The results relating to the influence of the increasing doses of nitrogen (0-200 kg/ha), with the constant amounts of phosphorus (80 kg/ha) and potassium (50 kg/ha), on the yield and quality of peppermint (Mentha piperita L.) are presented in this paper. The application of 150 kg N/ha from mineral fertilizer had the significant influence on the increase of the peppermint's dry leaves and herb yield. The application of 150 and 200 kg N/ha had positive influence on the increase of menthol content in the peppermint's essential oil.

Key words: peppermint, nitrogen fertilizer, herb yield, essential oil.

Cultivated mint (Mentha piperita L.) is widely grown medicinal plant species on our planet. In our climate, a peppermint has been cultivated in Vojvodina and in some smaller extends in central Serbia (regions of Tamnava and Mačva). In Yugoslavia, the most grown cultivar is English mint (Mitcham), which is still the most highly rated and the most requested one in European and global market (Mihajlov, 1998).

A fertile soil and favorable rainfall conditions are the most important factors for achievement of stable yields and good quality of cultivated mint. Mineral nutrition is factor that might have great influence on the yield increment, along with preserving, or even improving a quality of achieved raw material (Singh et al., 1989; Stepanović, 1993; Piccagila et al., 1993). Our intention was to investigate the influence of nitrogen fertilizer on the yield and quality of cultivated peppermint in our climate (Vojvodina region).

The field experiment was conducted on humogley soil, at the Experimental station of the Institute for Medicinal Plant Research "Dr Josif Pančić" in Pančevo. The experimental plots were arranged by randomize block system, with six variants:

1. Control (Ć)
2. N₀ - P₈₀ - K₅₀
3. N₅₀ - P₈₀ - K₅₀
4. N₁₀₀ - P₈₀ - K₅₀
5. N₁₅₀ - P₈₀ - K₅₀
6. N₂₀₀ - P₈₀ - K₅₀.

Each variant was replicated four times. The area of each experimental plot was 35 m². The stolones (cultivar Mitcham) were planted in the middle of October 1998 in rows (space between rows was 70 cm). Total amounts of phosphorus and potassium fertilizer and 1/3 of nitrogen fertilizer were applied immediately before planting. Another 1/3 of nitrogen fertilizer was applied in the middle of April 1999, and the last 1/3 of the planed nitrogen fertilizer dose was applied after the first harvest in June 1999. The usual agro-technique was used for the crop maintenance. The second harvest was done in August 1999.

After the harvests qualitative and quantitative parameters of the peppermint's yield were determined. In dried (80°C) and ground leaves and stem samples the content of essential macroelements (N, P, K) was determined, by usual laboratory methods. Dried (40°C) aerial plant parts were subjected to the essential oil analysis. The essential oil content was determined by hydrodistillation in Clevenger type apparatus, while isolated oil samples were subjected to gas chromatographic analysis. The obtained results are presented as the average values from both harvests.

Chemical properties of the humogley soil were also determined by usual laboratory methods.

The basic agrochemical properties of the experimental plots are quite uniform (Tab. 1). Soil reaction (according to pH in nKCl) is slightly acid to neutral. Humus content is rather high. Soil is very well supplied with available K, but amounts of available P are very low (average content 5.7 mg/100 g).

As it is shown in Tab. 2, the applied doses of nitrogen fertilizer had significant influence on the peppermint dry leaves yield. The achieved yields varied from 2136.67 to 2943.33 kg/ha. Statistically the most significant yield increase, comparing both to control variant and variant without nitrogen, was obtained with the application of 150 kg N/ha (var. 5). This is in agreement with the results obtained by Singh et al. (1989).

The achieved amounts of dry herb per hectare have also varied according to application of different nitrogen doses. The highest yield was obtained with the application of 150 kg N/ha (var. 5).

The essential oil content ranged from 1.88 to 1.95% (Tab. 2). The influence of added fertilizers on the essential oil content is not clearly expressed. But, since the highest yield of dried peppermint leaves was obtained on var. 5, the greatest yield of essential oil per hectare was also recorded in that variant. It was for 49.22 % higher then in the variant without nitrogen (var. 2).

Regarding the essential oil quality, only the most significant oil components: menthon, menthofuran, menthol and methyl acetate were presented. The obtained results are similar to those for cultivar Black Mitcham (Mitchell, 1996). It is evident that greater content of menthol (41.6-44.5%), which is the most important component of the peppermint essential oil, was obtained on the variants where the higher N-doses were applied (var. 5 and 6).

Macronutient content (Tab. 3) in peppermint's leaves and stem don't disagree from the previously reported data (Džamić and Krstić-Pavlović, 1983; Maksimović, 1991). Statistically significant increase of the nitrogen content in the peppermint's leaves under the influence of added fertilizers was obtained only for var. 5 (150 kg N/ha). Since it is the same variant where the highest yield of leaves and stem were recorded, the greatest removal of nitrogen by yield could be expected. The contents of phosphorus and potassium in both, leaves and stem, are quite similar in all variants.

On the bases of the conducted experiment the following conclusion can bee made.

The application of 150 kg N/ha from mineral fertilizer had the significant influence on the increase of the yield of peppermint's dry leaves and herb.

The increase of menthol content in the peppermint's essential oil was recorded after the application of 150 and 200 kg N/ha.

The obtained results point to the necessity of further research regarding the peppermint's, as well as the other medicinal plant's mineral nutrition.

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