Container plants are especially greedy and need more fertilizer than other plants. That’s because they live under unique conditions that don’t quite have their equivalent in nature.
First, they are usually grown in a potting mix that contains few natural minerals (most potting mixes are based on peat, coir, and/or bark, all of which are very poor in minerals).
Plus they are more exposed to moving air than plants in the ground and thus dry out more rapidly, forcing the gardener to water them much more often. And frequent waterings leach the soil of any minerals it originally did contain.
Do note that, when it comes to leaching, the situation is very different for plants growing in the ground. When it rains or you water them, true enough, soluble minerals in the soil tend to descend lower in the soil, carried by the downward flow of water, but then they start to move right back up into the root zone through capillary action as the soil starts to dry out. Thus, minerals tend to remain more or less within easy reach of most in-ground plants.
In a pot, though, dissolved minerals go straight out the drainage hole when water flows out and the plant no longer has access to them.
Add to this the fact that we usually tend to pack plants grown in containers or baskets much more densely than we would in the ground to give the container garden a fuller look or to increase yields. As a result, there are lots of hungry plants sharing the same growing space, all looking for their share of minerals.
Once final point: when plants growing in the ground lack minerals, they can send their roots out to look for more, often quite a distance from the mother plant. The roots of the potted plants remain imprisoned inside the container and can’t go anywhere searching for a snack.
A Different Environment
What you’ve seen from the information above is that container plants live in an environment far removed from their natural environment, one where minerals, if any there were any in the soil, are quickly exhausted. That’s why you need to fertilize container plants more often.
Most experienced container gardeners will agree that the best way to fertilize potted plants is to apply, at the beginning of the season, a slow-release organic fertilizer, as it will help to provide minerals gradually as the season progresses. Apply it according to the manufacturer’s recommendations. Then complement that by fertilizing regularly with a soluble fertilizer all through the summer. This is especially true if you notice a decrease in growth or in bloom or if the foliage shows signs of yellowing..
Personally, at the beginning of the season, I give each container a handful of homemade compost and apply whatever organic slow-release fertilizer I have on hand. I never even consider whether it’s a flower fertilizer, a vegetable fertilizer or even a lawn fertilizer. As if plants could really tell the difference! (Read Just Use Any Fertilizer to better understand that point.) Then, during the summer, I apply a soluble fertilizer (again, any kind) about every two weeks. And I’m very satisfied with the results!