As we progress in the summer, it's a good time to assess the fertility of many of our garden plants, especially containerized plants such as those in pots and hanging baskets. Most plants grown in containers have potting media as their rooting substrate, not soil.
Why does this matter? Soil has significant cation exchange capacity (CEC). Think of CEC as a ‘magnet’ that can hold nutrients. Potting media has very little CEC and so is very poor at holding nutrients that are available to plants for uptake.
The difference between fertilizing soil and potting media.
In soil we primarily focus on applying nitrogen, phosphorus, and potassium. Calcium is generally the 3rd most abundant nutrient in plants, with magnesium being the 4th or 5th most. In most soils in Iowa we do not need to routinely apply calcium and magnesium since they have enough to grow most plants. If our soil is in the right pH range, we often have enough micronutrients, though they may need to be applied in some situations.
Since potting media is not soil, it requires a different approach to fertilization. Given that potting media doesn’t have much CEC, it does not have a reservoir of nutrients, unless a type of controlled or slow release fertilizer has been used. Commercially most greenhouse plants are fertilized with low doses of water-soluble fertilizer at every watering. It’s important that you use low doses, since these nutrients will mostly leach out of the potting media with watering.
Whether you use controlled release fertilizers or repeated low doses of water soluble fertilizers, the products we need to use differ from those used fertilizing plants in the soil.
Plants grown in potting media need all the 12 required macro and micronutrients applied to them to perform since there is no reservoir present. All of them may not need to be applied at every application, but the closer you can get to that, the better.
While every plant has unique requirements, a fertilizer or combinations of fertilizers in a ratio of N: P205: K2O: Ca: Mg of around 4:1:4:2:1 (Example: 20-5-20-10 Ca-5Mg) closely resembles the macro nutrient content of many plants. Depending on your water source, you may be getting small amounts of calcium and magnesium in your irrigation water as well, but it's unlikely to be enough to meet all your plants needs of those nutrients. Micronutrients should also be in these fertilizers, with iron and manganese being the most prevalent.
Controlled release fertilizer release
It's becoming more common for plants to be grown or sold with controlled release fertilizers (CRF). Additionally some potting media mixes are also sold with CRF inside of them. Many resin coated CRF or potting media with CRF are labeled with an average release time. That release time is based on a standard soil temperature, usually 70 degrees, though this may vary based on the company. Common soil temperatures in the summer are in the 70s, so that release time is pretty accurate for soil applications.
The challenge is that the temperature of potting media in above ground containers is generally a lot warmer than average soil temperatures. This causes these fertilizers to release more quickly. For example a CRF labeled for 6 months may only last for 4 or 5 months at 80 degrees, 3 or 4 months at 90 degrees. In cooler spring temperatures, the release time may be extended. For example, if the average soil temperature was 60 degrees, it might release for 7-8 months. One example of this application is for potted garden mums that are planted in late spring and sold 3-4 months later. Growers often use a 6 or 8 month CRF to compensate for that more rapid fertilizer release in the summer.
Without specifically measuring the salt content (electrical conductivity) of our potting media, it can be challenging to know exactly where we are at in terms of fertilizer availability.
Things to look for
Typically, vigorously growing plants like petunia are some of the first to show nutrient deficiencies in the summer. Some of this can be related to a general lack of fertilizer, like a nitrogen deficiency. Nitrogen deficiencies look like a general yellowing of the foliage and is usually more pronounced on the old leaves. Unfortunately, not all leaf yellowing is due to a low amount of nitrogen but it can also be due to the low availably of iron. Even if iron is being applied, some plants like calibrachoa and petunia need a more acidic rooting media than others. At a high pH, nutrients like iron become converted into forms that the plants can’t use, and interveinal chlorosis of the young leaves is common.
What to do
If you notice your plants have a nitrogen deficiency, you can either apply a second dose of CRF with the appropriate release time, or start regular applications of low doses of water soluble fertilizer. If your plants have an iron deficiency, they could potentially only need additional iron, or they may need their pH lowered. Lowering the pH of potting media can be accomplished with using ammonium-based nitrogen ‘acidic’ fertilizers, low doses of aluminum sulfate, or commercially an acid is injected into irrigation water. Adding a chelated form of iron can also cause iron to stay available at a somewhat high pH.
One additional thing to consider is that nutrient deficiencies occur more quickly if we overwater plants. So if you are seeing nutrient deficiencies, it's also good to evaluate if you may be overwatering as well.
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