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.