This study was carried out during 2012 and 2013 seasons at the experimental farm of Medicinal and Aromatic Plants Research Department in El-Kanater El-Khairia. The investigation aimed to study the effect of irrigation at 25, 50 and 100% from field capacity (FC) combined with diatomaceous earth (DDM) at 0, 2 and 4 g/10 kg soil in order to regulate water irrigation and to study their effect on vegetative growth and volatile oil production of spearmint plants (Mentha viridis L.). The obtained results are summarized as follows: in all cuts in the two seasons, the plants received DDM at 4 g/10 kg soil + 100% FC, showed stimulation in growth characters, i.e plant height, branching, fresh and dry weights of spearmint plants.
Although the highest volatile oil percentage/plant through the two seasons was achieved when the plants were treated with 25% and 50% FC but the volatile oil yield was the highest at 4 g DDM /10 kg soil + 100% FC. The highest percentage of Carvone (the main component of the volatile oil) was produced with 4 g DDM/10 kg soil +100% FC.
Proline content was decreased with increasing irrigation and diatomaceous earth rate, this refers to response of mint plants to treated by diatomaceous earth with the irrigation at 50% or 100% FC and not exposed to stress at 25% FC compared to the control (the highest proline content).