Fertilizer in Potting Soil – Good or Bad?

In the good old days, commercial potting soil (media) contained no added fertilizer. Today, more and more products contain some fertilizer, either in synthetic or organic form. Gardeners have been led to believe they can fertilize less and skip fertilizing seedlings. Some believe they can stop fertilizing houseplants entirely. But no one seems to discuss the elephant in the room!

How much fertilizer has been added? Should you stop fertilizing? When does the added fertilizer run out? How quickly are the nutrients available to plants? This blog will answer these and other questions about this mysterious fertilizer.

What Kind of Fertilizer is Added to Potting Media?

The information provided by manufacturers is very limited, even if you contact their support office, as I did. None provides the name of the fertilizer added except in very vague terms like “compost”, “slow release”, or “synthetic”.

Some companies promote this aspect of their product fairly strongly and print it right on the front of their product. Others like Premier, the makers of Pro-Mix, don’t even list it with their product. You have to dig through well-hidden specs to find it.

I selected some popular North American products and summarized them in the following table.

Type of Potting Soil	Type of Fertilizer	Amount of Fertilizer	Wait Time before Fertilizing
Pro-Mix HP	symthetic	not specified	7-10 days
Espoma Organic Potting Mix	organic	not specified	zero days
Miracle-Gro Potting Mix	slow-release synthetic	0.21-0.11-0.16	1 month or 6 months – they’re not sure!
Vigoro All-purpose Potting Mix	slow-release synthetic	0.07-0.04-0.03	9 months or sooner
FoxFarm Happy Frog Potting Soil	organic	0.3-0.3-0.05	not specified – they suggest feeding after planting

Without an NPK value, you have no idea how much fertilizer was added to the mix.

Pro-Mix HP Growing Media

Espoma Organic Potting Mix

Miracle-Gro

Vigoro All-Purpose Potting Mix

FoxFarm Happy Frog Potting Soil

How Much Fertilizer is In The Bag?

The NPK numbers seem low compared to bags of fertilizer or compost, but remember that this material is used straight out of the bag without dilution.

How do the numbers compare to the suggested fertilizer concentration for potted plants, namely 100 ppm nitrogen?

Peat moss absorbs about 20 times its weight of water. If we assume potting media will do the same, 100 g of soil will produce 2,000 g of mix, or 2 liters. If it has a nitrogen level of 0.2%, it would produce a mixture of 200 mg / 2 L, or 100 ppm.

This is an approximate value, and wood products may absorb a different amount of water, but a nitrogen level of 0.2% is in line with the suggested nitrogen level. Unfortunately, manufacturers do not even provide the nitrogen level in a newly potted plant, which would be more useful than an NPK value.

Keep in mind that this assumes all of the nitrogen is in a soluble, plant-available form. Nitrogen in a slow-release form, or nitrogen in organic material, will NOT give you this level of nitrogen.

If you have another look at the above table, there is not a single product that provides the nitrogen level and uses a soluble form of nitrogen. What does this mean? For the products listed, a gardener has no idea how much nitrogen is available to their plants at planting time.

Use this handy Fertilizer PPM Calculator to prepare your fertilizer to the desired value.

Soluble Synthetic Fertilizer

This is your typical plant fertilizer. It comes as a solid that is added to water in a prescribed amount. As soon as the crystals dissolve, the nitrogen is available to plants.

Once in a plant pot, the nitrogen is easily washed out as you water the plant. This form of nitrogen is a short-term feed for the plant.

Liquid Synthetic Fertilizer

This is similar to the soluble synthetic fertilizer except that the manufacturer has already dissolved the crystals for you and has charged you a hefty price for the convenience. You might still have to dilute the material before you use it.

This nitrogen is also available immediately to plants.

Slow-release Synthetic Fertilizer

This fertilizer is similar to the above two types except that the nutrients are locked away in a small particle, either a plastic bead or a sulfur-coated particle.

A small amount of nitrogen is released each time you water the plant.

The benefit of this fertilizer over the above two is that it lasts longer, releasing nitrogen for an extended period of time, anywhere from weeks to months. The release rate is controlled by the chemical design of the bead.

Slow-release Organic Fertilizer

This category includes most of the popular organic fertilizers, such as compost, worm casts, manure, fish extract, blood meal, etc.

Very little of the nitrogen is available when the potting media is first used because nutrients are tied up in large molecules. It takes time to decompose these molecules and release the nitrogen.

The rate at which nitrogen is released depends on the source. For example, compost releases nitrogen very slowly over several years. Blood meal, on the other hand, decomposes quickly and releases its nitrogen over a couple of months.

When Should You Start Fertilizing Plants?

Assume that you want to keep the nitrogen level at 100 ppm, because that is the level that grows the best plants. You should then start fertilizing once the level drops below 100 ppm.

Nitrogen levels in a pot or container change for a number of reasons.

• Leaching out the bottom of the pot when you water.
• Absorption by plant roots.
• Bacterial conversion of nitrate to nitrogen gas through denitrification.
• Release from slow-release forms of fertilizer, both synthetic and organic.

Unfortunately, potting soil manufacturers provide no information about this process for their products. You have zero knowledge about the nitrogen level a week after potting up a plant or 2 months after potting up a plant. It is therefore impossible for you to know when to start fertilizing.

Most products give you a rough suggestion, such as 2 months or a week, but then others say they contain fertilizer, and you should start fertilizing right away. Miracle-Gro gives you two values: 1 month and 6 months. Vigoro has the lowest NPK and the highest wait time of 9 months.

To be honest, I don’t trust any of their recommendations because none of them provide a chart showing the change in nitrogen over time. Such a chart will vary from user to user, but at least an approximate chart is better than nothing. I want to see some real data.

I asked several of them for such data, and they don’t have it. But if they don’t have it, how do they come up with their recommendation? The only way they know when you should start fertilizing is to have this data.

Vigoro suggested that you wait until the leaves start going yellow and then start fertilizing. By the time you see yellow leaves, it is way past the point where you should fertilize. The plant is already stressed.

It is also important to understand that slow-release fertilizers, either synthetic or organic, only provide a small part of their nutrients on any given day. It is one of the major limitations of organic fertilizer.

For example, the Happy Frog product has an NPK of 0.3-0.3-0.05, but only 0.03% nitrogen is soluble. That means on day one, the nitrogen level is about 15 ppm, not nearly enough to grow plants. How does that change over time? Nobody knows!

Is the Fertilizer in Potting Soil Good or Bad?

Fertilizer in a potting mix is neither good or bad, but it can cause gardeners a lot of trouble. I wish manufacturers would either not add it, or be more transparent about their fertilizer.

Here is the problem with it. Gardeners pay extra for it, and the marketing hype leads them to believe that they no longer need to fertilize. That results in poorly growing plants and disappointment.

The potting soil on the market today can’t be trusted on their recommendation to withhold fertilizer, because they don’t provide the information needed. In several cases, the nutrient levels are so low that you really need to fertilize as soon as you pot up a plant.

Assume that any fertilizer in the potting mix adds no real value to the plant. This is especially true of organic fertilizers since they are very slow-release products. Pretend there is no fertilizer in the potting soil.

What should you do? Do what I do. I always water with a fertilizer solution at about 100 ppm nitrogen. I ignore any fertilizer that may have been added by the manufacturer.

Fertilizing Seedlings

A lot of the seedling starting mixes also include fertilizer, but they have the same problem as discussed above. It is added at very low levels. Fertilize seedlings as soon as you see the first true leaf. I fertilize with the first watering, even before the seed germinates.