Amino acid interference with ammonium determination in soil extracts using the automated indophenol method

Abstract

The formation of a colored indophenol complex is commonly used as a quantitative measure of the ammonium content of soil extracts. The potential interference with ammonium determination from co‐extracted amino acids was examined. The extent of color development was examined for 22 amino acids by subjecting pure solutions to ammonium determination by both the indophenol method and steam distillation. Apparent detection of amino acid as ammonium ranged from 0 to 94 % of total nitrogen for the indophenol procedure, whereas steam distillation resulted in little apparent ammonium recovery. With the exception of threonine, the extent of color development was inversely related to amino acid molecular weight. The range in recoveries for the indophenol procedure suggests both size and composition of the co‐extracted amino acid pool is important in determining the extent of interference.

Significantly (p=0.001) greater estimates, averaging 0.4 μg mL^‐1, were found in indophenol estimates of mineral‐N content of moist, fresh soil samples. Air drying, oven drying or chloroform fumigation significantly increased the difference (0.3 ‐ 0.7 μg mL^‐1) in estimates of ammonium content. At 10: 1 extract: soil ratios this could cause ? to be overestimated by 3–7 μg g^‐1soil. The increased interference was attributed to a release of amino acid as a result of pretreatment. The difference between distillation and indophenol estimates of ammonium content of 0.5 M K₂SO₄was found to be dependent upon ammonium content. The use of procedures employing a distillation step (manual or automated) is recommended to avoid amino acid interference when precise NH₄+‐N determinations are needed on dried or fumigated samples.     

An automated turbidimetric method for total sulfur in plant tissue and sulfate sulfur in soils

Abstract

An automated turbidimetric method has been developed for the rapid and accurate determination of sulfate. The method is practical and useful for accurately measuring total sulfur in plant tissues, and extractable sulfate in soils. The principle of intermittent reagent addition is used which eliminates drift and sensitivity changes caused by coating of BaSO₄ on tubing and cell walls. Also, the appropriate chemistry is used to minimize interactions of the wash with the sample at a sampling rate of 30/H. The sensitivity of the method is excellent with a working range of 0 to 15 ppm sulfur for soils. For plant digests the sample solutions are diluted to 0–35 ppm S. The precision as determined by repeated analysis of a soil sample extract was 0.58% RSD with a mean of 9.26 pg/g extractable SO⁼ ₄‐S. On another soil sample using a different extractant and extraction procedure the RSD was 0.64%, mean of 9.26 μg/g. Multiple automated sulfur analyses on a plant tissue digest resulted in an RSD of 0.41% for a sample containing 0.21% S. The automated turbidimetric method for sulfate has excellent precision and sensitivity in plant tissue and soil analyses where gravimetric BaSO₄ assays are not practical.     

Colorimetric method for phosphorus measurement in ammonium oxalate soil extracts

Abstract

Ammonium oxalate extractable phosphorus represents a potentially useful measure of the P associated with the amorphous iron and aluminum oxides in soils. However, because oxalate complexes molybdate, the colorimetric determination of phosphorus in ammonium oxalate extracts by the traditional molybdenum‐blue procedure is not possible. In this study, a method was developed whereby excessive molybdate was added to the extracts to complex the oxalate and eliminate the interference of oxalate with the formation of the phospho‐moblydate complexes necessary for color development. The method development and recommended procedure for colorimetrically measuring phosphorus in ammonium oxalate soil extracts are described.     

An acid dichromate digestion procedure for total nitrogen determination in soils

Abstract

Modified version of a dichromate digestion procedure, earlier found suitable for simultaneous determination of organic carbon (C) and potentially mineralizable nitrogen (N) in soils, was evaluated for total N determination in 44 diverse soil samples from the semi‐arid region of India. The soils belonged to Vertisols and Alfisols and had a wide range in total N contents. The method involves digestion of the soil sample with an acidified dichromate solution for 30 minutes in a digestion tube in a block digester preheated to 165°C followed by determination of N in the digest by steam distillation procedure. The efficacy of the dichromate procedure for determining total N differed for the two soil types. Excellent correlations were observed between the values of total N obtained by the dichromate procedure (Dichro‐N) and by Kjeldahl method (Kjeldahl‐N) for Vertisols (R²=0.995; n=21) and Alfisols (R²=0.952; n=23). The regression equations describing the relationships between the two soil types can be used to estimate Kjeldahl‐N from Dichro‐N. Both Kjeldahl and the proposed acid dichromate methods were found to be more precise in determining total N in Vertisols than Alfisols. The proposed procedure is rapid and simple and can be used for routine total N determination especially, in soils such as Vertisols. Based on our results the acid digestion procedure certainly deserves a wider evaluation with a range of soil types.     

Microscale determination of inorganic nitrogen in water and soil extracts

Abstract

Rapid, sensitive analysis of NH₄ ^‐ NO₃ ^‐, and NO₂ ^‐ in 1–150 μL of soil extract or water was achieved using a modified indophenol blue technique adapted to microtiter plate format. The microplate technique was similar to conventional steam distillation in accuracy and precision. By varying aliquot volume, a wide linear dynamic range (0.05 to 1000 mg of NH₄ ⁺‐ or NO₃ ^‐‐NL^‐1) was achieved without the need for sample dilution or concentration. High sample throughput (250–500 NH₄ ⁺ analyses d^‐1) was accomplished manually, but could be significantly increased by automation. Of considerable importance was the very low waste stream produced by the method. All equipment and supplies required are commercially available and need no modifications for this use. The microtiter plate format could be used for other soil colorimetric analyses with little or no down time for equipment setup, a major consideration for commercial soil‐testing laboratories. The method and equipment used are well suited to quality control and quality assurance programs, as required under FIFRA Good Laboratory Practices.     

A rapid dichromate procedure for routine estimation of total nitrogen in soils

Abstract

A simple procedure for routine estimation of total nitrogen (N) in soils is described. It involves (a) digestion of the soil sample with an acidified dichromate (K₂Cr₂O₇‐H2SO₄) solution for 45 minutes in a Pyrex digestion tube in a 40‐tube block digester preheated to 170°C, (b) titrimetric determination of the ammonia‐N liberated by direct steam distillation of the digest with alkali (NaOH), and (c) calculation of the total‐N value by application of a correction factor (1.1) to the measured NH₃‐N value. Studies using 14 soils selected to obtain a wide range in total N content and other properties showed that the uncorrected recovery of N by the procedure described ranged from 88.5% to 92.3% and averaged 90.5%. When a correction factor of 1.1 was applied to the results, the recovery of N ranged from 97.4% to 101.5% and averaged 99.6%.

The dichromate procedure described is rapid and precise, and it permits at least 80 analyses in a normal working day. The only equipment required for its use is equipment commonly used for total N analysis of soils by the Kjeldahl method (40‐tube block digester and steam distillation unit), and the acidified dichromate solution used for digestion of the soil is, the reagent commonly used for determination of organic carbon in soil by a modified Mebius procedure. Besides being simpler and more rapid than the Kjeldahl method, the procedure described has the advantage that it does not require the expensive fume disposal system needed for Kjeldahl digestion.     

Spectrophotometric determination of silicon in soil solutions by flow injection analysis: Reduction of phosphate interference

Abstract

A flow injection analysis (FIA) procedure for the determination of dissolved silica (0.04–20 mg/L Si) in aqueous solution has been optimized to reduce phosphate interference. Determinations are based on measurement of absorbance at 790 nm of heteropoly molybdenum blue formed by reduction with ascorbic acid at room temperature. Phosphate did not interfere in a 15‐fold excess. The optimized procedure was tested on soil solutions isolated by centrifugation of various horizons from a Typic Haplohumod. Si concentrations of 1.3–4.8 mg/L Si were found with a variation coefficient of about 2. Results obtained compared well with those obtained by a manual reference method and a proprietary FIA method except in solutions high in dissolved humic material where slightly higher values were obtained by the optimized method. In a standard addition mode the optimized method yielded 5–15% lower values than in the ordinary mode. This difference was reduced by persulfate oxidation of organic matter. Soil solutions investigated were very low in phosphate but phosphate spiking experiments demonstrated that phosphate interference was less than in model solutions matched in metal ion concentrations and insignificant in solutions low in humic material and with less than 10 mg/L P. Dissolved silica was unstable in a solution isolated from an organic horizon of high biological activity.     

Volatilization of Ammonia Derived from Fertilizer and Its Reabsorption by Coffee Plants

Abstract

Volatilization of ammonia derived from nitrogen (N) fertilizers and its possible reabsorption by crops depend on specific soil, climate, and atmospheric conditions, as well as the method of fertilizer application and plant architecture. In an experiment carried out in Piracicaba, State of São Paulo, Brazil, the volatilization of ammonia derived from urea, ammonium sulfate, and natural soil were quantified using static semi‐open N‐ammonia (NH₃) collectors. Fertilizers were top‐dressed under the plant canopy on top of dead leaf mulch. In another experiment, the reabsorption of the volatilized ammonia by plants was quantified using ¹⁵N‐labeled urea. Results showed, as expected, that volatilization derived from urea was seven times more intense in relation to ammonium sulfate, whose volatilization was very low, and slightly more than the natural volatilization from soil at pH 5.3. The loss of ammonia from the ammonium sulfate was very low, little more than twice of that of the natural soil. Through isotopic labeling, it was verified that 43% of the volatilized N‐NH₃ was reabsorbed by coffee plants, which gives evidence that volatilization losses are greatly reversed through this process.     

Evaluation of methods for determining sulphur‐35 and sulphur‐32 in the same trapping solution of Johnson and Nishita method

Abstract

A laboratory study was conducted to investigate whether the Zn‐Na acetate solution, used for trapping H₂S in the Johnson‐Nishita method (1952), could be used to quantitatively determine ³⁵S and ³²S in the same trapping solution, as the Zn‐Na acetate solution allows more sensitive S determination using methylene blue than does the bismuth sulphide colour development of H₂S trapped in NaOH solution. Results obtained using the Zn‐Na acetate solution were variable compared with those obtained from the normal procedure of using NaOH as the H₂S trapping solution, indicating that the Zn‐Na acetate solution was unsuitable, probably because of the rapid transformation of H₂S to water‐insoluble ZnS.

The applicability of methylene blue development for H₂S trapped in NaOH solution was also investigated. Results showed that ³²S could be determined quantitatively provided methylene blue colour development was conducted less than 30 minutes after the removal of H₂S‐trapping NaOH solution from the Johnson‐Nishita digestion‐distillation unit. A delay of more than 30 minutes caused a significant linear reduction (R² = 0.86 ‐ 0.92***) in methylene blue colour, indicating that H₂S in NaOH solution was readity: oxidized to other S forms which did not undergo methylene blue development.

Toluene and dioxane were compared as liquid scintillants for assaying ³⁵S activity trapped as H₂S in either NaOH or Zn‐Na acetate solution before and after methylene blue development. Before colour development, toluene improved the counting efficiency of ³⁵S in NaOH solution by 23 to 27%. After colour development, toluene improved the counting efficiency of both NaOH and Zn‐Na acetate solutions.     

An Improved Procedure for Determining Magnesium Fertilizer Dissolution in Field Soils

Abstract

The sequential extraction procedure currently used to measure magnesium (Mg) fertilizer dissolution in soils consists of removing dissolved Mg (step 1), and partially dissolved Mg (step 2), followed by an 18‐h extraction with 2 M HCl at room temperature to determine undissolved Mg (step 3). This procedure is satisfactory for soluble and moderately soluble Mg fertilizers but is not an accurate procedure for slightly soluble fertilizers, such as serpentine. When step 3 is replaced by a digestion procedure using 2 M HCl for 4 h at 90–95°C (improved step 3), the total serpentine Mg recovery (dissolved and undissolved Mg) from soil samples, either immediately after serpentine was added to soil or after a 21‐day incubation with moist soil, was about 100% compared to 40–50% by the original procedure. The improved procedure also increased the recovery of serpentine Mg applied to field soils. Therefore, this study recommends that the third step of the sequential extraction procedure be replaced by a 4 h digestion using 2 M HCl (90–95°C).     

Volatilization of sulfur from unamended and sulfate-treated soils

Volatilization of sulfur from unamended and sulfate-treated soils was studied by sensitive gas chromatographic techniques using a flame photometric detector fitted with a sulfur filter. The soils employed were surface samples of 25 Iowa soils selected to obtain a wide range in properties. No release of volatile sulfur compounds was detected when 11 of these soils were incubated under aerobic or waterlogged conditions before or after treatment with sulfate (400 μg sulfate S/g soil). Fourteen soils released volatile sulfur compounds when incubated under waterlogged conditions before and after addition of sulfate, but only 4 of these soils released volatile sulfur compounds when incubated under aerobic conditions. Where volatilization of sulfur was observed, the volatile sulfur detected was identified as dimethyl sulfide or as dimethyl sulfide associated with smaller amounts of carbonyl sulfide, carbon disulfide, methyl mercaptan, and (or) dimethyl disulfide. No trace of hydrogen sulfide was detected. Where release of volatile sulfur was observed, the amount of sulfur volatilized at 30°C in 60 days under aerobic or waterlogged conditions was very small and did not account for more than 0–05% of the sulfur in the unamended or sulfate-treated soils studied. It is concluded that gaseous loss of sulfur from unamended or sulfate-treated soils is insignificant under conditions likely to be encountered in the field.     

A manual colorimetric procedure for measuring ammonium nitrogen in soil and plant Kjeldahl digests

Abstract

The measurement of NH4⁺‐N in soil, and plant digests is one of the greatest needs in laboratories conducting agricultural and environmental research. Many laboratories do not have access to automated equipment for colorimetric analysis of soil and plant digests. The objective of this research was to modify an automated colorimetric analysis procedure for determining NH₄+‐N in soil and plant digests for manual use, and compare the proposed technique with the standard distillation‐titration technique. The modified procedure is based on the color reaction between NH₄ ⁺‐ and a weakly alkaline mixture of Na salicylate and a chlorine source in the presence of Na nitroprusside. Wavelength scans indicated a very well defined peak for determinations at 650 nm. Time scans showed that color development in the manual procedure was rapid, 12 to 40 minutes depending on temperature, and that the color development remained stable for at least 120 minutes. Regression analysis of the results from 18 soil and 20 plant tissue sample determinations by distillation‐titration and the proposed method indicated NH₄ ⁺ ‐N recoveries of 99% or higher. The results obtained using the colorimetric procedure were very similar to the values obtained by distil ling and titrating the digests for both soil and plant samples as indicated by the large coefficients of determination (R² = 0.99).     

Effect of moisture variation on sulfate concentration in a buck solonetz soil

Abstract

Sulfate concentration in the equilibrium solutions of a black solonetz soil was different at each moisture content. In the sub‐surface horizons a decrease in soil moisture up to about 35 percent resulted in a gradual increase of sulfate concentration. However, with further decrease in moisture the sulfate concentration increased abruptly. Variation of sulfate concentration appears to alter significantly certain cation ratios in the equilibrium solutions.     

In situ determination of sulfate turnover in peatlands: A down‐scaled push–pull tracer technique

Bacterial sulfate reduction (BSR) is a key process in anaerobic respiration in wetlands and may have considerable impacts on methane emissions. A method to determine sulfate production and consumption in situ is lacking to date. We applied a single‐well, injection‐withdrawal tracer test for the in situ determination of potential sulfate turnover in a northern temperate peatland. Piezometers were installed in three peat depth levels (20, 30, and 50 cm) during summer 2004, and three series of injection‐withdrawal cycles were carried out over a period of several days. Turnover rates of sulfate, calculated from first‐order‐reaction constant k (–0.097 to 0.053 h^–1) and pore‐water sulfate concentrations (approx. 10 µmol L^–1), ranged from –1.3 to –9.0 nmol cm^–3 d^–1 for reduction and from +0.7 to +25.4 nmol cm^–2 d^–1 for production, which occurred after infiltration of water following a heavy rainstorm. Analysis of stable isotopes in peat‐water sulfate revealed slightly increasing δ³⁴S values and decreasing sulfate concentrations indicating the presence of BSR. The calculated low sulfur‐fractionation factors of <2‰ are in line with high sulfate‐reduction rates during BSR. Routine application will require technical optimization, but the method seems a promising addition to common ex situ techniques, as the investigated soil is not structurally altered. The method can furthermore be applied at low expense even in remote locations.     

Sodium hypobromite digestion for boron analysis of plant and soil materials

Abstract

A sodium hypobromite digestion procedure was developed for total boron content of plant tissue. The method completely released total plant boron and took much less time than a dry ash method. The sodium hypobromite digestion was compatible with a modified curcumin colorimetric method. The sodium hypobromite digestion eliminated interference by nitrate, fluoride and iron, which were previously shown to be potential problems with the colorimetric method. Aluminium eliminated the interference of fluoride on boron analyses by the modified curcumin method without the sodium hypobrornite digestion step.

The sodium hypobromite digestion procedure was applied to a number of different soils after the volume of sodium hypobromite to soil sample weight ratio was optimized to release a constant amount of boron. The boron measured did not appear to be total boron in every soil. However, two other methods (sodium carbonate fusion and hydrofluoric acid treatment) recommended for total boron were not entirely successful either for the same soils. Correlations of soil boron values with selected soil chemical and physical measurements suggested that the hydrofluoric acid method was more successful in measuring boron associated with soil organic matter while the sodium hypobromite and sodium carbonate methods were more successful in measuring boron associated with the mineral portion, especially the silt fraction. The sodium hypobromite digestion appeared to be less efficient than sodium carbonate fusion in determining “total”; boron with increasing pH of the soil. The sodium hypobromite digestion boron values of soils may be useful as a boron fraction measurement but should not be considered as total boron.     

Shortcut Approach Alternative to the Step‐by‐Step Conventional Soil Phosphorus Fractionation Method

Abstract

In an attempt to characterize the phosphorus (P)–supplying capacity of a soil and to understand the dynamics of soil P, a procedure was followed whereby consecutive extraction procedures were carried out on a soil sample, first by dialysis membrane tubes filled with hydrous ferric oxide (DMT‐HFO), followed by subsequent P fractionation procedure. However, this combined method is lengthy and time‐consuming, and an approach to shorten these P desorption studies in soils was important. The major objective of this article, therefore, was to present a shortcut method as an alternative approach to the combined fractionation method. Comparison of the sum of DMT‐HFO‐P_i, sodium bicarbonate (NaHCO₃)‐P_i, sodium hydroxide (NaOH)‐P_i, D/hydrochloric acid (HCl)‐P_i, and C/HCl‐P_i extracted by a conventional step‐by‐step method with the sum of DMT‐HFO‐P_i and a single D/HCl‐P_i extraction as a shortcut approach for all extraction periods resulted in a very strong and significant correlations. Both these methods were correlated with maize grain yield, and it was found to be highly significant. This study revealed that this shortcut approach could be a simplified and economically viable option to study the P dynamics of soils especially for soils where the P pool acting as a source in replenishing the labile portion of P is already identified.     

A rapid steam distillation method of assessing potentially available organic nitrogen in soil

Abstract

A rapid steam distillation of assessing potentially available organic nitrogen in soil is described. It involves determination of the ammonia‐N produced by steam distillation of the soil sample with pH 11.2 phosphate‐borate buffer solution for 8 min. The method is simple and precise, and its results are not significantly affected by air‐drying or air‐dry storage of the soil sample before analysis. It is well suited for use in soil testing laboratories because it does not require extraction, filtration or transfer steps. Studies using 33 Brazilian soils showed that the results obtained by this method were highly correlated with those obtained by aerobic and anaerobic incubation methods of assessing potentially available organic nitrogen in soil.     

Formation of hydroxy‐sulfates from pyrite in coastal acid sulfate soil environments in Malaysia

Abstract

Pyrite in hydromorphic soils is oxidized when it is exposed to the atmosphere. The sulfide oxidation releases hydrogen (H⁺) ions and other ions into the aqueous solution, and subsequently hydroxy‐sulfates are formed. A laboratory aging experiment was conducted using coastal sulfate‐rich soils in Malaysia to identify and determine the nature and composition of the hydroxy‐sulfates and to explain the mechanism of their formation. Powder X‐ray diffraction (XRD) analysis showed that incubating the pyrite‐bearing soils in the presence of added electrolyte (KCl and NaCl) resulted in the formation of jarosite, natrojarosite, and/or alunite. Subsequent transmission electron microscopy‐energy dispersive X‐ray (TEM‐EDAX) analysis showed that a hydroxy‐sulfate crystal was composed mainly of hydrogen (H), oxygen (O), sodium (Na), aluminum (Al), sulfur (S), potassium (K), and iron (Fe) which was accounted for as jarosite, natrojarosite, and/or alunite by powder XRD. The small amount of fluorine (F), nickel (Ni), titanium (Ti), and manganese (Mn) occurring within the same hydroxy‐sulfate crystal was presumably originated from pyrite. This result points to the formation of hydroxy‐sulfates in acid sulfate soils via psuedomorphic replacement of pyrite under an oxidizing environment.     

The determination of extractable plant available selenium from soils by electrothermal atomic absorption spectroscopy

Abstract

Plant available selenate was measured in a soil extract by electrothermal atomic absorption spectroscopy. Selenate in soil was first extracted with potassium sulfate then extracted into toluene fron an iodide‐sulfuric acid media. Aliquots of the toluene were injected into the graphite furnace for selenium determination. Quantities of extractable selenium measured using the electrothermal atomic absorption method compared favorably with the results of determinations using a fluorometric technique on aliquots of the soil extract. The simplicity of the procedure allowed for the precise determination of soil selenate in a brief period of time.     

Relative Efficiency of Zinc Sulfate and Zinc Oxide–Coated Urea in Rice–Wheat Cropping System

Abstract

Rice–wheat cropping system covers about 24 million hectares in China, India, Pakistan, Nepal, and Bangladesh, and zinc deficiency is widespread in rice–wheat belts of all these five countries. The current practice of applying zinc sulfate heptahydrate (ZnSO₄ · 7H₂O) to soil is problematic because of the poor quality of the nutrients available in the market to the farmers. Zinc (Zn)–coated urea is therefore being manufactured to guarantee a good‐quality Zn source. This article reports the results from a field study conducted to study the relative efficiency of zinc sulfate and zinc oxide (ZnO)–coated ureas in rice–wheat cropping system. The highest grain yield of rice–wheat cropping system was obtained with 2.0% coating of urea. Zinc sulfate was also a better coating material than ZnO. Partial factor productivity, agronomic efficiency, apparent recovery, and physiological efficiency of applied Zn decreased as the level of Zn coating was increased.