Though most herbs don't require much fertilizer, at some point they will need it. Herbs produce the most oils when they are given a steady feeding of a slow acting fertilizer such as fish emulsion, bone meal, cottonseed meal and blood meal. Add it to the soil at time of planting or potting. These natural fertilizers break down slowly and provide herbs with nutrition, but at a slow pace, thus providing healthy growth and helping produce the flavors we look for.
Evaluate Soil Conditions First
There's usually not a need to perform a soil test before planting or fertilizing herbs; however, it may be a good idea to research the various types of plants you have growing in your garden to see if there are any specific nutritional needs for specific types of perennial plants. Some herbs like an acid soil while others like an alkaline soil. You can use the Gardenality Search to find Herb Plant Files on which will you will find the soil pH requirements and other details. Simply type one word in the name of a plant into the search at the top of this page and a list of plants files will appear. For example: If you are looking for Salem Rosemary type the word "Salem" or "Rosemary" in the search to find the plant.
To test soil you can purchase a test kit at your local nursery and garden center or buy a soil test kit online here. Your local extension service might also provide soil testing services.
Fertilizing Herbs In The Garden
In general, herbs you grow in garden soil will not require much fertilization. Herbs can often get much of their nutrient needs from the soil.
Make sure to plant your herb plants in well-drained, porous garden soil, as most do not like wet feet. Soil conditioners or compost can be mixed in with poorer soils to provide better drainage and supply beneficial nutrients to the plants.
At time of planting, you can fetilize your herb plants with a solution of fish emulsion, and then on an as-needed basis throughout the season. Well-rotted, or composted manure can be used as fertilizer early in the season as a slow-released source of nutrients.
Be aware that many herbs will produce poor quality flavors and aromas if fertilized too heavily, particularly when synthetic, fast-releasing fertilizers are used.
Fertilizing Herbs Growing In Containers
In general, herbs you grow in containers will need more fertilization because they have no other way to get additional nutrients once they've used up what's in the pot.
But first things first, make sure you grow potted herbs in a light, well-drained, premium potting "mix "(not cheap dollar-a-bag potting "soils"). Too, make sure the pot has drainage holes and, if there are none, drill one or two.
For efficiency, your potted herbs can be fed every two weeks with a liquid fertilize such as fish emulsion, mixed at full strength as suggested on the product label.
Organic fertilizers are recommended over synthetic, because synthetic fertilizers usually contain a lot of salts, and this can build up over time in potting soil.
Be careful not to overwater. Allow herbs growing in pots to dry out a little between waterings.
Avoid Late Fertilizing and Pruning of Outdoor Herbs
Most herbs are more flavorful when the fertility is not too high. Pruning should be done during spring and summer; avoid excessively cutting the plants back in the fall. The growth serves to catch leaves that help insulate the plants.
Remember that herbs are similar to wildflowers. When they look good, they look good, but they don't all have long seasons like Rosemary does. No matter what, many herbs, such as Cilantro, are going to decline at some point during the season, whether fertilized, or not.
If you are really serious about growing herbs, the best thing you can do is buy a good book on herb growing. A book will not only introduce you to herbs you had no idea existed and provide specific growing and fertilization instructions, but you will also find important information on when to harvest each herb, and how to keep and store them. With some herbs, harvest time does make a difference, and if you wait too long, they may be past their prime.
Though there is a shallow learning curve, growing herbs isn't rocket science. A year or two spent growing them, along with a good book and articles such as this one found on Gardenality, and you'll be an expert. Then you can write some articles here and maybe even a book!