Over-fertilizing herbs in containers for spring sale is a common problem this time of year. Compared to most bedding plants, herbs are lite-feeders. When the herbs are mixed with bedding plants in a greenhouse the tendency is to fertilize all plants at one rate. Try to avoid this practice.
A standard recommendation for most herbs is 100 to 150 ppm nitrogen (constant liquid feed). A routine clear watering is a good practice if the soil mix is allowed to dry.
For more information on growing herbs: