Foliar application of calcium and magnesium improves growth, yield, and essential oil yield of oregano (Origanum vulgare ssp. hirtum)

Oregano is one of the most important spices, is used all over the world, and includes many species. One of the most important commercially grown species is Origanum vulgare ssp. hirtum (Link) Ietsw (syn.: O. heracleoticum.), which is endemic to the Mediterranean area. O. vulgare ssp. hirtum is a crop species which is well adapted to both dry land conditions and calcareous soils. The objectives of this study were to determine the effects of foliar Ca2+ and Mg2+ applications on growth, yield, essential oil content, and essential oil yield of oregano. Five treatments (0, 0.5% Ca, 1% Ca, 1% Mg, and 2% Mg) were used, and the experiment was repeated at two locations over two growth periods (2005 and 2006) in northern Greece in a sandy loam and sandy clay soil. Foliar applications with Ca2+ and Mg2+ increased the Ca2+ and Mg2+ concentration of the leaves. Ca2+ and Mg2+ applications affected plant height as the plants were shorter in the control treatment and increased with Ca2+ and Mg2+ applications by an average of 10% during the 2-year period and at the both locations compared with the control. Also, Ca2+ and Mg2+ applications increased the number of stems per plant by an average of 23% in both years and at both locations. Chlorophyll concentration was affected by Ca2+ and Mg2+ applications and subsequently increased an average of 23% with Ca2+ and 38% with Mg2+ compared with the control. Applications of Ca2+ and Mg2+ decreased the number of days required for oregano plants to flower by an average of 3–4 days compared with the control. Dry matter yield also increased 22% with Ca2+ and Mg2+ during the 2-year study and at both locations compared with the control treatment. In addition, the Ca2+ and Mg2+ applications affected the essential oil yield, but they did not affect the essential oil content. These results show that Ca2+ and Mg2+ applications can affect the growth and yield of oregano, especially when the plant is grown in acid soils. However, the physiological basis of this effect remains unknown.