Automated Modification of the Molybdenum Blue Colorimetric Method for Phosphorus Determination in Soil Extracts

A flow injection analysis (FIA) method capable of automation for molybdate reactive phosphorus (P) determination in soil extracts is described. Results obtained using this method in three soil extracts [calcium chloride (CaCl₂), Olsen, and Mehlich I] were the same as those provided by the manual molybdate blue colorimetric method. Linear range extending to 2 mg P L^?1, detection limits ranging from 6 to 26 µg L^?1 depending on the soil extract, and accurate recoveries from P‐spiked samples were achieved. The sensitivity of the system was around 0.3 absorbance units per mg P L^?1, and the sampling frequency was 72 samples h^?1, higher than those described for most of the flow injection methods.     

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).     

Adaptation and Validation of Colorimetric Methods in Determining Ammonium and Nitrate on Tropical Soils

Inorganic nitrogen (N) determination in soil extracts is a useful tool in studying soil–N interactions. The main method used worldwide is the steam distillation, but colorimetric methodologies show advantages of being quick, simple, sensitive, and higher yield. However, they are subject to disruption when the extract has interfering organic N, limiting its application in tropical soil. The aim of this study is to evaluate colorimetric methods for the determination of NH₄⁺ (through Berthelot reaction) and NO₃^? + NO₂^? (through Griess-Ilosvay reaction) in saline extracts of tropical soils by addition of activated charcoal during the extraction process. It is recommended to use extraction solution of 2 mol L^?1 potassium chloride (KCl) and reaction time of 30 min. Validation parameters confirmed the best performance of the colorimetric methods, revealing results favorable to the addition of the activated charcoal in the extraction process to increase the accuracy and precision of the colorimetric methods.     

Interference by magnesium in the determination of nitrate in 2 M KCl extracts of soil by steam distillation

Abstract

Steam distillation of 2 M KCl extracts of soil showed low recovery of NO₃‐N when compared with an automated method for NO₃‐N determination. The low recoveries were more pronounced in extracts where a soil:solution ratio of 1:2.5 had been used. In extracts where the Mg²⁺ content was in excess of 0.02 M Mg, recoveries of added NO₃‐N could be as low as 25%. Increasing the amount of Devarda's alloy or using a 1:10 soil:solution extraction ratio overcame the problem of low NO₃‐N recovery. Calcium salts did not interfere in the recovery of added NO₃‐N.     

Colorimetric interference and recovery of adsorbed ions from ion exchange resins

Abstract

Analytical interference in the colorimetric determinations of ammonium and nitrate was examined in various KCl extracts of several ion exchange resins. No analytical interference was found in the colorimetric NO₃ ^‐‐N determination in any extract of any resin. However, a mixed‐bed (cation + anion) exchange resin extract substantially affected the colorimetric determination of NH₄ ^‐ ‐N. Recovery of adsorbed ammonium and nitrate from ion exchange resins was also studied as a function of KCl extractant strength and number of extractions. The recovery of adsorbed NO₃ ^‐‐N in the first extraction increased with increasing KCl concentration, with a 2 M solution recovering about 80%. However, a 1 M KCl solution gave the greatest recovery of ammonium‐N, recovering about 75–80% of the adsorbed ammonium. The second extraction with the same concentration of KCl solution was greater with the 0.5 and 2.0 M than with the 1 M solution so that total NH₄ ⁺‐N recovery after two extractions was about the same for all three KCl concentrations. The recovery of resin‐adsorbed NH₄ ⁺‐N and NO₃ ^‐‐N appeared independent of their concentrations on the resins.     

The application of fluorescence detection to the determination and speciation of aluminium in soil solutions by ion chromatography

Abstract

The determination and speciation of aluminium in soil solutions and extracts by ion chromatography with post‐column reaction and fluorescence detection is described. The fluorogenic post‐column reagent consists of 4 x 10^‐3 M 8‐hydroxyquinoline‐5‐sulfonic acid and 2 x 10^‐3 M cetyltrimethylammonium bromide in a 1 M acetate buffer at pH 4.4. The method is applicable to soil solutions and extracts and is more sensitive than existing colorimetric methods. Aluminium species could be detected at concentrations of 35 nM. Nine metals were checked for interference; only Zn²+ and Cd²+ were found to give responses, which were however well separated from the Al³⁺ peak.     

A system for the rapid analysis of organic phosphorus in water samples or fractions from chromotographic columns

Summary

An automatic method for the colorimetric determination of Al in soil extracts using catechol violet was developed. The method is very reproducible and precise. The manifold arrangement covers a range of 0–1 μg ml^?1. By using a dilution loop, solutions containing up to 10 μg ml^?1 can be analysed. The method compares favourable with a manual catechol violet method and the alumin on method.     

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).     

Chloride interference in total nitrogen analysis by the Kjeldahl method

Abstract

Very low recovery of NH₄+‐N was observed in total N determination of (NH₄)₂SO₄ in KC1 solutions by a semimicro Kjeldahl method using permanganate and reduced iron to recover NO₃‐ and NO₂‐, whereas complete recovery was obtained in analysis of NH₄+‐N in water, and of NO₃ ^?‐N or NO₂ ^?‐N in either water or KC1 solutions. The loss of NH₄ ⁺‐N observed with KC1 was attributed to the formation of NCl₃ upon reaction of NH₄ ⁺ with Cl₂ generated during oxidation of Cl^? by MnO₄ ^?. This difficulty is avoided by using K₂SO₄ instead of KC1 for extraction of inorganic N from soil. Complete recovery was obtained by adding ¹⁵N‐labeled NH₄+, NO₃‐, or NO₂‐ to 0.5 M K₂SO₄ soil extracts, and total ¹⁵N analyses of the labeled extracts were in good agreement with values calculated from the additions of ¹⁵N and the total N contents of the soil extracts.     

Automated determination of micro amounts of phosphate in dilute calcium chloride extracts of soils

Abstract

An Auto Analyzer II method for the colorimetric determination of micro amounts of phosphorus (<100 μg/L) in 0.01 M CaCl₂ soil extracts is described. The automated procedure gave results not significantly different from manual determinations.

Using data derived from four soils extracted in triplicate on three separate occasions, the relative confidence intervals for a single sample determination were found to be ± 10.6%.     

Nationally Coordinated Evaluation of Soil Nitrogen Mineralization Rate using a Standardized Aerobic Incubation Protocol

Abstract

Aerobic incubation methods have been widely used to assess soil nitrogen (N) mineralization, but standardized protocols are lacking. A single silt loam soil (Catlin silt loam; fine‐silty, mixed, superactive, mesic, Oxyaquic Arguidoll) was subjected to aerobic incubation at six USDA‐ARS locations using a standardized protocol. Incubations were conducted at multiple temperatures, which were combined based on degree days (DD). Soil water was maintained at 60% water‐filled pore space (WFPS; constant) or allowed to fluctuate between 60 and 30% WFPS (cycle). Soil subsamples were removed periodically and extracted in 2 M potassium chloride (KCl); nitrate (NO₃) and ammonium (NH₄) concentrations in extracts were determined colorimetrically. For each location, the rate of soil organic‐matter N (SOMN) mineralization was estimated by regressing soil inorganic N (N_i) concentration on DD, using a linear (zero‐order) model. When all data were included, the mineralization rate from four datasets was not statistically different, with a rate equivalent to 0.5 mg N kg^?1 soil day^?1. Soil incubated at two locations exhibited significantly higher SOMN mineralization rates. To assess whether this may have been due to pre‐incubation conditions, time‐zero data were excluded and regression analysis was conducted again. Using this data subset, SOMN mineralization from five (of six) datasets was not significantly different. Fluctuating soil water reduced N‐mineralization rate at two (of four) locations by an average of 50%; fluctuating soil water content also substantially increased variability. This composite dataset demonstrates that standardization of aerobic incubation methodology is possible.     

A quick test procedure for soil nitrate‐nitrogen

Abstract

A procedure for extraction and measurement of nitrate‐nitrogen (NO₃‐N) in soil is described. Extracting solution [0.025M Al₂(SO₄)₃] and field‐moist soil are measured volumetrically, with NO₃‐N concentration measured by nitrate‐sensitive colorometric test strips or nitrate‐selective electrode. Across a range of soil texture, moisture content, and NO₃‐N concentration, the procedure was well correlated with conventional laboratory analysis of 2N KC1 soil extracts (r² = 0.94). This quick test procedure is proposed as an on‐farm monitoring technique to improve N management.     

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.     

A comparison of the effectiveness of various extractants for determining humus quality in a calcareous soil

Abstract

Humic substances from an Argentinian chesnut (Mollisol) soil were extracted with Na‐Dowex A‐1 resin, 0.1 N NaOH and 0.1 M Na₄P₂O₇ without decalcification and with 0.1 N NaOH after decalcification. Humic to fulvic (Ch/Cf) acid and E₄/E₆ ratios of the extracts were determined and compared.

A simpler and more rapid procedure than the classical ones for extracting and characterizing soil humus is proposed. The procedure is based on the use of a chelating resin as extractant and on the direct determination of analytical properties of humus in aliquots of the extract.     

Deep‐freezing pretreatment and time of extraction of soil samples for inorganic nitrogen determination

Abstract

Soil samples for inorganic nitrogen (N) determination are usually deep‐frozen to prevent microbial transformations of N between sampling and analysis. For analysis, frozen soils are thawed, which may also lead to transformations of N. A specially manufactured mill for grinding frozen soil was tested to minimize these transformations. Whether the time of extraction of the samples could be extended to 20 hr to better accomondate routine work and to make the clay aggregates to disperse better during extraction was also investigated. Freezing of the samples did not produce different results to fresh soils from ammonium nitrogen (NH₄ ⁺‐N) or nitrate nitrogen (NO₃ ^‐‐N) determination. Thawing of the samples increased the concentration of NO₃ ^‐‐N in the extracts and grinding increased that of NH₄ ⁺‐N. When either thawing or grinding was applied, the total inorganic nitrogen concentration was about the same. Thawing of the ground samples increased concentrations of NO₃’‐N and NH₄ ⁺‐N in the extracts. Extending the time of extraction from 0.5 or 1 hr to 20 hr increased the concentration of NH₄ ⁺‐N in the extracts, while NO₃ ^‐‐N content was also increased slightly. It was concluded that sample pretreatment may cause serious errors in the determination of inorganic N even by methods which have proven most successful to prevent microbial transformations of nitrogen, unless the soils are extracted immediately after sampling. The period of extraction should not exceed two hours.     

Determination of nitrogen by microdiffusion in mason Jars. I. inorganic nitrogen in soil extracts

Abstract

Simple microdiffusion methods are described for determination of NH₄ ⁺, NO₃ ^‐, and NO₂ ^‐ in soil extracts. These methods involve diffusion of NH₃ in a 473‐mL (1‐pint) wide‐mouth Mason jar, the diffused NH₃‐N being collected in 3 mL of boric acid‐indicator solution in a 60 mm (dia.) Petri dish suspended from the Mason jar lid, for quantitative determination by titrimetry (0.0025 M H₂SO₄). Magnesium oxide is used to liberate NH₄ ⁺; Devarda's alloy is used to reduce NO₃‐ and NO₂ ^‐ to NH₄ ⁺; and sulfamic acid is used to eliminate NO₂ ^‐. Depending upon the volume of soil extract (10–50 mL), diffusion at room temperature (a20°C) was complete in 18–72 h with orbital shaking, and in 24–86 h without shaking. The methods gave quantitative recovery of NH₄ ⁺, NO₃ ^‐, and NO₂ ^‐added to soil extracts. A potential source of interference in the methods involving use of Devarda's alloy is the liberation of NH₄ ⁺‐N from alkali‐labile organic‐N compounds.     

Cadmium copperization assays for the preparation of nitrate‐reducing columns

Abstract

An electrolytic method for the copperization of cadmium as a reducing agent for nitrate‐nitrogen (NO₃ ^‐‐N) determination is described. The conditions, medium, and time of copperization as well as the length of the cadmium reducing column has been studied in detail. This column was placed in a flow injection analyzer for the online determination of NO₃ ^‐‐N in soil extracts. The results obtained show that 1% copper sulfate (CuSO₄), 30 min and 55 mm are the optimal medium, time, and column length, respectively. With this reducing column, the method is linear between 0 and 12 ppm NO₃ ^‐‐N with a precision of 0.29% and a sampling frequency of 80 determinations per hour. The proposed method has been applied to the determination of NO₃ ^‐‐N in soil extracts and the results agree with those obtained by the reference method (r = 0.9998). The optimized electrolytic procedure for the copperization of cadmium permits more than 3000 determinations without any significant loss of sensitivity.     

Validation and Recalibration of a Soil Test for Mineralizable Nitrogen

Abstract

A soil test for mineralizable soil N had been calibrated for winter wheat in the Willamette Valley of western Oregon. Seventy‐eight percent of the variation in spring N uptake by unfertilized wheat was explained by N mineralized from mid‐winter soil samples incubated anaerobically for 7 days at 40°C. Mineralizable N (Nmin) ranged from 10 to 30 mg N kg^?1 and was used to predict N fertilizer needs. Recommended rates of N were correlated (R²=0.87) with maximum economic rates of N fertilizer. Subsequent farmer adoption of no‐till sowing and a high frequency of soil tests>30 mg N kg^?1 prompted reevaluation of the soil test. Four N fertilizer rates [0, 56, G, and G+56 kg N ha^?1] were compared in 12 m×150 m farmer‐managed plots. Grower's N rates (G) ranged from 90 to 180 kg N ha^?1 and were based on N_min and NH₄‐N plus NO₃‐N soil tests. Averaged across ten no‐till and five conventionally tilled sites, grain yield and crop N uptake were maximized at the recommended rate of N. Results demonstrate that N fertilizer needs for winter wheat can be predicted over a wide range of mineralizable soil N (10 to 75 mg N kg^?1) and that the same soil test calibration can be used for conventionally sown and direct‐seeded winter wheat.     

Determination of iron in soils by flow injection atomic absorption spectrometry

Abstract

A single‐channel flow injection system was optimized for the determination of available iron (Fe) in soil extracts by atomic absorption spectrophotometry. This method of introducing the samples in the spectrophotometer worked particularly well in preventing blockage of the burner head which was observed in the conventional introduction of Fe for its determination by atomic absorption spectrophotometry. The appropriate selection of the manifold parameters, such as injection volume, tube length and flow rate, allowed introduction of any soil extract without requiring any pre‐treatment. This system allowed determinations at a detection limit of 0.36 mg L^‐1 to 5 mg L^‐1, with an output of 300 determinations per hour. The results obtained for analysis of 15 soil extracts were in good agreement with those provided by the colorimetric method, with average relative deviations of 1.6%. Relative standard deviations of 4.8, 2.5, and 2.3% were obtained for contents of 1.03, 1.85, and 3.99 mg Fe L^‐1, respectively.     

Evaluation of Several Indices of Potentially Mineralizable Soil Nitrogen

Abstract

A routine soil test that accurately predicts soil nitrogen (N) mineralization has long eluded researchers. Soil incubations, which are not practical for routine soil testing, are the only methods that have proven successful. Although several quick analytical methods have been proposed, no one method has correlated consistently enough to be implemented into a soil‐testing program. The objective of this study was to compare proposed quick analytical procedures with the amount of ammonium (NH₄)‐N mineralized after a 14‐d anaerobic incubation. The analytical methods of interest were

1. applying a mild acid oxidation to the soil using acidified permanganate;

2. analyzing a 1 M potassium chloride (KCl) soil extract in the ultraviolet (UV) spectrophotometric range before and after nitrate (NO₃) removal with Devarda's alloy; and

3. quantifying soil amino sugar‐N content using the Illinois soil N test (ISNT) diffusion method.

Fifty agricultural soils with different physical and chemical properties were utilized in this study. All methods displayed significant relationships with the anaerobic incubation procedure; however, some methods displayed higher, more acceptable, coefficients of determination. The ISNT and UV spectrophotometry measurement at 210 nm after NO₃ removal failed to accurately estimate N mineralization (R²=0.45 and 0.31, respectively). The acid oxidation procedure and UV measurement at 260 nm of soil extracts before NO₃ removal produced better results with coefficients of determination of 0.58 and 0.56, respectively. We suspect the ability of some methods to predict N mineralization was hindered because of the wide variety of geographic locations from which the soils were collected. Additional analyses were conducted on a subset of 16 silt‐loam soils from Arkansas. The coefficient of determinations increased for each method: acid oxidation procedure increased to 0.83, the ISNT increased to 0.71, and the UV method (at 260 nm before NO₃ removal) increased to 0.63. If anaerobic incubation is a true indication of N mineralization in the field, the aforementioned methods display promise to correlate with N uptake by field‐grown plant studies when adapted to a specific geographic location and/or soil series.