The best growing medium for container gardens is light and airy. It holds some moisture for the plants but has enough larger pores that excess water falls right through.
Most growing mixes are made from bark, peat or compost and aeration additives such as perlite. Being soilless, they lack minerals, so they're not rich in nutrients. You can add slow release fertilizer when you first fill the pot with the growing mix.
If the growing mix has no added fertilizer, or once it's past midsummer for pots that had fertilizer mixed in from the start, use a soluble fertilizer at one-half or one-quarter strength in the irrigation water every few days. (This supplement is needed for any soilless mix and even for fertilizer-added mixes since what's in a pot will almost always be diminished by leaching as we water thoroughly. Watering thoroughly means to add water until some drips out the drain holes. In hotter weather and for containers more exposed to drying wind and sun, we must water more frequently and more nutrient falls out the drain.)
- Fertilize throughout the season or supplement from mid-season by adding fertilizer solution to the irrigation water. If possible, choose an organic product so there is less likelihood of salt build up that could dehydrate plant root tips.
- Use a fertilizer solution that contains the standard nitrogen, phosphorus and potassium plus micronutrients -- also known as trace elements. Soilless mix has no minerals so plants can become wan or less floriferous from micronutrient deficiencies. These minerals are listed on the fertilizer label: sulfur, magnesium, iron, boron, etc.
Fertilizer solutions, examples:
Dr. Earth Organic liquid 3-3-3 (One quart is enough for twenty 10" baskets for the season)
Fish Emulsion 2-4-0 plus an equal amount of Seaweed 0-0-4.5 (Use one quart of each product, mixing both into every application. Enough nutrients for fourteen 10" baskets for the season.)
Miracid 30-10-10 (One pound box is enough for 200 ten-inch baskets for the season, or 100 square feet of garden, such as a 4' x 25' strip.)
(More about calculating proper fertilizer amounts, in Grow 522.)
- Make a compost tea from compost or organic granular fertilizer: Driconure, Groganic, Espoma "Tone", Milorganite, etc. Or cool the water in which vegetables were cooked. Use a tea for every watering. (Regarding amount: It's difficult to say how much nutrient a tea or cookwater delivers. It will not burn plants even in excess but could be insufficient to plant needs. So watch for pale color or poor growth and supplement as necessary.)
- Slow-release organic fertilizers and pelleted slow-release processed fertilizers can be added to a pot at planting time. Mix them throughout the potting mix since what dissolves from them must be available to as many roots as possible.
- Examples of slow release organic fertilizer: Driconure, Groganic poultry manure, Milorganite composted sewage sludge, mixtures of animal- and vegetable meal by Fertrell, Espoma and other companies, etc. Alternatives are slow release processed pellets such as Osmocote and Once. More about organic slow release fertilizer in Grow 559.
- Never add fertilizer to water for a very dry plant. If a plant is wilting, use clear water. Add fertilizer to the next watering.
Making compost tea: 1 part fertilizer to 4 parts water in a glass jar, put in the sun 2-3 days, shake it occasionally. More on compost tea http://www.dep.state.pa.us/dep/deputate/airwaste/wm/recycle/tea/tea1.htm