For less than the price of a bundle of fresh herbs at the supermarket you can grow your own supply. A single window box on a sunny sill or porch is all you need to grow a variety of herbs. The right growing medium can be the key to your success when growing herbs in containers.
Soil-less growing mediums are popular for potted plants, but soil-based mixtures are better for container gardening, according to the Arizona Cooperative Extension Master Gardener Manual (ref1). Soil provides a more constant stream of nutrients to herbs. Herbs that lack those nutrients are less flavourful and aromatic. Never dig up ground soil for a potting soil mix when growing herbs, as the clay content can compact roots and reduce aeration, and microscopic organisms can infect plants. Instead, use a sterilised loam. Loam is the perfect soil blend, containing a balanced amount of clay, sand and organic matter. If loam is not available, top soil from a garden centre will suffice, but for best results be certain it is sterile. A potting soil mixture should consist of at least one-third soil.
The second component for a potting mix should be partially decayed organic matter that is lightweight and will aerate your mixture so it holds water, oxygen and nutrients well. Peat is the best candidate as it takes a long time to decompose. There are different types of peat, made from different types of vegetation and existing in various stages of decomposition, and the National Sustainable Agriculture Information Service (ref2) warns that each will have different results in a soil mixture. For potted herbs, sphagnum peat moss is highly recommended because it is less decomposed than most peat, and therefore more stable. One-third of your growing medium should be organic matter. An alternative to peat is a mixture of Perlite and vermiculite, which are volcanic minerals. These are not as effective as peat moss in retaining nutrients in the pot, but can replace peat if necessary.
The third and final essential component to a good potting soil mix is one-third sand. Sand helps improve drainage in the pot, and since humus is so light, the weight of the sand can help anchor the pot better. The best type of sand to use is a clean, course sand, recommends the American Horticultural Society (ref3), such as builder's sand. Use gravel as an alternative to sand if desired. Beware of using fine sand, which can become compact and impede drainage.
Aside from soil, peat and sand, there are other optional elements you can add to a potting soil mixture in order to improve the quality. Since peat moss can be acidic, you can bring a better balance to your mixture if you use it by adding one tsp of dolomitic lime per gallon of potting mix. Compost, at approximately one-half cup per gallon of soil mix, can provide additional nutrients. If your region is dry or hot, you might save the life of your herbs by blending polymer water crystals with the potting mixture. Finally, time-release fertilisers can continuously feed your herbs for up to six months. The amount you need of polymer water crystals and time-release fertiliser will depend upon the size of the containers you use, so follow the guidelines on the packaging.