Solid Residues from the Distillation of Medicinal and Aromatic Plants as Amendments for Acid Soils: A Preliminary Study

Abstract Knowledge concerning the use of solid residues from the distillation of medicinal and aromatic plants (MAP) as amendments-fertilizers for acid soils, instead of their combustion or disposal to the environment, is too limited. The aim of the study was a preliminary evaluation of the use of solid residues from the distillation of selected medicinal and aromatic plants as amendments-fertilizers for acid soils. Specifically, in the perspective of further and extensive investigation, the fertilization capacity of solid residues from the distillation of selected MAP was initially realized and the optimal range of application rates of the residues to acid soils was initially determined. From that point of view, the present study was a preliminary study and lasted for six months. Solid residues from the distillation of hemp ( Cannabis sativa L.), helichrysum ( Helichrysum italicum (Roth) G. Don), lavender ( Lavandula angustifolia Mill.), oregano ( Origanum vulgare L.), rosemary ( Rosmarinus officinalis L.) and sage ( Salvia officinalis L.) were collected from industries of essential oils of northern Greece. Then the residues were added to a strongly to moderately acid soil at 0 (control), 1, 2, 4 and 8% rates (treatments), in three replications and the soil treatments were analyzed for chemical properties and fertility indices. pH significantly increased compared with control upon the addition of all residues at the 2% rate and above. Although pH remained acid in all cases, it increased more than one unit in soils treated with 4% and 8% flower-leaf residues from hemp distillation, making soil’s reaction slightly acidic. Similar results were obtained for electrical conductivity (EC) and the carbon/nitrogen (C/N) ratio. It is worth noting that EC increased six-ten times upon the 8% application of flower-leaf residues, whereas the C/N ratio increased considerably with the 8% addition of lavender and helichrysum residues. Regarding soil available macronutrients, nitrate-nitrogen (NO 3 -N), phosphorus (P) and potassium (K) significantly increased from adequate to high levels compared with control. From the soil available micronutrients, only boron (B), zinc (Zn) and manganese (Mn) consistently affected by the application of residues and significantly increased. It is worth noting the multiple increases of B and Zn in soils treated with 8% residues compared with control, which were somewhat concerning. In almost all treatments, soil available B, Cu and Zn ranged at sufficiency levels, whereas Fe and Mn ranged at high levels. The solid residues from the distillation of the studied MAP, especially the flower-leaf residues, could be used as amendments-fertilizers for acid soils, at application rates between 2 and 4%. Rates lower than 2% are not expected to significantly affect soil properties, whereas rates higher than 4% should be avoided, because of risks of soil salinization, N immobilization and B, Zn and probable Mn phytotoxicities. Graphical Abstract