Summary of literature:
Organically produced vegetables has been found to have at least equal (9) but mostly lower nitrate contents (e.g. 5; 6; 11) than conventionally produced vegetables. The way of fertilization can be assumed to be the reason for these findings, since lower nitrate contents have been analysed in products after organic fertilization (e.g. farmyard manure, compost) compared to mineral fertilization (1; 2, except for carrots; 3; 7; 8; 10). In the trials quoted here fertilization was mostly based on application of equal amounts of total nitrogen. Therefore, often but not in every case higher yields have been achieved with mineral fertilization. A conclusion concerning the way of farming, however, is hardly possible without considering the yields obtained; not to consider it has even been made out to be a "deliberate or undelibarate attempt to mislead" (4). In this respect the results of our trial fill a gap of argumentation, as we found lower nitrate contents in manure than in minerally fertilized products at comparable yield levels.

References:

1. Ahrens, E.; Elsaidy, S.; Samaras, I.; Samaras, F.; Wistinghausen, E.v., 1983: Significance of fertilization for the post-harvest condition of vegetables, especially spinach. In: Lockeretz, W. (ed.); Proc. 4. Int. IFOAM-Conf., Cambridge, Mass., 229-246.
2. Habegger, R.; Fritz, D., 1990: Auswirkung von organischer Düngung auf einige Qualitätseigenschaften von Gemüse. VDLUFA-Schriftenreihe 30, Kongreßband 1989; 455-460.
3. Lairon, D.; Lafont, H.; Leonardi, J.; Spitz, N.; Reynier, M., 1983: Studies of yield and food quality of vegetables grown under organic and mineral fertilization. In: Lockeretz, W. (ed.); Environmentally Sound Agriculture; Proc. 4th Int. IFOAM-Conf., Cambridge, Mass.; 408-409.
4. Marschner, H., 1985: Einfluß von Standort- und Wirtschaftsbedingungen auf die Nitratgehalte in verschiedenen Pflanzenarten. Landw. Forschung 37, Kongreßband 1984; 16-33.
5. Pommer, G.; Lepschy, J., 1985: Untersuchungen über Inhaltsstoffe von Winterweizen und Karotten aus verschiedener Produktionsweise und Vermarktung. Bayer. Landw. Jahrb. 62, 550-563.
6. Regierungspräsidium Stuttgart (Hrsg.), 1987: Qualitätsvergleich von "biologisch" und "konventionell" erzeugten Feldfrüchten. Selbstverlag.
7. Smith, S.R.; Hadley, P., 1989: A comparison of organic and inorganic nitrogen fertilizers: Their nitrate-N and ammonium-N release characteristics and effects on the growth response of lettuce (Lactuca sativa L. cv. Fortune). Plant and Soil 115, 135-144.
8. Stopes, C.; Woodward, L.; Forde, G.; Vogtmann, H., 1989: Effect of composted farmyard and a compound fertilizer on yield and nitrate accumulation in three summer lettuce cultivars grown in an organic system. Agric., Ecosyst. and Environm. 27, 555-559.
9. Unteutsch, H.-G.; Finck, A., 1985: Nitratgehalte von Winterweißkohl, Möhren, Kohlrabi und Salat in Abhängigkeit von Düngung und Anbausystem in Schleswig-Holstein. Landw. Forschung 37, Kongreßband 1984; 250-257.
10. Vogtmann, H.; Temperli, A.T.; Künsch, U.; Eichenberger, M.; Ott, P., 1984: Accumulation of nitrates in leafy vegetables grown under contrasting agricultural systems. Biol. Agric. & Hortic. 2, 51-68.
11. Woese K., Lange D., Boese C. & Bögl K.W., 1995. Ökologisch und konventionell erzeugte Lebensmittel im Vergleich - Eine Literaturstudie, Teil I und II. Bundesinstitut für gesundheitlichen Verbraucherschutz und Veterinärmedizin, BgVV Hefte 4 und 5.