Aromatic plants as soil amendments: Effects of spearmint and sage on soil properties, growth and physiology of tomato seedlings

• Spearmint and sage were incorporated into tomato seedbeds without prior composting.
• Soil features and processes were greatly improved with spearmint.
• Spearmint enhanced emergence, physiology and growth of tomato seedlings.
• Sage effects on growth characteristics of tomato were mostly negative.
• Incorporating dried spearmint into the soil is an effective amendment practice.

Improvement of soil characteristics through the use of renewable inputs is fundamental to environmentally friendly farming systems. In the present study, the potential of improving soil properties and, consequently, growth of tomato (Lycopersicon esculentum L.) seedlings through a direct incorporation of aromatic plant tissues into seedbeds is assessed. Dried spearmint (Mentha spicata L.) and sage (Salvia fruticosa Mill.) tissues are incorporated at different rates into the soil of experimental field plots. At 0, 20, 40, 60, and 90 days following incorporation, soil samples are removed from the plots and used as substrates in tomato seedbeds. Growth and physiological parameters of tomato seedlings (emergence, size of the most robust leaf, shoot length, dry weight, net photosynthetic rate, stomatal conductance, photosynthetic yield) as well as soil attributes (pH, nitrogen and organic carbon content, organic matter decomposition rate, microbial populations, changes in essential oil content) are monitored. Spearmint incorporation into the soil improved emergence, physiology and growth of tomato seedlings. This was not the case with sage. Soil microbial populations and organic matter decomposition increased with increasing rate of incorporated aromatic plant tissues, especially in the case of spearmint which exhibited a more prominent increasing trend. Soil pH was not affected, remaining within the range for optimum tomato growth. Further, C:N ratio increased, yet it did not inhibit tomato growth. Lastly, the observed decrease with time of the essential oil content in soil was dependent on the aromatic plant incorporated, and is discussed in relation to the beneficial effects of spearmint on tomato growth. The herein undertaken study demonstrates that incorporating intact spearmint tissues into the soil is a promising tool for improving tomato seedling production. This practice circumvents the arduous composting process and, therefore, it can be more cost-and-time-effective compared to the currently applied techniques.