Illinois Fertilizer
Conference Proceedings
January 23-25, 1995
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H.M. Brown, R.L. Mulvaney, and R.G. Hoeft1
The Horiba Cardy nitrate meter provided a rapid, simple method of nitrate-N determination in soils and aqueous solutions. It provided quantitative recovery of nitrate-N added to soil/extract suspensions with an acceptable level of precision. Results obtained were in close agreement with nitrate-N determinations by extraction-steam distillation. Anions, such as chloride and nitrite caused interference at low concentrations of nitrate-N. However, use of an extraction solution containing 20 ppm nitrate-N provided a background level of nitrate-N which minimized anion interference. The battery-powered selective electrode is a reliable "on site" instrument for rapid determination of soil and solution nitrate-N concentrations.
The use of nitrogen fertilizer, whether organic or synthesized inorganic forms, provides impact from both an economic and environmental point of view on Midwest U.S. corn production. Under-applications of supplemental nitrogen may result in less than economic optimum yields for the grower, increasing per-unit costs. Over-applications may pose a risk to drinking water sources through surface and/or ground water contamination.
To avoid over-fertilization, proper credit must be given for the soil supply of plant-available nitrogen. Magdoff et al. (1984) found that recommendations based on a soil nitrate test called for less nitrogen that the traditional recommendation system while still providing enough nitrogen for maximum yield. Morris and Blackmer (1989, 1990) reported that of 45 fields tested in 1988 and 1989, most had greater concentrations of nitrate-N than was needed to obtain optimum corn yields. The use of a nitrate-N test at presidedress time (Blackmer et al., 1991), allowed credit for soil nitrate-N already present, reducing the need for supplemental N applications. Brown et al.(1993) suggested a soil nitrate-N test may be useful when determining nitrogen needs in fields where manure has been surface applied with delayed or no incorporation.
Analyses for soil NO3-N are normally performed in the laboratory
using specialized equipment (Keeney et al., 1982). Although such analyses are
generally reliable, this approach is not ideally suited for soil testing because
of the high cost and the unavoidable delay in supplying the results to the grower
(Hartz, 1993; Hoeft et al., 1992). Test kits have been developed for quick "on
farm" determination of soil nitrate-N by colorimetry (Hach Company, 1990),
however, difficulties have arisen because of differences in visual interpretation
of color development.
An alternative to test kits for "on-farm" determination of soil NO3-N
is the Cardy NO3 meter, manufactured by Horiba Instruments Company, Japan. The
meter is comprised of a NO3-selective and reference electrode built into a replaceable
sensor. Recent work by Hartz et al. (1993) suggests the Horiba Cardy NO3 meter
will offer "a quick and reliable way to measure crop and soil NO3-N in
an out-of-the laboratory setting". However, the work was limited to plant
sap NO3-N determinations and included little mention of work with soils.
Use of the NO3-selective electrode in laboratory determination of soil NO3-N is thoroughly discussed by Keeney et al. (1993). Limitations to NO3-N selective electrode use in the laboratory includes the affect of interfering ions on NO3-N determinations, and the stability of the electrode's measurement.
High concentrations of some anions interfere with NO3-N determinations when using a nitrate selective electrode, especially at the lower concentration of NO3-N. Myers and Paul (1968) found the major factor affecting the accuracy of the nitrate electrode was interference by other anions, such as nitrite, chloride, and bicarbonate. However, they suggested that interference from these anions would be minimal, since nitrite occurs rarely in field soils, and chloride and bicarbonate concentrations in most arable soils should have little effect as interfering anions.
The purpose of this study was to evaluate the Horiba Cardy NO3 meter's
accuracy and precision in measuring soil NO3-N concentrations. Meter
response to a range of interfering ion concentrations was also examined.
Soil samples were collected from 12 sites throughout the state of Illinois that varied in soil type, fertilizer level, and organic matter content. Samples were collected from a 12-inch depth, airdried, and crushed to pass a 2 mm screen. After thorough mixing they were stored at room temperature in air-tight plastic containers.
Nitrate-N solutions were prepared using reagent-grade potassium nitrate. Nitrate-N concentrations were evaluated in both deionized water and the standard extraction solution2 to evaluate any affect of the solution's composition. Reagent-grade sodium nitrite, potassium chloride, and sodium bicarbonate were used to prepare concentrations for the interfering anions.
The Horiba-Cardy Nitrate Meter consists of a miniature ion- selective and reference electrode mounted in a replaceable plastic insert, and a two-digit LCD display. The instrument weighs approximately 40 grams and is 3.7 inches long, 2.2 inches wide, and .35 inches thick. The Manufacturer claims the following resolutions for nitrate-N determinations: 1 ppm for samples in the range of 0-99 ppm, 10 ppm for 100-990 ppm, and 100 ppm for 1000 to 9900 ppm. NitrateN standards of 450 and 20 ppm were used to calibrate the meter. Both the high and low standards were checked every six samples to correct for fluctuations in electrode sensitivity.
Thirty-gram samples of soil were placed in plastic bottles and hand-shaken for 2 minutes after treatment with 30 ml of the extraction solution prepared by Ag Spectrum Technologies. The resulting suspensions were allowed to stand for 5 minutes and were then filtered using Whatman #1 filter paper placed in glass funnels. The filtrate was analyzed for NO3-N by placing a 0.7 ml aliquot on the meter's sensor with a pipet. The meter reading was recorded after allowing sufficient time for equilibration. Between samples, the sensor was rinsed twice with deionized water. Data presented are the mean of 5 replicate determinations.
Nitrate-N was also measured by the extraction-distillation method described
by Bremner and Keeney (1966). Data presented is the mean of 3 replicate determinations.
Figures 1 and 2
summarize the results obtained when NO3-N analyses were performed
with the Cardy Nitrate Meter on NO3 solutions of known concentrations
prepared in deionized water or the premixed extraction solution.
In both cases, the measured concentrations were highly correlated with the actual
concentrations. Measurements above 100 ppm showed greater variability than lower
measurements because the meter can only display two digits. A factor of 10 or
100, indicated by the display, is used when the NO3-N concentration exceeds
99 ppm.
Accuracy was also evaluated through recovery tests involving known additions of NO3-N to soil samples. The results (Table 1) indicate that the Cardy Card Nitrate Meter is generally accurate within a range of 0 to 60 ppm NO3-N. In most cases recoveries were accurate to within 1 ppm. The largest deviations were observed with soil #4, which was collected from a field with a history of manure applications and likely had a high content of readily available organic N.
The precision of the Cardy Nitrate Meter is illustrated by Table 2, which summarizes the data obtained when 10 replicate analyses were performed on two different soils. In both cases, the coefficient of variation was approximately 2%, which closely agrees with work performed by Varsa (1992).
Table 3 summarizes the results of a study to compare soil NO3-N analyses by the Cardy Meter to those by the most widely accepted method, involving steam distillation of 2M KCl soil extracts (Bremner and Keeney, 1966). Table 3 shows that the two methods were usually in close agreement and gave similar precision. Accuracy of the Cardy Meter at low NO3-N concentrations is limited by the fact that the display registers only to the nearest whole number.
Conventional NO3 electrodes are subject to interference by various anions, particularly Cl, HCO3, and NO2, (Meyers and Paul, 1968; Onken and Herbert, 1970). This study showed that the Cardy meter is subject to the same interferences (Table 4). Although NO2 causes the greatest interference, Cl interference is more serious, since NO2 does not normally accumulate in field soils and Cl is added to many soils through commercial fertilizer applications. The data in Table 4 show that interference by Cl must be considered when using the Cardy Meter to measure NO3-N concentrations less than 20 ppm. However, chloride interference would be minimized for soil determinations with the Cardy meter, since the extraction solution contains an initial background concentrations of 20 ppm NO3-N. Bicarbonate showed the least effect as an interfering anion of those anions tested. No interference was detected with up to 800 ppm of HCO3 at the lowest concentration of NO3-N (15 ppm) and the highest bicarbonate (800 ppm) during the initial screening.
The Cardy Card Nitrate Meter demonstrated its ability to be a reliable tool
to use in the determination of nitrate-N concentrations in soils with both accuracy
and precision. It should serve as a helpful tool to those who desire to monitor
on-farm soil nitrate-N concentrations.
Table 1: Recovery of NO3-N added to soils
Table 2: Precision of NO3-N analyses with the Cardy Card Nitrate Meter
Table 4: Effect of Cl and NO2 on NO3-N analyses with the Cardy Nitrate Meter
Figure 1. Recovery of nitrate-N from known concentrations prepared in deionized water
Figure 2. Recovery of nitrate-N from known
concentrations prepared in extraction solution
1H.M. Brown is Research Assistant and R.L. Mulvaney and R.G. Hoeft are Professors of Agronomy, Department of Agronomy, University of IL.
2Extraction solution obtained from Ag Spectrum Technologies composed of .25N Ammonium Sulfate, Sodium nitrate, and preservatives.
Blackmer, A. M., T. F. Morris, D. R. Keeney, R. D. Voss, and R. Killorn. 1991. Estimating nitrogen needs for corn by soil testing. Iowa State University Extension Bulletin. Pm-1381. Ames, Iowa.
Blackmer, A.M., D. Pottker, M. E. Cerrato, and J. Webb. 1989. Correlations between soil nitrate concentrations in late spring and corn yields in Iowa. J. Prod. Agric. 2:103-109.
Bremner, J.M., L.G. Bundy, and A.S. Agarwal. 1968. Use of a selective ion electrode for determination of nitrate in soils. Anal. Lett. 1:837-844.
Bremner, J.M., and D.R. Keeney. 1966. Determination and isotope-ratio analysis of different forms of nitrogen in soils: 3. Exchangeable ammonium nitrate, and nitrite by extraction-distillation methods.
Brown, H. M. , R. G. Hoeft, and E. D. Nafziger. 1993. Evaluation of three N recommendation systems for corn yield and residual soil nitrate. in Ill. Fert. Conf. Proc. (R. G. Hoeft, ed.) p43-49.
Hartz, T. K. , R. F. Smith, M. LeStrange, and K. F. Schulbach. 1993. On-farm monitoring of soil and crop nitrogen status by nitrate-selective electrode. Commun. Soil Sci. Plant Anal. 24(19 and 20) 2607-2615.
Hoeft, R.G., H.M. Brown, D. Mengel, D.J. Eckert, and M.L. Vitosh. 1992. Predicting N fertilizer needs for corn in humid regions. Bull. y-226. National Fertilizer and Environmental Research Center, Tennessee Valley Authority, Muscle Shoals, AL.
Kenney, D.R., B.H. Byrnes, and J.J. Genson. 1970. Determination of nitrate in waters with the nitrate-selective ion electrode. Analyst (London) 95:383-386.
Magdoff, F.R., W.E. Jokela, R.H. Fox, and G.R. Griffin. 1990. A soil test for N availability in the northeastern United States. Commun. Soil Sci. Plant Anal. 2:1103-1115.
Magdoff, F.R., D. Ross, and J. Amadon. 1984. A soil test for nitrogen availability to corn. Soil Sci. Soc. Am. J. 48:1301-1304.
Mahendrappa, M.K.. 1969. Determination of nitrate nitrogen in soil extracts using a specific ion activity electrode. Soil Sci. 100:132-136.
Milham. P.J., A.S. Awad, R.E.Paull, and J.H. Bull. 1970. Analysis of plants, soils, and waters for nitrate by using an ion selective electrode. Analyst (London). 95:751-757.
Myers, R.J.K., and E.A. Paul. 1968. Nitrate ioin electrode method for soil nitrate nitrogen determinations. Can. J. Soil Sci. 48:369-371.
Onken, A.B. and H.D. Sunderman. 1970. Use of the nitrate electrode for determination of nitrates in soil. Commun. Soil Sci. Plant Anal. 1:155-161.
Sommerfeldt, T.G., R.A. Milne, and G.C. Kozub. 1971. Use of the nitrate-specific ion electrode for the determination of nitrate nitrogen in surface and ground water. Commun. Soil Sci. Plant Anal. 2:415-420.
Varsa, E. C. , T. Laatsch, M. Davison, and N. Jan. 1992. Comparison of selected methods for soil nitrate. Presented at the Winter Meetingsw of the Illinois Soil Testing Association. Springfield, Illinois. Feb. 18-19, 1992.
