rainfall. This might have resulted from (1) the development of a solid phase with high SO₄ retention under intense weathering conditions, (2) more total SO₄ received by the soils from atmospheric deposition, and (3) past fertilization of sugarcane grown in high rainfall areas.

Low concentrations of soil solution SO₄‐S in relation to large amounts of P‐extractable SO₄ suggest that a S bearing mineral, such as basaluminite, may be controlling soil‐solution SO₄. Furthermore, SO₄ adsorption isotherms of these volcanic soils generally show a bi‐phasic property, and suggest that 40 to 80 mg SO₄‐S/kg is required to maintain 3 ‐ 6 mg SO₄‐S/L in the soil solution, a concentration range considered adequate for the growth of most crops.     

Characterization of sulfur retained by soils exposed to sulfur dioxide

Abstract

Soils have substantial capacity for sorption of sulfur dioxide (SO₂) but little is known about the nature of the sorbed S. Three surface soils varying in pH, organic matter, CaCO₃ equivalent and surface area were exposed to air containing 5% SO₂ and subsequently analyzed by ten different procedures to characterize the sorbed S. Most of the sulfur retained by soils after exposure to SO₂ could be recovered as CaCl₂‐extractable S, Ca(H₂PO₄)₂‐extractable S, or S released as H₂S by hydriodic acid (HI). Only small amounts of sulfur could be recovered as tetrachloromercurate (TCM)‐extractable S, S released as SO₂ by HCl, or S released as H₂S by HCl + Zn, HCl + Sn, or Raney Ni and NaOH. However, large amounts of S released as SO₂ by HCl were recovered from the air‐dry Webster and the moist Storden soils indicating that SO₂ sorption is influenced by organic matter in air‐dry soils and by CaCO₃ in moist soils.     

Determination of sulfate in soils by reduction with tin and phosphoric acid

Abstract

A rapid and precise method for determination of SO₄ ^2‐‐S in soils is described. It involves the extraction of SO₄ ^2‐ from soils and its reduction to H₂S by a reagent containing Sn and H₃PO₄ and subsequent determination as methylene blue. The results agreed closely with those obtained by reduction with the a reagent containing HI, H₃PO₂, and HCOOH and by ion chromatrography. Tests indicated that, in addition to SO₄ ^2‐, the Sn‐H₃PO₄ reagent reduces certain organic S and reduced inorganic S compounds, but these S compounds are not present in extracts of agricultural soils. By using a bank of 10 distillation units, a single operator can perform 60 analyses in a normal working day.     

在实验酸性硫酸盐条件和酸性硫酸盐土壤上可溶性铝的控制

The controls of soluble Al concentration were examined in three situations of acid sulfate conditions:1) experimental acid sulfate conditions by addition of varying amounts of Al(OH)3(gibbsite) into a sequence of H2SO4 solutions;2)experimental acid sulfate conditions by addition of the same sequence of H2SO4 solutions into two non-cid sulfacte soil samples with known amounts of acid oxalate extractable Al; and 3) actual acid sulfate soil conditions.The experiment using gibbsite as an Al-bearing mineral showed that increase in the concentration of H2SO4 solution increased the soluble Al concentration,accompanied by a decrease i the solution pH, Increasing amount of gibbsite added to the H2SO4 solutions also increased soluble Al concentration,but resulted in an increase in solution pH.Within the H2SO4 concentration range of 0.0005-0.5mol L^-1 and the Al(OH)3 range of 0.01-0.5g(in 25 mL of H2SO4 solutions),the input of H2SO4 had the major control on soluble Al Concentration and pH .The availability of Al(OH)3，however,was responsible for the spread fo the various sample points,with a tendency that the samples containing more gibbsite had a higher soluble Al concentration than those containing less gibbsite at equivalent pH levels.The experimental results from treatment of soil samples with H2SO4 solutions and the analytical results of acid sulfate soils also showed the similar trend.     

Effects of temperature on phosphate sorption isotherms and phosphate desorption

Abstract

Phosphorus sorption isotherms were constructed for two Idaho soils with widely different chemical properties. The soils were equilibrated with various amounts of Ca(H₂PO₄)₂ in 0.01 M CaCl₂ for 1, 3, 7, and 11 days at temperatures of 5°C and 20°C. The two soils which had been equilibrated previously for 11 days at 20°C with various amounts of Ca(H₂PO₄)₂ in 0.01 M CaCl₂ were desorbed at 5°C and 20°C.

The rates of sorption and desorption were decreased as the equilibration temperatures were lowered. The effect of temperature on these processes was detected during the first day of equilibration. Less P was found in the equilibrating solution at the lower temperature. The two soils varied widely in sorption and desorption properties.     

Factors affecting the adsorption of micro‐amounts of tagged phosphorus by soils

Abstract

Adsorption of a small amount of P by soil from a solution, which delivers concentrations approximating the composition of soil solution systems, was measured for a heterogeneous group of 343 soils using ³²P‐labelled KH₂PO₄ solution. The method allowed accurate determination of small quantities of P and identified the P under consideration as that added from solution. Simple correlations and stepvise linear regression analyses indicated that soil pH, Ca, P, Al, Fe, organic matter and particle size significantly influenced the amount of P adsorbed by the soils.     

Remediation of a Magnesium‐Contaminated Soil by Chemical Amendments and Leaching

The deposition of magnesium (Mg)‐rich dust from magnesite mining activities has resulted in serious land degradation. However, the main factors limiting plant growth in Mg‐contaminated soils are unclear. Moreover, little information is available on the remediation of Mg‐contaminated soils. In this study, remediation of soils contaminated with Mg‐rich dust was investigated in a pot experiment using maize as the indicator plant. There were five treatments: (i) control; (ii) leaching; (iii) application of CaCl₂; (iv) leaching + CaCl₂ application; and (v) application of Ca(H₂PO₄)₂ · H₂O. Soil properties and growth of maize (Zea mays L.) seedlings were measured. Leaching alone significantly decreased soluble Mg concentration. Leaching + CaCl₂ application greatly increased exchangeable Ca concentration and decreased soil pH by 0·3 units. Application of CaCl₂ alone increased soluble Mg concentration sharply, which directly inhibited the germination of maize seeds. Application of Ca(H₂PO₄)₂ · H₂O significantly increased the concentrations of exchangeable Ca and available phosphorus and decreased soil pH by 1·7 units. The biomass of maize seedlings increased in the order of control = leaching < leaching + CaCl₂ < < Ca(H₂PO₄)₂ · H₂O. These results suggested that the plant growth in Mg‐contaminated soils was limited primarily by Ca deficiency and secondarily by high soil pH when exchangeable Ca was sufficient. High soil pH suppressed plant growth probably mainly by inhibiting phosphate uptake from the soil. Applying acid Ca salt with low solubility is an attractive option for the remediation of Mg‐contaminated soils. Copyright © 2015 John Wiley & Sons, Ltd.     

从不同浸提剂所浸出的磷酸盐形态探讨几种速效磷测定方法的意义

在前一报告中^[1],作者通过室内化学分析及生物试验证明,在所试的六种测定土壤有效磷的方法中,以Olsen的0.5M NaHCO₃法与生物反应的相关性最好,马乞金的1%(NH₄)₂CO₃法和Radet的3.5%柠檬酸钠法次之,Egner-Rim的乳酸钙法、Burriel-Hern-ando的混合浸提液法和Joret-Herbert的草酸铵法又次之。另外,从浸提出的磷量看,碳酸氢钠法、马乞金法和柠檬酸纳法相当接近,可以列为同一数量级。乳酸钙法、混合浸提液法和草酸铵法浸出的磷量也很接近,可以列为另一较高的数量级。     

Ionic equilibria,selectivity and diffusion of cations in soil as influenced by anion additions

Abstract

An experiment was carried under controlled conditions to investigate the influence of the anions, H₂PO₄ ^‐. and Cl on the ionic equilibria, selectivity and effective diffusion of Rb, K, Na, Ca, Mg in two Indiana soils.

Additon of anions to the soils increased the concentration of cations in soil solution. In both the soils receiving H₂PO₄ ^‐, lower cation concentrations were found in the soil solution than in those receiving Cl^‐ . Additon of H₂PO₄ ^‐and Cl^‐ reduced the ion selectivity coefficient, k, for various homovalent (Rb/K, Rb/Na, K/Na, Ca/Mg) and mono‐divalent ion pairs (Rb/Ca, Rb/Mg, K/Ca, K/Mg). In Zanesville soil treatments receiving H₂PO₄ ^‐ had lower k values for mono‐divalent cations than treatments receiving Cl^‐. However, no such conclusions could be drawn for Raub soil. Soils treated with H₂PO₄ ^‐ had higher k values for homovalent cations than Cl^‐ treated soils. The differences in the selectivity of adsorption in these two soils might be attributable to the differences in the type and nature of exchange materials and cation concentrations on the exchange phase.

Addition of H₂PO₄ ^‐ or Cl^‐ enhanced the magnitude of effective diffusion coefficient. (De) of all the cations under considerations. The magnitude of effective diffusion coefficient for cations was lower for H₂PO₄ ^‐ treated soils than Cl^‐ treated soils. Such a reduction in De is related to the reduction in cation concentration in soil solution thereby increasing the buffer capacity for the ions under consideration.     

Comparison of Seven SO4-S Extraction Methods for Analysis by Turbidimetry or ICP Spectrometry

Soil sulfur (S) analyses for fertilizer recommendations in the northern Great Plains often do not reflect crop S requirements. Seven SO₄-S extraction methods with S determination by either turbidometry or inductively coupled plasma emission spectroscopy including Ca(H₂PO₄)₂ and KH₂PO₄ (both containing 500 ug/l P), 0.25 M KCl (40 ºC) and 0.25 M KCl (room temperature), H₂O, DTPA, and Mehlich 3 extractants. Three horizon depths of three soils from a previous field study were used for these comparisons. Average standard deviations for turbidometric determinations were 4.3 times greater than ICP determinations. With turbidometry, S values were H₂O > KH₂PO₄ > Ca(H₂PO₄)₂ > KCl (40 ºC) = KCl, while with ICP, the values were Mehlich 3 > KCl (40 ºC) = KCl > DTPA (diethylenetriaminepentaacetic acid) > KH₂PO₄ > H₂O > Ca(H₂PO₄)₂. Extraction with KCl at room temperature with ICP determination appears to show promise, but further method evaluation is necessary before it can be recommended as a SO₄-S test method.     

Sorption of calcium and sulfate by sandy soils from flue‐gas desulfurization gypsum and phosphogypsum

Abstract

Increasing concern on sulfur dioxide (SO₂) pollution has prompted many coal‐fired electric power generating plants to install SO₂ scrubbing units which generate large amounts of flue‐gas desulfurization gypsum (FGDG). This source of gypsum can be an excellent amendment to supply calcium (Ca) and/or sulfate (SO₄) to sandy soils which are deficient in these nutrients. In this study, reactions of FGDG, phosphogypsum (PG), reagent grade calcium sulfate (CaSO₄), and calcium chloride (CaCl₂) were investigated using a Candler fine sand (uncoated, hyperthermic, Typic Quartzipsamment), a typical well‐drained deep sand, and a Pineda fine sand (loamy, siliceous, hyperthermic, Arenic Glossaqualf), a typical shallow poorly drained sand. Concentrations of various water‐soluble elements were similar in FGDG as well as PG, except the latter contained 11 and 15 g/kg phosphate (PO₄) and fluoride (F), respectively, while these ions were undetectable in the former. In batch‐equilibration (4 h) technique using soilrsolution ratio of 1:2, pH of equilibrium solution decreased by 0.2 and 0.8 units, while ionic strength increased by 5‐ and 15‐fold in the Pineda and Candler sand, respectively, when using FGDG or PG solution as compared to using water for equilibration. Sorption of Ca by the two soils varied from 0.56 to 0.71 Cmolc/kg regardless of source of gypsum. As compared to sorption of SO₄, that of Ca was greater by 2.5‐ and 2.6 to 4.3‐fold by the Candler and Pineda sand, respectively. With the exception of SO4 sorption by the Pineda sand, this study demonstrated that the reactions of FGDG with these sandy soils were very similar to those of PG or reagent grade CaSO4. Since much of the past research on agricultural use of gypsum was done using PG and CaSCU, we could conclude that similar response could be expected with use of FGDG.     

Analysis of sulfur compounds in acid sulfate soils and other recent marine soils

Abstract

A refined scheme for the semi micro chemical analysis of sulfur fractions in soils is presented. Pyrite is analyzed, as iron, after extraction in HNO₃. Non‐pyrite iron is excluded by a pretreatment with HF/H₂SO₄. Water‐soluble sulfate and jarosite [KFe₃(SO₄)₂(OH)₆], the other dominant sulfur fractions in acid sulfate soils, are analyzed turbidimetrically, as sulfate, after successive extractions by EDTA.3Na (water soluble plus exchangeable SO₄) and by hot 4 M HCl (jarosite). These methods are simpler, less bulky and more specific than most existing procedures.

Introduction of elemental sulfur analysis permits estimation of organic sulfur fraction as well. Sums of individual sulfur fractions agree well with separate total sulfur determinations.

The proposed analysis of pyrite permits also distinction of the components Fe₂O₃, FeO and FeS₂ in soils and rocks².     

Chemical fractionation,water solubility and temporal distribution of sheep faecal sulphur fractions in grazed pastures receiving long‐term sulphate fertilization

Abstract

Although over 40% of excretal S is returned to intensively sheep ‐grazed pastures as faecal S, limited information is available on faecal S fractions, their water solubility and temporal distribution. This study reports results obtained from sheep faeces returned to grazed pastures which have received long‐term annual sulphate applications for 15–20 years. Five freshly‐voided sheep faecal samples (<100 g moist faeces per sample) per sampling were randomly collected at approximately one month intervals over a one‐year growing season. Faeces were fractionated into total S, inorganic SO₄ ^2‐, ester SO₄ ^2‐, Hi‐reducible S and C‐bonded S. Results obtained showed that faecal total S, ester SO₄ ^2‐ Hi‐reducible S and C‐bonded S fractions varied significantly throughout the year. Carbon‐bonded S was the dominant (>80%) faecal S fraction, regardless of faecal total S content or the time of year faecal samples were deposited. Faecal ester SO₄ ^2‐ and inorganic _SO₄ ^2‐fractions accounted for 3.3–7.1% and 0.1–14% of faecal total S respectively. Thus approximately 3.4–21.1% of faecal total S was estimated to be potentially leached or readily available to pasture plants. The Hi‐reducible faecal S fraction was significantly‐correlated (r = 0.59***; *** = P ≤ 0.001) with HCl‐extractable faecal inorganic S, which was considered to represent faecal total SO₄ ^2‐ (ester SO₄ ^2‐ and inorganic SO₄ ^2‐ fractions).

The solubility of different faecal S fractions was determined by sequential extraction of ground (< 1 mm) faeces three times (30 minutes per extraction) with water or 0.01 M Ca(H₂PO₄)₂ solution (1: 5 ratio of faecal DM: extractant). Both amounts of water‐extractable and Ca(H₂PO₄)‐extractable faecal S fractions were found to vary significantly throughout the year. Ca(H₂PO₄)₂ tended to extract more inorganic faecal S than water, attributed to the presence of phosphate and the low pH (pH=4) of Ca(H₂PO₄)₂ extractant. A significant proportion (15–25%) of faecal S was extracted by water and most (70%) of this water‐extractable faecal S was in the organic S fraction. Water‐extractable inorganic faecal S probably originated from the faecal total SO₄ ^2‐ fraction as shown by their significant correlation (r = 0.45** ‐0.63***; ** = P≤ 0.01; *** = P≤ 0.001). Some of the faecal S in water extracts may also originate from the faecal C‐bonded S fraction, as a significant correlation was obtained between C‐bonded faecal S and either water‐extractable faecal organic S (r = 0.53–0.57***; *** = P ≤ 0.001) or water‐extractable faecal inorganic S (r = 0.40–0.41*; * = P ≤ 0.05).

Significant amounts of faecal inorganic SO₄ ^2‐ and ester SO₄ ^2‐ fractions were removed by Ca(H₂PO₄)₂ extractant. The Ca(H₂PO₄)₂‐extractable faecal inorganic S was significantly correlated (r = 0.73***^; *** = P ≤ 0.001) with water‐extractable faecal inorganic S.     

Extractability and adsorption of sulphate in soils

Virtually all of the indigenous sulphate (SO₄) in a range of UK soils with moderately high pH values (> 6) was found to be present in the soil solution and, as a consequence, was highly susceptible to leaching. For acid soils containing adsorbed SO₄, the extractability of SO₄ in NaCl and CaCl₂ solutions was dependent on both the ionic strength and cation species. Addition of small amounts (<～ 10^?2M) of either NaCl or CaCl₂ actually decreased the amount of SO₄ extracted, but SO₄ extractability increased sharply with concentrations of NaCl or CaCl₂ higher than about 0.1 M. At a similar ionic strength, more SO₄ was extracted by NaCl than CaCl₂. Sequential extraction with 1 M NaCl removed essentially all of the absorbed SO₄. The release characteristics of SO₄ were very different to those of phosphate and this difference in behaviour is not easily reconciled with the view that SO₄ is chemisorbed, as is phosphate. Except for a few acid soils with high oxide contents, the capacity of the soils to adsorb added SO₄ was quite small. None of the soils with pH values higher than 6 adsorbed a significant amount SO₄. The results raise questions regarding the efficiency of SO₄-containing fertilizers in correcting and preventing S deficiency in situations where leaching is important.     

Indirect determination of micrograms of sulfate by barium absorption spectroscopy

Abstract

The indirect procedure for sulfate determination by Ba absorption spectroscopy was modified so that low concentrations of SO₄ in small volumes of solutions could be determined rapidly and precisely. Major modifications consisted of seeding the sample with BaSO₄, precipitating in ethanol solutions to lower BaSO₄ solubility, and determining Ba in a N₂O‐acetylene flame using the absorption mode.

The results showed: complete SO₄ precipitation as BaSO₄ after 15 min of shaking, little or no effect from solution Al on SO₄ determination, quantitative recovery of SO₄ from 0.01 M Ca(H₂PO₄)₂ soil extracts, and greater precision of SO₄ measurement with indirect method than with turbidimetric method.     

Evaluation of soil sulfate extractants and methods of analysis for plant available sulfur

Abstract

A steady decline in sulfur additions to Atlantic Canadian soils has prompted the need for an accurate method of determining their plant available sulfur status. Three soils were extracted with five soil extractants ‐ 0.01M Ca(H2PO₄)₂‐H₂O in 2M HOAc, 0.1M CaCl₂, Bray‐1 and de‐ionized water. The soil extracts were analyzed for sulfur or sulfate using inductively coupled argon plasma emission spectrometry (ICAP), AutoAnalyzer (AAN), anion exchange‐high performance liquid chromatography (HPLC‐CD) or atomic absorption spectroscopy (AAS). Results were compared with plant response of sulfur treatments to red clover, ryegrass, canola and wheat in a growth room. Instrument reproducibility and crop response indicated the ideal method of determining plant available soil sulfur was HPLC‐CD using the extractant Ca(H2PO₄)₂‐H₂O.