effect of irrigation intervals, type of fertilizers and harvesting time on essence content and yield of three medicinal plants: lavender (lavandula angustifolia), rosemary (rosemarinus officinalis) and hyssop (hyssopus officinalis) in mashhad condition.

in order to investigate the effect of fertilizer types and irrigation regimes on quality criteria of three medicinal plants: lavander, rosemary and hyssop, an experiment was conducted at research field of faculty of agriculture, ferdowsi university of mashhad, during two growing years of 2007-2009. a split plot design with three replications was used. treatments were three irrigation intervals (10, 20, 30 days) as main plots and six fertilizers: including (control, nitroxin (5lit/ha), nitrogen fertilizer (50 and 100 (kg/ha), cow manure (10 and 20 ton/ha)) and three medicinal plants as sub-subplots. animal manure and chemical fertilizer were applied at the time of transferring seedlings to the field and nitroxin was used with the first irrigation. results indicated that the effect of irrigation intervals on yield of essential oil in all species was significant (p≤0.01). the highest essential oil content was from rosemary (1.5%) but the highest yield of essential oil was obtained from three species, lavander (10kg/ha), rosemary (7kg/ha) and hyssop (12kg/ha) with application of biological fertilizer. application of fertilizer affected significantly (p<0.01) essential oil yield of three species. however, the best fertilizer treatment and irrigation intervals for three spices plants were 20 days irrigation intervals with 10 ton/ha manure application.