Fertilizing Container Gardens: A Beginner’s Guide

urban-container-garden-with-potted-tomatoesGrowing food without a garden is a labor of love. My own life as a vegetable gardener started out on a series of tiny balconies and in small window boxes.

While I have since been blessed with much larger areas for gardening, I still hold a special place in my heart for potted herbs and greens.

Among people like me with a love for growing vegetables and herbs in pots, one of the more common questions is about the best way to fertilize container-grown edibles. The answer is a bit complicated, and really varies based on your level of gardening expertise.

If you regularly fertilizer your container plants already, but you’re interested in finding organic substitutes for store-bought fertilizers, read Joe Urbach’s article, “15 Simple and Inexpensive Homemade Fertilizers.” If you want to find organic substitutes for granular fertilizers, this resource is helpful: “How to Measure Your Favorite Organic Fertilizers.”

But if you are just trying to get started—if, for example, you purchased a basil plant and put it in a pot on your kitchen windowsill, and now you’re wondering what to do next—this article is especially for you.

Why You Should Fertilize Your Container-Grown Edible Plants

Plants grown in boxes and pots are a little bit like a baby in a bubble. They’re cut off from the natural world around them by a container.

Several feet away, in the ground, there is a flurry of organic activity taking place in the soil. But in an isolated container, there is much less organic microbial activity, and the plants in that container aren’t getting all of the beneficial nutrients, minerals, and enzymes that they would get if they were in the ground.

Fertilizing is about supplementing the soil in your pot, to make it more like the soil in the ground.

In the ground, there is always some presence of the key macro nutrients that are required for plant growth and fertility. Most likely, your potting soil has some concentration of these nutrients—but in theory, you could create a container environment that is completely devoid of key plant nutrients. Fertilization is how you can manage the levels of these nutrients in your potting soil.

Finally, when a plant in the ground uses available nutrients to grow and fruit, those nutrients are slowly replaced by natural processes in the soil around it.

A plant in a container doesn’t always have that advantage, and it is much more prone to “using up” its available nutrients, creating a situation where one or more nutrients are not available in the amount required by the plant to keep growing and fruiting.

When I worked at The Natural Gardener, I would demonstrate this to people by using my hands to estimate the size of a good slicing tomato. I would say, “Your plant is going to make tomatoes this big using only water, sunlight, and the nutrients in the soil. How many tomatoes this size do you think it can make before it runs out of nutrients in its little pot?” I think that’s an effective illustration. In a container, the nutrients get used up relatively quickly, and so it becomes important to keep adding fresh nutrients to the soil.

Compost in Potting Soil

Perhaps the best way to prevent the “baby in a bubble” situation described above is to use fresh, finished compost in your potting soil.

Growing vegetables is a different game than growing houseplants, and in this case you’ll have better success with a healthy, living soil.

  • If you can get some fresh, finished compost from a friend or neighbor’s bin, this would be an awesome addition to your potting soil. If not, get some from a nearby nursery.
  • If you can get some fresh, finished worm castings from anywhere, definitely add those.
  • If you can get some aerobic compost tea, absolutely use that. (More on that below.)
  • After you incorporate compost, castings, and compost tea, add a little molasses to feed the microbes and get them growing strong.

Click Here to See How to Make Your Own Worm Castings at Home

Now, a caveat: You can somewhat recreate a healthy, living soil by adding all the right stuff. If you do this, you’ll need fewer supplements. You can somewhat “fake” a healthy soil by continually adding artificial nutrients, but the plants grown in that way won’t be as good nutritionally as plants grown in a healthy, living soil.

There’s just no substitute for the real thing.

Don’t, however, just dump your garden soil in a pot. Do use a potting mix, any potting mix—homemade is good.


Ready to learn more about fertilizing container gardens? Be sure to check out part 2 of this series, where I give my four-part recommendation for fertilizing container-grown vegetable and herbs, including three sample schedules for different circumstances.

Then, in part 3, I discuss the four types of fertilizer in depth, including specifics on the products I like to use.


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(This is an updated version of a post that was originally published in October 2015.)

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