Vitamin and mineral contents of carrot and celeriac grown under mineral or organic fertilization.
Leclerc, J., M.L. Miller, E. Joliet and G. Rocquelin

Biological Agriculture and Horticulture 7:339-348. 1991

This study adds to the body of research that compares the influence of organic production methods and conventional agriculture on the nutritional quality of produce. Specifically, this study focuses on the effects that the type of fertilizer- either organic or mineral-may have on the vitamin and mineral contents of carrot and celeriac (celery root). Zinc levels in both vegetables were also measured since an increase in the concentration of zinc may indicate agricultural or atmospheric contamination.

Methods

Twelve market gardeners using organic production methods were paired with twelve using conventional methods for each vegetable: carrots and celeriac (six pairs per crop in 1987 and a different six pairs per crop in 1988). Each pair was in the same geographical area (in the Burgandy area and in Dole, in the Jura region of France), had the same soil type, used the same variety, same growing period and when possible, the same sowing date. The organic or "biological" farmers used cattle or stable manures or commercial organic fertilizers, while the conventional farmers used manures and mineral fertilizers. The total nitrogen, potassium and phosphorus fertilizer added to the soil was calculated for each farm from information furnished by the farmers. At harvest, 5 kg of carrots or 12 celery roots were randomly collected from each field. Carrots or celery were blenderized, homogenized and divided into aliquots to determine mineral content (Ca, Mg, K, P, Cu, Zn, Fe, Mn, NO3 and total N), and vitamins (C, beta carotene, B vitamins and pantothenic acid). Vitamin and mineral determinations were averaged for the organic and for the conventional growers and reported with their standard errors (see Table 1). A two-way analysis of variance was performed on the results with the fertilization technique as one factor and blocks of farmer pairs as the second factor.

Results and Conclusions

Carrots. This study showed that the organically grown carrots were significantly higher (P <0.05) in beta-carotene (+12 percent) and in vitamin B1 than those grown with mineral fertilizers. Other nutrients were not affected. These results agree with some, but not all of the previous research in this area. The data for beta-carotene is particularly variable. The authors note that since carrot beta-carotene is dependent on soil nitrogen, it is interesting that the organically grown carrots had higher beta-carotene levels since the organic farmers generally used less fertilizers than the conventional farmers. Thus, the authors conclude that cultural techniques appear to be the principal cause of the variation in beta-carotene content.

Correlations showed that vitamin B2, niacin, zinc, phosphorus, iron and manganese were positively correlated with weight, and nitrate was negatively correlated. Also, dry matter was positively correlated with Ca, P, Zn, Cu, Mg and Mn. Thus, for an equal growing period, the heavier the carrot, the higher its nutrient content and the lower its nitrate content. Yet, younger carrots are generally richer in many vitamins (except beta-carotene). The overall nutritional value of the carrot depends both on its "age" and its weight.

Celeriac. The study showed that the organically grown celery roots generally weighed less and had lower total nitrogen, nitrate (-56 percent) and zinc (-19 percent), but were higher in dry matter, phosphorus and vitamin C (+11 percent) than conventionally grown celery roots. The authors note that the zinc level was higher in the conventionally grown celeriac due to application of a zinc-based pesticide. Correlations showed that phosphorus was negatively correlated with weight, thus explaining why phosphorus was higher in the organic celeriac. Since nitrate did not correlate with either dry matter or weight, the authors conclude the difference observed was due to the fertilization method.

The authors conclude that the nutritional differences found in these two vegetables due to the use of organic fertilizer are significant. They recommend extending this study to other commonly eaten vegetables such as leafy greens and tomatoes.

Reviewer's Comment

Although the authors conclude that organic fertilization methods improve the nutritional quality of carrots and celeriac, it is difficult to make valid comparisons with other studies without more information. Indeed, the development of a widely accepted protocol for doing these kinds of experiments would greatly facilitate comparison and interpretation of results. First, it would be helpful to know more about the specific cultural techniques and irrigation practices used by the organic and conventional growers. Could it be possible that other cultural techniques used in organic agriculture, aside from fertilization methods, are responsible for some or all of the nutritional differences? Did all of the organic growers use the same techniques? It would have been interesting to see more specific data on the nutrient contents of the vegetables by farm to examine whether particular farms yielded consistently different results. Information on the conventional growers' cultural techniques, use of pesticides and other petrochemical inputs is also lacking. Second, we do not have information on how long each of the organic growers had been practicing organically. This time factor could affect the soil quality and potentially, the nutritional quality of the vegetables.

Finally, despite the interest this type of study attracts, it is important to bear in mind that the differences between organic and conventional produce must be considered within a broad context. Although consistent differences in specific nutrients may eventually be found, their contribution to overall health is questionable, given most North Americans' and Europeans' access to food. Choosing organically grown produce for its contribution to the long-term health of the soil and our capacity to produce food sustainably may ultimately be more important than its contribution to individual nutritional health.

For more information write to: Unite de Toxiologie Nutritionnelle, Station de Recherches sur la Qualite des Aliments de l'Homme, I.N.R.A., 17 rue Sully, F- 21034 Dijon-Cedex, France.

(GWF.OO7) Contributed by Gail Feenstra