Why Good Potting Soil is Important (and the Story of My Dead Tomato Seedlings)

Last spring, I couldn’t get my tomato seedlings to grow.

I tried everything. After using multiple different seeds and seed companies, trying different pots, changing the lighting, watering differently, and even replanting everything, I threw up my hands in despair. 

Maybe I just wouldn’t be growing tomatoes for the year…

But then I thought of the one variable that I hadn’t considered before: the potting soil.

Later into the season, I made a YouTube video all about my seedling troubles and many of you shared similar experiences in the comment section.

I also shared in that video the various experiments and research I did in order to figure out if it really was the fault of the potting soil (it ended up being kinda fun to dig into the mystery…check out the video below if you’re curious!).

When I did this deep dive into the factors that made all my seedlings die last year and the potting soil issues that started it, it made me very aware of the importance of good potting soil for seed starting. So in this post, I’m going to share with you some tips on how to know if your potting soil is bad and how to find good potting soil options. 

If you enjoy podcasts, check out my episode The Mystery of the Poisonous Potting Soil to get the scoop on my potting soil woes.

Does Potting Soil Matter?

I didn’t used to think so. Whenever I was feeling ambitious, I would make my own homemade potting soil. But other years, I would just grab whatever potting soil was cheapest off the shelf of my local garden supply store and called it good. And until last year, I didn’t have any major issues.

But now I know better.

As I devastatingly watched all my seedlings slowly stop growing, then turn yellow, and then start dying all together, I resolved to not let anyone else make the same mistake.

Lesson learned: If you want healthy and thriving seedlings, then you need to be careful with the soil you plant them in. 

Unhealthy or unbalanced potting soil just won’t have what is necessary for thriving seedlings and plants. Sometimes there are signs that the potting soil is bad, however, like I found last year, some potting soil mixes can look perfectly fine and yet can actually be totally missing the vital components of healthy soil. 

The Year of My Dead Tomato Seedlings

I always start my own tomatoes from seed and I’ve gotten fairly decent at seed starting after many years of practice (here’s my seed starting setup). This time, however, after two weeks of good growth, I started to notice that my plants stopped growing. They would get their first set of leaves and then stop. And then the few leaves my tomato seedlings had started to turn yellow.

At first, I thought my grow lights were too close or too far away from the seedlings. Then I wondered if I was watering too much or not enough. After moving the lights closer and then farther away, and watering more and then watering less, I wasn’t seeing any changes in their growth, no matter what I tried.

The next thing I wondered was if I had some sort of fungus or mold issues from improperly-cleaned seed trays (here’s my tips on how to disinfect seed trays). But I double-checked and knew that I was using clean seed trays. So all of the common reasons for poor health in seedlings were being ruled out. 

I was stumped. After some worry that I was just an awful gardener with a ‘black thumb’, I put my energy into researching the issue and finally figured it out: it was the potting soil.

Once I realized that the potting soil was the issue with my tomato seedlings, I started nerding out over potting soil information and decided to check the quality of multiple potting soil companies to see if they were similar to each other. I used the Redmond Soil Testing Kits and mailed off a bunch of different potting soil samples.

After anxiously checking my email over and over again waiting for results, I finally got my results and one thing I found is that most of the potting soils I tested were really low in nitrogen and phosphorous. 

So my seedling troubles made sense. I was able to get my seeds to germinate, but then they would seem to just…. stop growing. They were totally stunted. They just couldn’t grow properly since they weren’t getting enough nitrogen and phosphorous from the potting soil. And the yellowing that my plants were experiencing was also probably due to the lack of nutrients from the poor-quality potting soil.

IMPORTANT NOTE: There are other reasons for struggling seedlings so make sure you check all the factors for your issues; for example, plants can’t retrieve nutrients from the soil if their roots are too dry. So check to make sure your yellowing plants are also getting consistent water. And stunted seedling may need more warmth. If tomato seedlings are in too cold of an environment, then they just won’t grow well until they get warmer. So make sure that other things aren’t impacting your plants and then proceed with your investigation into the potting soil. 

I reached out to the company who manufactured the potting soil that produced the worst results and they said that materials were hard to come by and in short supply, so they were doing their best despite shortages and supply chain issues. I’m glad that they were so upfront about things, but it was still quite disappointing to learn this information about some of the potting soil options on the market right now.

So let this be a lesson to you all: the soil that you plant your seeds in matters. It’s a good idea to research your potting soil options and make sure you’re giving your seedlings a good-quality potting soil that they can thrive in. 

Signs That Your Potting Soil Might Be Bad

There are some things you can look for to know if the potting soil you’ve purchased is bad.

• Foul odor: some soils that have fermented, rotted, or harbored mold will smell foul. A rotten egg smell in soil is a hard pass.
• Compaction: soil that has been sitting around the store for years or has too high a percentage of peat moss will tend to over-compact. Dense, dry, compacted soil isn’t ideal.
• Mildew and/or mold: avoid soils that have produced mold or mildew. This is often caused by soil being contained in plastic bags for long periods of time, and moldy soil can lead to root rot and other issues in your plants. Avoid soils that are ripe with mold and mildew.
• Bugs: not all soil with bugs are bad (in fact, we know that some life is essential for healthy soil), but the presence of fungus gnats might be an indicator that the potting soil won’t be ideal for your plants. This is because these types of gnats feed off the nutrients in the soil, leaving behind soil that is lacking in nutrients.
• Sitting around for a long time: potting soil naturally loses nutrients and compacts as it sits and ages. This process is often sped up when the soil is contained to plastic bags or tubs. Keep this in mind when purchasing potting soil, and try to buy the freshest soil possible.
• Dying plants: finally, the last clue that your soil might be bad is if your plants simply don’t thrive. Obviously there is more than one reason why your plants might not be thriving (too much or too little water, old seeds, pots too small without enough room for root expansion, etc.), but checking your potting soil might be a good idea if your plants just aren’t wanting to grow and thrive.

 

What to Look for in a Good Potting Soil

Just like there are things to be aware of when looking for bad potting soil, there are also things you can look for in good potting soil.

• Ability to hold water: soil naturally dries out, but we want to look for soil that holds water well (doesn’t dry out too quickly), while also allowing drainage at the same time (not staying moist too long). If your potting soil is drying out too quickly, try mixing in some compost or coconut coir and see if that helps with drainage. Good potting soil will hold moisture well, but also won’t stay soggy for extended periods of time.
• Aeration/drainage: roots not only need good nutrients in the soil, they also need good aeration and the ability to drain. If your potting soil is too compacted or too loose, your plants won’t be able to thrive. Good potting soil will be soft but not too loose.
• The pH level: good potting soil will be formulated for an ideal pH level. If this is off, plants can be stunted, burned, or just not thrive. In general, look for potting soil that has between 5.8 and 7% acidity. You can check the pH level of your potting soil using pH strips that are available from most garden stores (and online) or through a soil test.
• Nutrients: look at the labels on your potting soil for natural and nutrient-rich ingredients like earthworm castings, sea minerals, and bat guano that will give your plants a boost in their early days. A good potting soil will contain a mix of nutrient-rich components to give your plants more than one option to pull from.
• No Shrinkage: a good potting soil will keep its shape, even as it’s moistened and dried. Cheaper potting mixes will shrink away from the sides of the container as they dry.

If you are struggling to find good potting soil near you (or you cannot afford the good ones), you can always try making a homemade potting soil recipe. My recipe is very flexible and you can add various natural nutrients if you want (or you can just add a good-quality homemade compost). If you buy the homemade potting soil ingredients in bulk, you can usually get a ton of potting soil for a similar price as what you could find in the store, plus you’ll know exactly the ingredients in your potting soil if you make your own.

IMPORTANT: If you make your own potting soil, make sure you don’t add contaminated compost to your recipe!

Watch my video below to learn how we ended up with contaminated compost so you can be aware of this issue (and also learn how to fix it):

Final Thoughts on Potting Soil…

If your seedlings and plants are mysteriously dying, please understand that it’s probably not because you have a gardening ‘black thumb’. Don’t be afraid to ask questions and investigate. Research the symptoms of your seedlings first and see if you can figure out what went wrong. After that, if you are still struggling to figure out the reason for your seedling woes, consider expanding your efforts. For example, I contacted some potting soil companies for more details in my investigative process and I also testing my potting soil with a soil test (perhaps that’s a bit extreme, but it might be worth doing so you can get some answers).

If you find yourself in a similar situation, one thing that might help is watering your seedlings with a liquid fertilizer (like compost tea made with good-quality compost). In my case with the dead tomato seedlings, the potting soil that I was using wasn’t contaminated or poisonous to the plants, it was just missing key nutrients that the seedlings needed. So giving your plants those nutrients just may be exactly what they need and you might be able to save your seedlings with just a little bit of extra care.

If you are curious about either your potting soil or your garden soil and if they are missing any important nutrients, my favorite soil testing kit option on the market is this one: theprairiehomestead.com/soiltest. If you want to try that soil test, use the code HOMESTEAD & save 15% off your entire order.

Wondering which potting soils that I found to be good and which ones I found lacking? Check out my youtube video to see my soil testing and investigative results on different potting soils.

More Garden Woes (and cures):

• Top Reasons for Tomato Leave Curling (blog)
• What We Learned from Having our Garden Soil Tested (podcast)
• How to Recover After a Major Garden Failure (podcast episode)
• The Time When I Poisoned my Garden (blog)