A high sample volume procedure for the colorimetric determination of soil organic matter

Abstract

A modified procedure for routine determination of soil organic matter is described. The procedure employs a wet combustion method of reducing the carbon in soil organic matter with dichromate in an acid media. The modification will permit 700–800 organic matter determinations per day with samples containing less than 8% 0M without undue difficulty.     

A rapid accurate wet oxidation diffusion procedure for determining organic and inorganic carbon in plant and soil samples

Abstract

A method is presented for determining organic carbon content of soils, extracts and plant tissue utilizing an acidic wet oxidation in modified culture tubes; samples containing carbonates are pretreated with dilute acid. Evolved CO₂ is trapped in a NaOH solution and quantified titrimetrically. The method described has a range of 0.4–11 mg. carbon and the range can be extended in either direction by modifying the CO₂ trapping/titration procedure. This method is as accurate and precise as more time consuming dry oxidation procedures.     

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.     

Determination of carbon,hydrogen, and nitrogen in soils by automated elemental analysis (dry combustion method)

Abstract

The use of the LECO CHN‐600 Elemental Analyzer (based on a dry combustion method) for the analysis of soils for their content of organic and total carbon, hydrogen, and nitrogen is described. Organic carbon determined by this means was closely related to organic carbon determined according to the Tyurin method, and total carbon related to organic matter content determined by the loss‐on‐ignition of soil samples. Precision for the carbon and hydrogen determination were acceptable. It was lower in the case of the total nitrogen determination. The time of analysis was approximately 4 to 5 minutes.     

Rapid and sensitive determination of nitrate in plant tissue using flow injection analysis

Abstract

The determination of nitrate in waters and soil extracts by the reduction of nitrate to nitrite by metallic or liquid reductants followed by the colorimetric determination of NO₂ ^‐using the Griess‐Ilosvay reaction has been automated for use with air‐segmented auto‐analyzers or flow injection techniques. However, this technique is not applicable to plant extracts as organic species in the extracts inactivate the reduction columns. The objective of this study was to develop an automated procedure that would allow the determination of NO₃ ^‐in plant extracts without the necessity of prior manual treatment. A flow injection technique was developed that successively traps and releases NO₃in an anion exchange column thereby removing pigments and other non‐ionic and cationic species that otherwise interfere with conversion of NO₃ ^‐ to NO₂ ^‐on a copperized cadmium column. This reduction step is subsequently followed by standard Griess‐Ilosvay colorimetric detection of this ion at a wavelength of 530 nm. The technique uses relatively simple and inexpensive equipment, principally a spectrophotometer equipped with a flow‐through cell and a pen recorder output, a 6‐channel peristaltic pump with accompanying tubing and a Perspex injector/commutator valve made in a laboratory workshop. The technique was found to avoid any significant interference of pigments or other organic compounds in the plant extracts, and the results compared favorably with those obtained using the manual transnitration technique. Analysis time was approximately 1.5 min per sample and could detect NO₃’ concentration as low as 0.1 ug NO₃’‐N ml/¹ in plant extracts (10 ug NO₃’‐N g‘¹of plant material).     

A comparison of some methods for soil organic carbon determination

Abstract

This study compared three dichromate‐oxidation methods adapted for use with 100‐mL digestion tubes and 40‐tube block digester (for controlled heating), the Walkley‐Black method, a loss‐on‐ignition procedure and an automated dry combustion method for the determination of organic carbon in soils of the northwestern Canadian prairie. The Walkley‐Black method required a correction factor of 1.40. The modified Tinsley method and the Mebius procedure, adapted for use with 100‐mL digestion tubes, recovered 95% and 98%, respectively, of soil carbon against the dry combustion procedure. The presence of elemental carbon in some soils probably caused, at least partially, the slightly incomplete recovery; thermal decomposition of dichromate may not have been accurately corrected for. A dichromate‐oxidation procedure with controlled digestion at 135°C gave 100% recovery, but somewhat more variable results. The loss‐on‐ignition procedure, even when allowance was made for clay content of the soils, was the least satisfactory of the methods tested. All procedures produced correlation coefficients of 0.980 or better against the dry combustion method.     

Determination of organic carbon in soils and fertilizers

Abstract

Many methods have been proposed to determine the total amount of organic carbon in soil; some of them determine only a percentage of the actual carbon content and therefore the results need the application of a correction factor. Methods for the determination of organic carbon in soil have been rarely extended to organic fertilizers and amendments.

We propose a rapid method based on a modification of the original Springer and Klee¹⁰procedure. Samples are oxidized for exactly 10 minutes with a mixture of 20 ml 2N K₂Cr₂O₇and 26 ml conc. H₂SO₄at 160±2°C; the excess dichromate is titrated either potentiometrically or manually with FeSO₄.

The method is fast, accurate, and more reliable than other commonly used procedures. The procedure can be easily adopted for serial determination of carbon in both soils and organic fertilizers or amendments     

Determination of total organic‐C in soils by an improved chromic acid digestion and spectrophotometric procedure

Abstract

An improvement to the Walkley‐Black wet digestion method for the rapid determination of organic carbon over the range 0.2–5.5% in air‐dry soil is described. It permits total recovery of the organic‐C in finely ground soil samples digested with the heat of dilution from mixing N K₂ Cr₂ O₇ with concentrated H₂SO₄. in test tubes followed by external heating from a hot‐plate digestor. The organic‐C concentrations are determined directly, as the Cr product in diluted soil digests, by absorptiometry at 600 nm with calibration against similarly treated sucrose standards in solution. For the soils tested, there were negligible interferences from carbonates, wood charcoal, coke, Fe⁺² and readily reducible Mn; Cl does not interfere with the organic‐C assay in non‐saline soils but for saline soils a correction based on 1/12 Cl^‐ assay of the soil is necessary. The present method is compared with Tabatabai and Bremner's dry combustion procedure and Allison's manometric adaptation for calcareous soils. The procedure described here does not require carbonate to be determined and is therefore simpler. In addition it is cheaper, faster and more effective in controlling interferences than dry combustion procedures.     

Automated analysis of 15N and 14C in biological samples

Abstract

An automated method for the simultaneous analysis of total N, total C, ¹⁵N and ¹⁴C in small plant and soil samples is described. A commercial C‐N analyser ‐ continuous flow isotope ratio mass spectrometer (ANCA‐MS) has been extended to also measure CO₂ and collect ¹⁴CO₂ produced by sample combustion. Samples containing 20 ‐ 200 μg N and up to 5 mg C can be analysed directly with no sample preparation other than drying and fine grinding. The precision of total elemental analysis is comparable to that by conventional methods. The average standard deviation of ¹⁵N analyses of plant material at natural abundance was ±1 ‰. This is accurate enough for all ¹⁵N studies except those using natural abundance and possibly long term studies of soil organic matter. Recovery of ¹⁴C in test samples was 100%. The instrument can be operated by graduate students under supervision and operating costs are primarily for sample cups, combustion catalyst and quartz tubes.     

Rapid preparation of plant samples for cation analysis

Abstract

In the dry ashing of plant material for chemical analysis, the hot digestion on the steam bath of the ashed sample was replaced by the quantitative addition of dilute HCl and stirring very briefly to dissolve the ash. By waiting a few minutes for the particles to settle, there was no need to filter the solution prior to dilution with an automatic diluter and the determination of K, Na, Ca and Mg by atomic absorption spectrophotometry. The new procedure permits the handling of large batches of samples conveniently, eliminates the use of pipettes and volumetric flasks and decreases the risk of inadvertent contamination by sodium and therefore enhances accuracy. Precision was at least as good as with the standard procedure and the method is expected to be more suitable for use in conjunction with automated analytical techniques.     

Determination of carbon and nitrogen in samples of various soils by the dry combustion

Abstract

Carbon and nitrogen (N) content of various soils in the world were analyzed using a CNS‐2000 (LECO, Corp., St. Joseph, MI) analyzer. The results were in good agreement with those obtained by a laboratory proficiency test at the International Soil‐Analytical Exchange, organized by Wageningen Agricultural University, Wageningen, The Netherlands. The best agreement for both elements was observed at a combustion temperature of 1000°C. Results of organic carbon (C) determined by the Tyurin method were closely related to results of C determination at this temperature. Higher C contents were obtained with samples rich in carbonates when analyzed at higher temperatures >1000°C. These results confirm the suitability of automated dry combustion in soil analysis for C and N.     

Comparison of Analytical Methods for Organic Matter in Composts and Organic Mulches

Abstract

The conventional dichromate (Walkley and Black), carbon analyzer, and weight loss‐on‐ignition (WLOI) methods are compared for determination of organic matter contents in composts and organic mulches. The objective of this study was to evaluate these three methods for their reliability in determining the organic matter contents of composts and organic mulches that also contain inorganic carbon. The carbon analyzer method overestimated organic matter contents for samples containing inorganic carbon (C) as carbonate or charcoal C. The removal of inorganic C improved the correlation coefficients (r) of results obtained by the carbon analyzer method and the Walkley and Black method (0.95 vs. 0.89). The WLOI method produced results more similar to those obtained with the Walkley and Black method than with a carbon analyzer. Oven drying samples for 16–24 h at 105°C as a basal temperature for WLOI improved results compared with a basal temperature at 70°C, which is commonly used. A heating temperature of 500°C for 12 h resulted in organic matter determinations by the WLOI method in the closest agreement with those obtained by the Walkley and Black method.     

Loss‐on‐ignition as an estimator of soil organic carbon in a‐horizon forestry soils

Abstract

The determination of soil organic matter by wet digestion techniques is a slow and laborious analysis. Loss‐on‐ignition (LOI) provides a simple alternative technique for the estimation of soil organic carbon in non‐calcareous A horizon soils of the Natal midlands and Zululand forestry regions. Using multiple regressional techniques, the relationships between loss‐on‐ignition, Walkley organic carbon and soil texture for 55 soils were determined over a range of ignition temperatures. The relationships hold best for soil samples with relatively low organic carbon contents (< 5%). The optimum temperature for ignition was found to occur at 450°C and resulted in the relationship: Soil organic carbon = 0.284*LOI percent. No advantage is gained through ignition at higher temperatures due to the loss of clay mineral structural water, even if the soil texture is accurately known.     

Accurate and Precise Measurement of Organic Carbon Content in Carbonate-Rich Soils

Accurate measurement of soil organic carbon (SOC) is dependent on precise and fast methods for the separation of organic and inorganic carbon. The widely used methods involving thermal decomposition of soil samples at a specific temperature in an automated carbon (C) analyzer are susceptible to interference by carbonates and overestimation of organic C, and thus removal of carbonates by acid pretreatment of samples is recommended. Two carbonate-removal pretreatments including hydrochloric (HCl) acid addition and HCl fumigation are compared using the calcium carbonate (CaCO₃) standard and soil samples of varying SOC contents. Both pretreatment methods provided similar measurements of organic C, indicating that both methods are efficient in removal of carbonates present in the soil. However, the HCl fumigation method exhibited greater accuracy and precision compared to the HCl addition method. Hence, SOC measurement procedure involving HCl fumigation as a pretreatment for the removal of carbonates is recommended for carbonate-rich soils.     

Soil analysis procedures using 0.01 M calcium chloride as extraction reagent

Abstract

This publication gives details of laboratory procedures for the determinations of bioavailable (e.g., plants) quantities of nutritional and polluting inorganic elements in 0.01 M CaCl₂ extracts of air‐dry soil samples. Air‐day soil samples are extracted for two hours with a 0.01 M CaCl₂ solution of 20°C in a 1:10 extraction ratio (W/V). After measuring the pH in the settling suspension, the concentrations of nutritional and polluting elements are measured in the clear centrifugate or filtrate. The procedure is simple, easy to perform, and cheap (labor, chemicals) in daily use in routine soil laboratories. The method receives internationally more and more attention as an alternative for the many extraction procedures for a single nutrient or pollutant that are still in use nowadays. The soil is extracted with a solution what has more or less the same ionic strength as the average salt concentration in many soil solutions. Various nutrients and metals can be measured in a single extract that allows considering relationships between them during interpretation of the data. For most elements, different detection techniques are described in detail in this publication. Detailed laboratory procedures are described for the determination of pH, total dissolved organic carbon, nitrate, ammonium, total dissolved nitrogen, sulphate, total dissolved sulfur, ortho‐phosphate, total dissolved phosphate, sodium, potassium, magnesium, cadmium, copper, nickel, lead, aluminum, iron, arsenic, boron, and phenols. Since only one extract of soil samples is used, profitable use can be made of multi‐element detection techniques like segmented‐flow analysis spectrometry, ICP‐OES, and ICP‐MS.     

Methods for determining organic carbon in podzolic sands

Abstract

We compared three methods of estimating soil organic carbon (dichromate oxidation with and without external heating and loss‐on‐ignition) with a method for total carbon measured in an automated resistance furnace. In 38 surface (0–150 mm) podzolized sands the concentration of total carbon ranged from 0.39 to 4.57% and was highly correlated with all three methods (r²=0.99, p<0.0001, in each case). Dichromate oxidation with and without external heating recovered 99.1 and 88.8% of total carbon respectively, indicating that all carbon was organically bound, and that the ‘heat of reaction’ was not sufficient for total oxidation of organic carbon. Because the organic carbon content of the organic matter varied from 38% to 55% a mean value of 46% would be more appropriate for these soils than the Van Bemmelen factor of 58% for converting organic matter to organic carbon concentration The loss‐on‐ignition procedure represents a precise technique for the estimation of organic matter and, when calibrated, organic carbon in surface sandy soils.     

A rapid and precise method for routine determination of organic carbon in soil

Abstract

A simple method for routine determination of organic carbon in soil by a modified Mebius procedure is described. It involves (a) digestion of the soil sample with an acidified dichromate (K₂Cr₂O₇‐H₂SO₄) solution for 30 minutes in a Pyrex digestion tube in a 40‐tube block digester preheated to 170°C and (b) estimation of the unreacted dichromate by titration of the cooled digest with an acidified solution of ferrous ammonium sulfate with use ofN‐phenylanthranilic acid as an indicator. The method is more rapid and precise than the Mebius procedure commonly used for routine analysis of soils for organic carbon, and the only equipment required for its use is equipment now commonly used for routine Kjeldahl analysis of soils for total nitrogen.     

Evaluation of a rapid microwave digestion method for determination of total organic carbon in soil

Precise measurement of soil organic carbon (SOC) is essential for constructing regional inventories, developing best agricultural management practices, and modeling purposes. Currently, the automated dry combustion method is considered standard, but the method is both costly and time-consuming. There is a need for a simple, easy to use and cost-effective method of organic C determination in soil. A simple method of total organic carbon (TOC) determination in soil that involved wet digestion of K₂Cr₂O₇-H₂SO₄-soil mixture in a commercial microwave oven followed by spectrophotomteric measurement of Cr (III) was evaluated. The method was compared with automated dry combustion and two other wet digestion methods. The method showed close agreement with dry combustion method (R² = 0.90; root mean square error = 0.70) and the TOC measured with the two methods did not differ for a range of soils drawn from lowland and upland land-uses and varying in pH (6.2–9.3), TOC (2.8–14 g kg^?1), and calcium carbonate content (0–6.7%). The recovery of the added organic C by the microwave method was 98.6 ± 4.2%. The results suggested that microwave-spectrophotometric method could be easily adopted in routine soil analysis as it is not only precise, rapid, and cost-effective but also produced small volume of reagent waste.     

Determination of organic matter content in arid‐zone soils using a simple “loss‐on‐ignition” method

Abstract

A simple and rapid procedure for the determination of organic matter content in mineral soils by loss‐on‐ignition without pretreatment was studied. Attention was given to the possible effect of inorganic compounds abundant in mineral soils on the estimation of organic matter content by this method. Both fast heating (DTA‐TGA type) studies and prolonged heating procedures were employed on natural and “synthetic”; soils. The results were compared to those obtained by the dichromate wet‐oxidation method widely used in soil laboratories for organic matter determination. In a group of 91 soils collected from various mineral soils in Israel, and having OM contents between 0.09 and 13.23%, a correlation coefficient of 0.972 was obtained for the linear regression between organic matter content measured by the proposed method and organic carbon measured by the dichromate wet‐oxidation method.     

A simple manual method for the determination of phosphate‐extractable (ion‐exchange membrane) sulphate in soils

Abstract

A simple manual method for measuring phosphate‐extractable sulphate in soils using basic laboratory equipment is described. Sulphate is extracted by a phosphated anion‐exchange membrane which, after elution, is measured turbidimetrically as barium sulphate. The results obtained by the proposed manual method compare well with a continuous flow analyser method using a range of Pacific Island soils.