Answer: If you are growing a medicinal herb to be eaten or consumed, you need to be very careful about what is added to your soil or applied to the plant -- just as you would if you were growing fruits or vegetables for human consumption.
Commercial fertilizer and/or a good quality compost can be used if your soil requires supplemental nutrients. The best way to know that is to run some basic soil tests.
Seedlings as a rule do not require heavy amounts of fertilizer, often a top dressing of compost, or an application of compost tea or water soluble fertilizer at a low rate is enough. You also do not want to overfertilize your plants; many herbs seem to grow well in a leaner soil. The nutrient needs will vary depending on the specific plant you are growing and the overall growing conditions for it.
Many herb growers prefer to use organic products such as compost rather than a synthetic fertilizer. If you make your own compost you will be able to control what goes into the mix and can be sure nothing harmful is added as well as know that the quality is high. Certainly do not use any product that includes chemicals for weed or disease or insect control, or is not labeled for use on edible plants. Be sure to carefully read the label of any product you are considering using.
The chemical content of medicinal herbs will depend on many factors including the seed strain you are planting and the overall soil type and its fertility. To verify the quality or strength of the product would require laboratory testing and chemical analysis. I hope this helps.
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