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.     

Use of diffusion for automated nttrogen‐15 analysis of soil extracts

Abstract

Studies to evaluate the use of diffusion for automated ¹⁵N analysis of inorganic N in soil extracts showed that serious error can arise from use of the Devarda's alloy recommended for steam distillations and that the error can be avoided by using a commercial product of higher purity. These studies showed that serious error can also arise when NO₃ ^‐‐N is diffused following NH₄ ⁺‐N and that separate diffusions should be performed for NH₄ ⁺‐N and (NH₄ ⁺ + NO₃‐)‐N. Other work demonstrated that the plastic specimen containers employed for diffusion can be reused if acid‐washed, that diffusions can be performed using either light or heavy MgO without ignition to decompose carbonate, and that labeled NO₂‐is completely removed from soil extracts by treatment with sulfamic acid before diffusion. A comparison of ¹⁵N analyses by steam distillation and diffusion using extracts from two soils revealed better agreement for the soil having a lower content of organic matter. Substantial differences in analyses by the two techniques for the soil having a higher organic‐matter content were attributed to enzymatic conversions of inorganic N during the 6‐d diffusion period.     

Evaluation of methods for nitrogen‐15 analysis of inorganic nitrogen in soil extracts: I. Steam‐distillation methods

Abstract

A study was conducted to evaluate conventional steam‐distillation techniques for N‐isotope analysis of inorganic forms of N in soil extracts. Extracts obtained with 2 M KCl from 10 diverse soils were treated with: (i) (¹⁵NH₄)₂SO₄ and KNO₃, (ii) (NH₄)₂SO₄ and K¹⁵NO₃, or (iii) KNO₃and Na¹⁵NO₂. Steam distillations were performed sequentially to determine NH₄ ⁺‐N and NO₃ ^‐‐N, and were also carried out to determine (NO₃ ^‐ + NO₂ ^‐)‐N or (NH₄ ⁺ + NO₃ ^‐ + NO₂ ^‐)‐N; a pretreatment with sulfamic acid was used to determine NO₃ ^‐‐N in the presence of NO₂ ^‐‐N. Recovery of added N ranged from 95 to 102%. Significant isotopic contamination was observed in sequential distillation of unlabeled NO₃ ^‐‐N following labeled NH₄ ⁺‐N; otherwise, analyses for ¹⁵N were usually within 1% of the values calculated by isotope‐dilution equations.     

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.     

Yield of take‐all infested winter wheat as influenced by inhibiting nitrification with dicyandiamide

Abstract

The potential for using dicyandiamide (DCD) to enhance yield of take‐all‐infested winter wheat (Triticum aestivum L.) was evaluated in six field experiments on four acid soils (pH 5.7–6.2). Ammonium and NO₃ ^‐ concentrations and NH₄ ⁺: NO₃ ^‐ ratios in 0–10 and 10–20 cm soil depths were measured for ten weeks after spring topdressing 180 kg N/ha as urea with 0, 13, or 27 kg DCD/ha. Nitrification was strongly inhibited for 6 to 10 weeks by either 13 or 27 kg DCD/ha. Averaged over the ten‐week sampling period, NH₄ ⁺: N0₃ ^‐ ratios in the 0–10 cm depth of soil were 36: 1 for DCD‐treated plots as compared to 2: 1 for plots receiving only urea. Ratios in DCD‐treated plots were considerably wider than ratios associated with take‐all suppression (10: 1 to 3: 1) in earlier studies. Extractable NH₄ ⁺ + NO₃ ^‐ concentrations in soil were high in DCD‐treated plots after 30 to 40 days, suggested that DCD had reduced crop uptake of N because of the lower mobility of NH₄ ⁺ as compared to NO₃ ^‐. In four of the six studies, grain yields tended to be reduced by DCD. Results suggest that lower rates of DCD and/or application of some NO₃ ^‐ will be necessary if DCD is to be used as a tool for suppressing take‐all.     

Effects of solution pH,temperature, nitrate/ammonium ratios,and inhibitors on ammonium and nitrate uptake by Arabica coffee in short‐term solution culture

Solution pH, temperature, nitrate (NO₃ ^‐)/yammonium (NH₄ ⁺) ratios, and inhibitors effects on the NO₃ ^‐ and NH₄ ⁺ uptake rates of coffee (Coffea arabica L.) roots were investigated in short‐term solution culture. At intermediate pH values (4.25 to 5.75) typical of coffee soils, NH₄ ⁺ and NO₃ ^‐ uptake rates were similar and nearly independent of pH. Nitrate uptake varied more with temperature than did ammonium. Nitrate uptake increased from 0.05 to 1.01 μmol g^‐1 FWh^‐1 between 4 and 16°C, and increased three‐fold between 16 to 22°C. Between 4 to 22°C, NH₄ ⁺ uptake rate increased more gradually from 1.00 to 3.25 μmol g^‐1 FW h^‐1. In the 22–40°C temperature range, NH₄ ⁺ and NO₃ ^‐ uptake rates were similar (averaging 3.65 and 3.56 umol g^‐1 FW h^‐1, respectively). At concentrations ranging from 0.5 to 3 mM, NO₃ ^‐ did not influence NH₄ ⁺ uptake rate. However, NO₃ ^‐ uptake was significantly reduced when NH₄ ⁺ was present at 3 mM concentration. Most importantly, total uptake (NO₃ ^‐+NH₄ ⁺) at any NO₃ ^‐/NH₄ ⁺ ratio was higher than that of plants fed solely with either NH₄ ⁺ or NO₄ ^‐. Anaerobic conditions reduced NO₃ ^‐ and NH₄ ⁺ uptake rate by 50 and 30%, respectively, whereas dinitrophenol almost completely inhibited both NH₄ ⁺ and NO₃ ^‐ uptake. These results suggest that Arabica coffee is well adapted to acidic soil conditions and can utilize the seasonally prevalent forms of inorganic N. These observations can help optimizing coffee N nutrition by recommending cultural practices maintaining roots in the temperature range optimum for both NH₄ ⁺ and NO₃ ^‐ uptake, and by advising N fertilization resulting in a balanced soil inorganic N availability.     

An evaluation of some manual colorimetric methods for the determination of inorganic nitrogen in soil extracts

Abstract

A number of manual colorimetric methods for the determination of inorganic nitrogen in 1 M KCl soil extracts were investigated to find techniques that were inexpensive, rapid, versatile and suitable for laboratories with limited analytical equipment. Three colorimetric methods for No^? ₃‐N determination were evaluated and only the copperised/cadmium reduction technique suffered no significant interference from the Cl^? present in the extracting solution. A phenol‐hypo‐chlorite (Berthelot) procedure for NH⁺ ₄‐N determination and the Griess‐Ilosvay method for NO^? ₂‐N determination were both found suitable for N determination in 1M KC1 soil extracts. The reliability and accuracy obtainable with the manual colorimetric methods described was shown to be comparable with that obtained from colorimetric analyses performed using an AutoAnalyser.     

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 surface charge density of soils

Abstract

A simple and precise method for determination of surface charge density of soils is described. It involves saturating the negative and positive exchange sites with NH⁺ ₄ and NO^‐ ₃ ions, respectively, removing the excess solution by centrifugation, and determining the ions on the exchange sites by a steam distillation method. Results showed that the concentration and type of saturating ions and the extractant used significantly affected the surface negative charge densities. The average value of surface negative charge densities of 10 surface soils from Iowa, Chile, and Costa Rica by the proposed method agreed closely with that obtained by the original method proposed by Schofield in 1949 (14.6 vs. 13.7 cmol(‐) kg^‐1 soil). The advantages of the proposed method are no need for the laborious extraction steps and simplicity of the steam distillation method for determination of NH⁺ ₄ and NO^‐ ₃ in soil samples.     

Method for the quantification of acid production by plants in gel

Abstract

Hydrogen (H⁺) and hydroxyl ion (OH^‐) production by the tropical grass, Brachiaria humidicola, is quantified using a method in which the plants are grown in soil then transferred to agar gel for 24 h. The amount of H⁺ and OH^‐produced was calculated from the pH of the melted gel and the gels’ buffer curve. Values were obtained for plants of different ages and with nitrogen (N) supplied in the gel as nitrate (NC₃ ^‐), ammonium (NH₄ ⁺), or ammonium nitrate (NH₄NO₃) and compared with data calculated using the sum of H⁺ changes in differently colored zones of the gel. Daily H⁺ and OH^‐ production increased with plant age and total dry matter for the NH₄ ⁺‐ and NO₃ ^‐‐fed plants, respectively. By integrating the data over time, a value of 0.33 mmol H⁺ plant^‐1 was obtained for the total H⁺ production over 62 d. The proposed method was sufficiently rapid and versatile to allow the comparison between plant species or genotypes, which were grown using a variety of nutrient supplies. This procedure may indicate how acid production affects plant nutrient acquisition and aid the prediction of soil acidification by different plant species or cultivars.     

Diffusion of NH+4 and NO−3 mineralized from organic N in soil

The mineralization of native soil organic matter and the simultaneous diffusion of zero NH⁺₄ and NO^?₃ to a solution sink of zero N concentration was analysed experimentally and theoretically for a fine sandy loam soil. Experimentally, the NH₄ and NO₃ ions produced in an incubated unsaturated soil column were allowed to diffuse through a sintered glass plate into a stirred solution sink. The distribution of NH⁺₄ and NO^?₃ in the soil column was measured after various incubation times. The rate of ammonification was measured directly during incubation and the rate of nitrification modelled from nitrifier growth kinetics. A Freundlich equation was used to describe the equilibrium between soluble and exchangeable NH⁺₄ in the soil. Terms for the microbial transformation of N and the adsorption-desorption of NH⁺₄ were combined with diffusion equations which were solved numerically using finite difference methods. The model constructed was used to predict the NH⁺₄ and NO^?₃ con-centration distributions in the soil column, and good agreement was obtained between the experimental and predicted concentration profiles. The use of the model for predicting the diffusive flux of mineral N to the outer surfaces of soil peds, where it is vulnerable to leaching, was demonstrated.     

Effects of cold storage on anion,ammonium, and total nitrogen concentrations in soil water

Abstract

Researchers frequently must store water samples >24 h after collection until chemical analyses can be accomplished. Samples are commonly stored in darkness at near‐freezing temperatures until analysis, but effects of this storage method on soil water chemistry are not well‐documented. Soil water samples were collected from eastern hemlock [Tsuga canadensis (L.) Carriere] stands and analyzed initially and over time to determine if nitrate (NC₃ ^‐), chloride (Cl^‐), sulfate (SO₄ ^2‐), ammonium (NH₄ ⁺), and/or total nitrogen (TN) concentrations changed when samples were stored in darkness at 2–4°C. Subsamples were analyzed <24 h after collection and reanalyzed five times in a 12 to 24 wk period following sample collection. Nitrate concentrations remained stable for at least one wk and were only slightly lower than initial concentrations after 3–16 wk of storage. Chloride and SO₄ ^2‐ remained stable for 16 wk, while TN remained stable for 8 wk and decreased slightly after 24 wk. Most NH₄ ⁺ concentrations were below the limit of quantification, but concentrations of samples with quantifiable NH₄ ⁺ decreased in the first week of storage. Overall, cold storage in darkness was effective for preserving NO₃ ^‐, Cl^‐, SO₄ ^2‐, and TN concentrations in soil water, but NH₄ ⁺ analyses should probably be performed as soon as possible after sample collection.     

Effect of tebuthiuron on soil N mineralization and nitrification

Abstract

Herbicides have potential for economical and efficient site preparation following timber harvest. The effects of tebuthiu‐ron, one of the herbicides approved for this use, on soil nitrogen (N) mineralization and nitrification were determined in laboratory incubations. Tebuthiuron was added at rates from 0 to 1000 μg g^‐1 to three soils. There was no effect of tebuthiuron additions of less than 1 μg g^‐1 on soil N mineralization and nitrification. Tebuthiuron reduced nitrification in all soils at 1000 μg g^‐1 and in two of the soils at 100 μg g^‐1 . All soils had increased net mineralization with tebuthiuron added at 100 and 1000 μg g^‐1. The addition of 50 μg NH⁺ ₄‐N and 1000 μg tebuthiuron g^‐1 resulted in increased net mineralization in the three soils. Nitrification was affected differently in each of the three soils by the addition of both NH⁺ ₄‐N and tebuthiuron. The added NH⁺ ₄‐N either removed the inhibition of nitrification by the herbicide or had no effect on the inhibition in two of the soils. In the third soil, nitrification was reduced by the addition of NH⁺ ₄‐N.

The presence of NO^‐ ₃‐N in these acid soils and the effects of added NH⁺ ₄‐N on NO^‐ ₃‐N production suggest that heterotrophic nitrification occurs in at least two of the soils. The findings of this study indicate that any effects of tebuthiuron on N mineralization and nitrification at the currently recommended application rates are likely to be transient and localized.     

Evidence that fungi can oxidize NH4+ to NO3− in a grassland soil

The contribution of bacteria and fungi to NH₄⁺ and organic N (N_org) oxidation was determined in a grassland soil (pH 6.3) by using the general bacterial inhibitor streptomycin or the fungal inhibitor cycloheximide in a laboratory incubation study at 20°C. Each inhibitor was applied at a rate of 3 mg g^?1 oven‐dry soil. The size and enrichment of the mineral N pools from differentially (NH₄¹⁵NO₃ and ¹⁵NH₄NO₃) and doubly labelled (¹⁵NH₄¹⁵NO₃) NH₄NO₃ were measured at 3, 6, 12, 24, 48, 72, 96 and 120 hours after N addition. Labelled N was applied to each treatment, to supply NH₄⁺‐N and NO₃^?‐N at 3.15 μmol N g^?1 oven‐dry soil. The N treatments were enriched to 60 atom % excess in ¹⁵N and acetate was added at 100 μmol C g^?1 oven‐dry soil, to provide a readily available carbon source. The oxidation rates of NH₄⁺ and N_org were analysed separately for each inhibitor treatment with a ¹⁵N tracing model. In the absence of inhibitors, the rates of NH₄⁺ oxidation and organic N oxidation were 0.0045 μmol N g^?1 hour^?1 and 0.0023 μmol N g^?1 hour^?1, respectively. Streptomycin had no effect on nitrification but cycloheximide inhibited the oxidation of NH₄⁺ by 89% and the oxidation of organic N by more than 30%. The current study provides evidence to suggest that nitrification in grassland soil is carried out by fungi and that they can simultaneously oxidize NH₄⁺ and organic N.     

Correlation of soil test nitrogen with potato yields

Abstract

Yield response of Idaho Russet Burbank potatoes to nitrogen fertilizer was related to soil test for inorganic N (NO₃ ^‐ and NH₄ ⁺) in a total of 27 field experiments over a 3‐year period using polynomial correlation and regression analysis. Nitrate plus ammonium nitrogen content in the surface foot of soil was found to be useful in predicting yield response of potatoes to applied nitrogen.

Correlations between yield and extractable N were considerably better when the data from each cropping system were analyzed separately than when all locations were analyzed as one group. Additional improvement was obtained by including extractable ammonium nitrogen and nitrogen in the second foot of soil respectively. The best correlation with yield was found using (NO₃ ^‐ + NH₄ ⁺)‐N in the surface foot following grain and the top two feet following non‐grain crops with R² values of 0.875 and 0.821 respectively.     

Inorganic N analysis of soil extracts by automated and distillation procedures

Abstract

A comparison was made of automated procedures and steam distillation for analysis of NH₄ ⁺ and NO₃ ^‐ in KCl extracts of soils. Automated analysis of NH₄ ⁺ utilized the phenate‐hypochlorite reaction. Nitrate was analyzed by E. coli reduction of NO₃ ^‐ to NO₂‐which was then determined colorimetrically by reaction with sulfanilamide and napthylethylenediamine diHCl. Soil extracts were made with 1 M and 2 M KCl. Extracting with 1 M KCl was as effective as 2 M KCl. Extracts in 1 M KCl did not interfere with E. coli reduction of NO₃‐, but 2 M KCl did. The automated procedures when compared to distillation gave similar if not better precision and reduced variation particularly at low N concentrations. The automated procedure tended to give higher values at higher NO₃‐concentrations. Reduced time required for the analyses provided an additional advantage for the automated procedures.     

Influence of ammonium,nitrate, and chloride on solution pH and ion uptake by ageratum and salvia in hydroponic culture

The influence of nitrogen (N) forms and chloride (Cl) on solution pH and ion uptake in the hydroponic culture of Ageratum houstonianum [ammonium (NH₄ ⁺)‐tolerant] and Salvia splendens (NH₄ ⁺‐sensitive) for a period of 216 hours was investigated. The pH of the hydroponic solution (initially 6.50) containing either NH₄ ⁺ or NH₄ ⁺+nitrate (NO₃ ^‐) was drastically lowered (3.08), whereas that of the same solution containing NO₃ ^‐ was raised (7.74). Solution pH changed more by ageratum than by salvia. The solution Cl^‐ concentration did not influence pH significantly. However, addition of Cl^‐ in the solution lowered transpiration rate in both NH₄ ⁺ and NO₃ ^‐ treatments. Total N uptake was the greatest in the NH₄ ⁺ + NO₃ ^‐ treatment and the lowest in the NO₃ ^‐treatment. In the NH₄ ⁺ + NO₃ ^‐ treatment, NO₃ ^‐ uptake was suppressed by NH₄ ⁺ (to about 50%), while NH₄ ⁺ uptake was not affected by NO₃ ^‐. The rate of Cl^‐ uptake was the lowest in the NH₄ ⁺ treatment, but was similar in the NH₄ ⁺ + NO₃ ^‐ and NO₃ ^‐ treatments. Uptake of potassium (K⁺), dihydrogen phosphate (H₂PO₄ ^‐), sulfate (SO₄ ^‐2), manganese (Mn⁺²), and zinc (Zn⁺²) was significantly enhanced in the NH₄ ⁺ treatment. The uptake rate of calcium (Ca⁺²) and magnesium (Mg⁺²) was the highest in the NO₃ ^‐ treatment. Absorption of copper (Cu⁺²) and boron (B) was not affected by N source. Ion uptake was more stable in the solution containing both NH₄ ⁺ and NO₃ ^‐ than in the solution containing either NH₄ ⁺ or NO₃ ^‐. The uptake rate of total N, NH₄ ⁺, NO₃ ^‐, Mn⁺², Cu⁺², and Zn⁺² was higher, whereas that of Cl^‐ and molybdenum (Mo) was lower in ageratum than in salvia. Amounts of total anion (TA) and total cation (TC) absorbed, the sum of TC and TA, and the difference between TC and TA (TC‐TA) were affected by N source, Cl^‐ level, and their interactions. The NO₃ treatment, as compared to the NH₄ ⁺ or the NH₄ ⁺ + NO₃ ^‐treatment, reduced total cation and anion uptake while increasing TC‐TA, especially in the absence of Cl^‐. Plant tissue ion contents were also affected by N source and Cl^‐ level.     

Rate of nitrapyrin and soil ph effects on nitrification of ammonium fertilizer and growth and composition of burley tobacco

Abstract

Laboratory and greenhouse experiments were conducted to determine the effects of rate of nitrapyrin and soil pH on nitrification of NH₄ ⁺ fertilizer in soil, and growth and chemical composition of burley tobacco (Nicotiana tabacum L. cv. ‘KY ‐14'). Such experiments were needed to develop information for increasing efficiency of N fertilizer use and to lessen the fertilizer‐induced soil acidity and salt effects on tobacco plants.

Results for laboratory and greenhouse incubations indicated that nitrification proceeeded slowly below pH 5.0 and the nitrapyrin necessary to delay nitrification increased with both increasing soil pH and length of incubation time. Generally, nitrification could be delayed 30 days by nitrapyrin rates of 0.25 or 0.5 μg g^‐1 regardless of soil pH. but rates of 1 μg g^‐1 nitrapyrin or higher were required for 60 days and longer incubation times, particularly at higher soil pH.

Growth and morphology of tobacco plants were either unaffected, or affected positively, by low rates of nitrapyrin (up to 2 μg g^‐1). However, rates of 4 μg g^‐1 and above reduced total plant dry weight, reducing sugars and contents of mineral elements. Concentrations and content of plant NO₃ ^‐ N and Mn were greatly decreased by application of nitrapyrin. Values for most parameters measured increased with increasing soil pH. The data show that low rates of nitrapyrin may be used to alter the ratio of NO₃ ^‐ to NH₄ ⁺ N absorbed by tobacco and possibly improve growth and safety of tobacco.     

Presidedress soil nitrogen test for corn in Virginia

Abstract

The presidedress soil nitrate test (PSNT) and the presidedress tissue nitrogen test (PTNT) have been developed to assess residual soil nitrogen (N) sufficiency for corn (Zea mays L.) in the humid eastern U.S. We conducted field studies at 47 sites during 1990 and 1991 to evaluate the use of the PSNT and PTNT for corn in Coastal Plain, Piedmont, and Appalachian Ridge and Valley regions of Virginia. Seven rates of fertilizer N (0, 45, 90, 135, 180, 225, and 270 kg/ha) were applied at corn height of 0.40 to 0.50 m and replicated four times in a randomized complete block design. Whole corn plants and soil to a depth of 0.30 m were sampled when corn height was 0.15 to 0.30 m to estimate available soil N prior to the application of fertilizer N treatments. Corn grain yield response to fertilizer N was used to assess residual soil N availability. Nitrogen concentration of whole corn plants at 0.15 to 0.30 m height was not an accurate indicator of plant‐available soil N. Corn yields were maximized without sidedress N at the 19 sites where soil NO₃‐N was at least 18 mg‐kg^‐1 and at the 17 sites where soil (NO₃+NH₄)‐N was at least 22 mg‐kg^‐1. The PSNT predicted corn N sufficiency regardless of soil physiographic region or surface texture; however, the critical values for NO₃‐N and (NO₃+NH₄)‐N were 3 to 5 mg‐kg^‐1 lower than those established in Pennsylvania and Maryland, where cooler soil temperatures may permit greater residence time of inorganic N.     

Development of a New Soil Extractant for Simultaneous Phosphorus,Ammonium, and Nitrate Analysis

Abstract

A new soil extractant (H³A) with the ability to extract NH₄, NO₃, and P from soil was developed and tested against 32 soils, which varied greatly in clay content, organic carbon (C), and soil pH. The extractant (H³A) eliminates the need for separate phosphorus (P) extractants for acid and calcareous soils and maintains the extract pH, on average, within one unit of the soil pH. The extractant is composed of organic root exudates, lithium citrate, and two synthetic chelators (DTPA, EDTA). The new soil extractant was tested against Mehlich 3, Olsen, and water for extractable P, and 1 M KCl and water‐extractable NH₄ and NO₂/NO₃. The pH of the extractant after adding soil, shaking, and filtration was measured for each soil sample (5 extractants×2 reps×32 soils=320 samples) and was shown to be highly influential on extractable P but has no effect on extractable NH₄ or NO₂/NO₃. H³A was highly correlated with soil‐extractable inorganic N (NH₄, NO₂/NO₃) from both water (r=0.98) and 1 M KCl (r=0.97), as well as being significantly correlated with water (r=0.71), Mehlich 3 (r=0.83), and Olsen (r=0.84) for extractable P.