The GardenMyths.com blog is mostly focused on gardening in the ground. Comments may or may not apply to container gardening and I rarely make special comments about potted plants.. A few weeks ago a reader asked me several questions about fertilizing plants in containers and it got me thinking about the various differences between container gardening and gardening in the ground. In this post I’ll be looking specifically at fertilizing potted plants grown in a soil-less mix.
Xeriscaping and Potted Plants
Two significant gardening trends have developed over the last 10 years or so. On the one hand people are very interested in growing drought tolerant gardens–Xeriscaping. This makes a lot of sense since many areas are running out of water and if there is a shortage of water it is difficult to justify watering landscape plants.
At the same time there is a tremendous interest in container gardening–the exact opposite of Xeriscaping. Potted plants need a tremendous amount of water to make it through the summer.
We certainly live in a strange world.
What does this have to do with fertilizing container plants? Not very much, however, pots that get a lot of water because they dry out quickly have some special fertilizer requirements that need to be considered.
What Happens When a Container Dries Out?
When a container of soil has lots of water, the nutrients, ie the salts in the fertilizer, stay in solution–for the most part. So the phosphate, iron, magnesium etc exist as ions floating around in the water that surrounds the soil particles. In this form the nutrients are easily absorbed by plant roots.
As the water starts to evaporate, the ions in solution become more and more concentrated. Water escapes into the air, but the nutrient ions can’t go anywhere–they stay in the pot. As the concentration of ions increases, some of the ions start getting too crowded and they move out of the water and form solid salts attached to the soil. You have seen this happen if you have old pots around. The white/yellow deposit on the pot are solid salts that have been deposited as water evaporates. A similar thing happens in your water kettle. As you boil water, some salts deposit in the bottom as a white/yellow film that gets thicker over time.
Which nutrients become solids first? It depends on the fertilizer you use, and on the quality of your water. Each ion behaves differently, but calcium, phosphate and magnesium tend to become solids (precipitate) more quickly than other ions.
Once these nutrients have precipitated into a solid, they tend to remain that way even if more water is added. Think of your old pots and the white deposit on them. The white stuff does not easily wash away with fresh water. You usually have to scrape the white stuff off to get rid of it.
Plant roots react negatively to both high concentrations of ions in solution and to the dry salts. This is one of the reasons that over fertilizing in pots is bad for your plants. Too little fertilizer may stunt your plants, but too much can kill them.
Fertilizing Container Plants – Which is the Best Fertilizer?
There are several types of fertilizer you can consider.
Dry Garden Fertilizer
Dry garden fertilizer, the type you might use on your lawn is probably not a good choice. These fertilizers are designed to dissolve quickly with the first watering, and they tend to contain high nutrient levels. It is too easy to over fertilize with these.
Slow Release Fertilizer
Slow release fertilizer, also called timed release fertilizer, consist of small plastic balls which contain the nutrients. They are designed to release a bit of fertilizer over an extended period of time. These work quite well and are used by many nurseries since they save time. You only need to apply them once or twice a season, a good option for those of us that forget to fertilize.
Slow release fertilizers do have some issues. It is impossible for you to know when they are used up. You are left with waiting for the plant to show you it is suffering and needs to be fed. Another problem is that nutrients are washed away each time you flush the pot–this is discussed in Container Gardening – Selecting the Right Soil.
Don’t be fooled by the little plastic balls. They remain long after the fertilizer has been emptied from them. Just because you see the little balls does not mean you are still fertilizing the plants.
Water Soluble Fertilizer
Water soluble fertilizer is available as either a powder or a liquid. These can be added directly to the pot or to the water you use for watering the plants. If you are rinsing the pot you can skip a feeding. The down side of this type of fertilizer is that you need to feed much more often than with the slow release fertilizer.
For most people slow release fertilizers are the easiest to use. I use water soluble fertilizer because it is cheaper, and it gives me more control.
Fertilizer numbers consist of 3 numbers which indicate the nitrogen, phosphorus and potassium in the fertilizer. So you might buy a 10-5-5 fertilizer which is roughly 10% N, 5% P and 5% K. Actually it is 10%N, 5% P2O5 and 5% K2O. but that is a topic for another post.
Which fertilizer number is best?
The soil used most often for containers is a soil-less mix which contains very few, if any, nutrients. It is therefore important to use a fertilizer that contains micro-nutrients. The actual amounts of these is not that important, and most commercial products that contain micro-nutrients will provide them in acceptable amounts.
Which fertilizer ratio is best–I asked this question on the Garden Professors Facebook page and the consensus was that a ratio of 3-1-2 is best. So the fertilizer could be a 3-1-2, or 6-2-4, or 9-3-6 etc. Each of these has the same ration of nutrients.
How important is it that you match it exactly? I don’t think it is all that important. In potted plants, much of the fertilizer that is added, gets washed out the bottom of the pot. If you have a bit too much N, P, or K, the excess gets washed away.
If you think about plants growing in the ground, you find that many different plants grow just fine in the same soil, and you rarely know the exact ratio of the nutrients. For the most part, provided that nutrients levels don’t get extremely low or extremely high, plants are able to take up the nutrients they need. The exact formulation is not that critical.
The nutrient that plants need most is nitrogen, and having more P and K than required is a waste of your money, and it is a waste of our natural resources. Try to stick to a ratio of 3-1-2 or something close to it. To be honest, I usually buy something that is close to the right ratio and at the same time, low in price. It’s similar to buying cosmetics–a high price means a fancy bottle and fancy name, but it does not work any better than a lower priced product.
How Much Fertilizer?
The recommended amount from the Garden Professors is 100-150 ppm nitrogen. If you need help with this calculation see reference #2 below. Alternatively use the recommendations on the fertilizer label. I tend to use about 1/2 of what the label recommends since it is always better to under fertilize than to over fertilize.
It is important to realize that the amount of fertilizer you add depends very much on your watering habits and the amount of rain you get. That is why it is difficult to give you exact numbers.
If you are using a slow release fertilizer, follow instructions on the bottle.
For many people growing plants in containers is all about getting flowers–lots of them. Will a bloom booster give you more flowers?
A bloom booster is a fertilizer with a high middle number–it has a lot of phosphorus compared to nitrogen.
The answer is NO. The idea that plants need a huge injection of phosphorus in order to bloom is an old myth that just won’t die. Plants need a good steady diet, just like you and me. Bloom boosters do not give you more flowers.
I have touched on some aspects regarding the fertilization of container gardening, but there is more to the story. In my next blog I will discuss organic fertilizers, the proper way to water pots, reusing soil and the magic (or not) of adding mycorrhizae fungi.
1) Watering and Fertilizing Container Plants: http://web.extension.illinois.edu/ccdms/yg/080209.html
2) Fertilizer Calculations for Greenhouse Crops: https://extension.umass.edu/floriculture/fact-sheets/fertilizer-calculations-greenhouse-crops
3) Selecting Fertilizer for a Soil-less Mix: https://www.facebook.com/groups/GardenProfessors/permalink/10153367893571490/
4) Photo Source: melgupta