Experimental site: Experiment was conducted to study the dry matter content and quality of oil obtained from different plant parts harvested at various growth stages [pre- flowering, 50% flowering, 100% flowering and post-flowering (beginning of seed setting)] at the Central Institute of Medicinal and Aromatic Plants (CIMAP), Lucknow (25.5° N, 80.5° E, 120 m altitude), India. The study area falls in the sub-tropical plains in the northern part of the country. The soil of experimental plot was a sandy loam. Plant material: Tagetes minuta plants were grown in 0.02 ha area during winter season of 2002-2003. Forty day-old seedlings of T. minuta were planted in rows 60 cm apart at 30 cm plant to plat distance within rows in the second week of December, 2002. The crop received fertilizers at the rate of 100 kg N, and 60 kg each of P₂O₅ and K₂O/ha. Nitrogen 40 kg/ha and full quantity of P₂O₅ and K₂O were applied before planting of crop. Remaining of nitrogen application was through topdressing in two equal splits, 30 days after transplanting and at pre-flowering stage. Tagetes minuta plants from 1.20 m row length were harvested from four different places in the experimental area at each date of study. The dates for pre-flowering, 50% flowering, 100% flowering and post-flowering (beginning of seed setting) were March 15, March 27, April 4 and April 18, 2003, respectively.

The maximum essential oil yield (1.01 g/plant and 56.1 kg/ha) was obtained upon harvesting at post fowering stage (seed setting stage). Advantage in essential oil yield at this stage was 130%, 34.5% and 36.5% yield enhancement over pre-flowering, 50% and 100% flowering stages, respectively. Flowers and leaves contained the maximum essential oil 2.14% and 1.89% (dry weight basis), respectively. Relative contribution of leaf, flower and stem to essential oil production was 40.7%, 37.8% and 21.5%, respectively. In the oil quality analysis, dihydrotagetone (77.1%) was the predominant component in oil from the leaves while (Z)-beta-ocimene, limonene and (Z)-tagetone were the major constituents in the flower oil. The study suggests that T. minuta should be harvested at the post-flowering stage for realizing maximum essential oil yield and its desirable major chemical constituents. The study also differentiated the quality of leaf and fower oil for chemical constituents.

The plants were cultivated in the Botanical Garden of the University of Agriculture, Plovdiv/Bulgaria on meadow-carbonate soil with neutral reaction (pH 7.1-7.2) and were harvested during the flowering period (July and September). The reserves of nitrogen, phosphorus and potassium in the soil (NPK) were: NH⁴⁺ 31.42 mg/kg; NO^3- 16.66 mg/kg; P₂O₅ 12.3 mg/100 g and K₂O 11.4 mg/100 g. Ammonium nitrogen and nitrate nitrogen were extracted from the soil with a 1% solution of KCI and were determined by consecutive distillation on a Parnas-Vagner apparatus. Phosphorus was determined by the Egner-Ream method and potassium was extracted with 2 n HCI and was determined by flame photometry. The plants were grown as seedlings and were planted in a two-row bed 50 × (20 + 20) cm in patches of 5 m². A scheme on the increase was used for the introduction of N, P and K fertilizers. The experiment was carried out as a randomized block modus - 15 variants in 4 replications (6 levels for N and 4 levels for P and K). Phosphorus (triple superphosphate - 50 % active substance) and potassium (potassium sulphate - 50 % active substance) were introduced only once before planting. Nitrogen (the ammonium nitrate - 33 % active substance) was introduced in three portions - during preparing of the soil, during the plants drafting (active vegetation) and at the full bloom.

The highest oil yield was obtained from leaves in July for N4P2K4 (0.63%) and the basic components were piperitenone (29.4%) and piperitone (13.5%). The mineral fertilization had no effect on the oil yield from the flowers. Regardless of the variant, it was found that the flower oils in July were rich in caryophyllene oxide (12.0-48.4%) and piperitenone (3.0-7.0%), while the oil composition in September was different.