Validation of Analytical Methods Used for Determination of Nitrate in Soil

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  • Topic: Nitrate, Nitrite, Measurement
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Accred Qual Assur (2005) 10:172–176 DOI 10.1007/s00769-005-0914-6

PRACTITIONER’S REPORT

V. Kmecl J. SuÐin ˇˇ L. Zupancic-Kralj

Validation of analytical methods used for determination of nitrate in soil

Received: 7 April 2004 Accepted: 3 November 2004 Published online: 24 March 2005  Springer-Verlag 2005

V. Kmecl ()) · J. SuÐin Agricultural Institute of Slovenia, Hacquetova 17, 1000 Ljubljana, Slovenia e-mail: veronika.kmecl@kis.si Tel.: +386-1-2805164 Fax: +386-1-2805255 ˇˇ L. Zupancic-Kralj Faculty of Chemistry and Chemical Technology, University of Ljubljana, ˇ AÐkerceva 5, P.O. Box 537, 1000 Ljubljana, Slovenia

Abstract The standard method (ISO/ DIS 14255) and the quick test were used for determination of nitrate in soil. We validated both methods using parameters such as accuracy, reproducibility within 1 day and between days, limit of detection and limit of quantification. The accuracy of the results was determined using the analysis of samples from the international interlaboratory scheme WEPAL. The accuracy of the standard method was good, while for the quick test the results were not accurate. The standard method showed a solid reproducibility of measurement results (in 1 day, relative standard deviation, RSD=0.2%; between days,

RSD=0.8%). The quick test gave poorer results (in 1 day, RSD=6%; between days, RSD=7%). We tried to established the compatibility of both methods on real soil samples and we were satisfied to obtain the correlation coefficient 0.98 using the regression straight line. The analyses with the quick field instrument are much simpler and cheaper than the standard laboratory analyses and can be used for advising on nitrogen fertilisation. Keywords Nitrogen · Validation · Segmented flow analyser · Quick field instrument

Introduction
Nitrate ions are the subject of numerous investigations today owing to their mobility and a relatively high content in soil. Unprofessional handling with nitrogen fertilisers often results in an inexpedient use of nitrogen. Sometimes, the crop is fertilised with a larger quantity of nitrogen than is really needed by plants. A great part of this nitrogen is mobile in soil (NH4+, NO2Ÿ, NO3Ÿ) and may be transported in underground waters which could cause contamination of drinking water or an excessive content of nitrogen in products. On the other hand, a complete lack of mineral fertilisation with nitrogen cuts the yield in two [1]. Measurement of mineral nitrogen in soil was introduced into agricultural practice in the 1970s as a basis for wheat fertilisation [2]. Soon afterwards doubts were raised concerning its practical usefulness because of price and the time-consuming sampling and analytical proce-

dures. These facts triggered the introduction of rapid soil tests which may be carried out in the field. The main advantage of rapid field tests is a simple and rapid measurement allowing the determination of the rate of soil supply with nutrients in the field and, on the basis of these measurements, prompt advice on fertilisation. Measurement with RQflex is one such method. The subject of trials conducted at the Agricultural Institute of Slovenia for several years was the study of the influence of mineral nitrogen in soil on the growth of field crops. For that purpose we have introduced and tried to validate two methods for determination of nitrate nitrogen in soil (ISO/DIS 14255 and a quick field method).

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Table 1 Accuracy of the results of analyses of NO3 N following the standard method (segmented flow analysis, SFA) International sample WEPAL 1 2 3 4 5 6 7 8 Average (mg NO3 N/kg) 5.4 1.3 57 8.0 4.8 56 4.7 6.2 Standard deviation (mg NO3 N/kg) 0.2 0.6 2.8 0.5 0.2 0.4 0.7 1.1 Minimum (mg NO3 N/kg) 4.9 0.0 51 6.9 4.3 55 3.2 4.0 Our result (mg NO3 N/kg) 5.6 1.3 58 7.7 5.0 56 4.4 5.6 Maximum (mg NO3 N/kg) 5.8 2.6 63 9.1 5.3 57 6.2 8.5 Comment

Accurate Accurate Accurate Accurate Accurate Accurate Accurate Accurate

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