Soil Test - Which Potting Mix is Best? - GreenStalk Garden

Soil Test - Which Potting Mix is Best?

We are always getting questions on what type of potting mix is best for the GreenStalk Vertical Planter, so we decided to test 6 different mixes side-by-side. The results have really surprised us!

Please keep in mind that this is not a scientific test and results may vary. You may love a potting mix that didn't work well for us and vise-versa and that's ok! We also want to point out that this was a very short test and may not tell the full story.

You'll notice that there's both 4 and 5 Tiers in this soil test. We ended up with a combo of sizes because we didn't have enough soil of each type to do a full 5 Tier. For the 5 Tiers, we just used the same seeds as the first tiers - a mixed lettuce blend. All of the seeds used are from our Organic Leafy Greens Seed Bundle.

Mixes We Used

Miracle Gro Potting Mix

We planted 5 GreenStalk Tiers of Miracle Gro Potting Mix - straight from the bag.

Good Dirt Potting Mix

We had enough Good Dirt to fill 4 GreenStalk Tiers. Plus we used some of their plant food later on in the growing process.

Our Mix

This is Jack's blend of potting soils from the past few years. It contains mostly Happy Frog Potting Soil along with some Pro-Mix and fertilizer. Jack added Espoma's Tomato Tone, Crab and Lobster Shell, Kelp Meal, Sulfur, Worm Castings and Gypsum. We are using this mix to plant everything this season.

Pro-Mix with Worm Castings

Pro-Mix straight from the bag with our 3lb bag of worm castings mix into the top 2 inches.

PittMoss - Commercial Blend

We got this blend straight from PittMoss. It's a combo of recycled paper and peat moss. Unfortunately, it isn't widely available.

Happy Frog Potting Soil

Happy Frog straight from the bag.

The Process

This is not a scientific or exhaustive test. We simply used different mixes for the same seeds and planted at the same time. We didn't even water in the same amounts. That may sound crazy, but they dried out at different rates and we didn't want to flood some to benefit others.

To summarize:

  • Same sun exposure
  • Planted at the same time by the same person
  • Watered if needed at the same time
  • Except for Our Mix, all soils are straight from the bag

Each of the 6 testers were left unstacked for a couple of weeks so we could easily see the germination differences. After the seeds sprouted, we stacked them up one by one and began watering with the GreenStalk's watering system.

We didn't add any fertilizer through the process - with one exception. After a few weeks of stunted growth with the Good Dirt blend, we used their company's plant food to feed the plants and it did make a difference in their rate of growth. We only fed them once using their recommended dose through the GreenStalk's watering system.

In order of the quickest to sprout: Good Dirt, Pittmoss, Our Mix, Happy Frog, Pro-Mix, Miracle Gro.

The order of plants (bottom to top): mixed lettuce blend, romaine lettuce, arugula, spinach, and if there is a 5th tier we planted it with more mixed lettuce. All seeds used are from our Organic Leafy Green Seeds Bundle.

The Results

This is what you all can't wait to see, right?! There was some major differences in growth and like I said earlier, it did surprise me. We ranked them from worst to best based on which mix we would purchase. This is totally subjective and isn't meant to bash any product or brand. Gardening is very personal and results definitely vary from garden to garden. Below, we share what we would do differently next time and what we learned in the process.

Please ignore the white dust on the planters - we tried using diatomaceous earth for a couple of days around the bottom of the planters while they were unstacked to keep ants away but it didn't work. Some of the planters still have some residue from that. The diatomaceous earth wasn't applied to the plants themselves or the soil.

#6Miracle Gro Potting Mix

This is one of those that surprised us because we had heard such positive things about quick growth from fellow GreenStalk Gardeners about Miracle Gro blends. But as you can see, they never really looked healthy or grew very well. After seeing the mix up close and seeing how poorly the seeds germinated then grew in the Miracle Gro mix, we can't recommend this. However, so many of our customers say they use this potting mix and love it. Please let us know your thoughts below!

#5 Good Dirt Potting Mix

We love this company's mission, style and product line. If there was one blend we were rooting for it was this one! Good Dirt is becoming more readily available and is a truly unique product. The texture of the mix is unlike anything else we've seen. However, we rank it at the bottom because the plants sprouted the quickest but then did nothing. They don't look terrible, but we could have gotten three harvests from the plants in the top soils before we got one from Good Dirt.

If we were to purchase this mix, we would add more organic matter to it. Good Dirt's plant food seemed to help, but for the purpose of this experiment, we tried to stick with reviewing just the mixes and this one is supposed to have starter nutrients already added. I would recommend this more as an amendment then a straight out of the bag potting mix.

#4 Pro-Mix with Worm Castings

So this one isn't bad, it's actually pretty good! But using this alone is just not our first choice. The seeds were slower to germinate but they did grow at a decent pace. We would say this was what you could expect from a decent potting mix. Like Good Dirt, if we were purchasing Pro-Mix, we would add amendments to the blend. Pro-Mix is a good lightweight soil that works well for most plants we've grown through the years. Another thing to keep in mind with Pro-Mix is that when it dries out it can be tough to re-hydrate in our experience.

#3 PittMoss - Commercial Blend

We’re getting to the top performers! This PittMoss mix is super light and fluffy. It was one of the first to germinate and maintain consistent growth. This would be our top pick for germinating seeds – you can see pretty much all of the seeds germinated. Those pockets are full! It also didn’t require any extra effort. It needed water about as often as the others and didn’t flood when we watered it unstacked. We would buy this if we were looking for potting mix at the store and it was available. Unfortunately, we can’t find this exact mix online. 

#2 Happy Frog Potting Soil

Happy Frog and PittMoss were pretty close but we ended up ranking Happy Frog a little higher really just because it's much easier to find.
We also have the most experience with this blend. But if they were both easy to pickup, we'd probably rank Pittmoss as #2. Happy Frog was in the middle of the range as far as germination goes and put on healthy, steady growth every week since. It's a great out of the bag mix that has been reliable for us.

#2 Our Mix

Ok, ok, we know what you're thinking. Of course we choose "our mix" as the best! We honestly didn't think this blend would be our favorite. We threw it in there as kind of a baseline so we'd know how our mix this year was doing.

This mix is really just a bunch of old potting soil (mostly Happy Frog and Pro-Mix) with some fertilizers added. We think that made all the difference. This is great news because it goes to show that even potting soil that is 2 years old can be re-purposed into a great mix with just some easy to find soil amendments.

For reference, this mix included: Espoma's Tomato Tone, Crab and Lobster Shell, Kelp Meal, Sulfur, Worm Castings and Gypsum.

What we recommend

We still love our Happy Frog mix and we still recommend growing with Pro-Mix, and if you can get your hands on the PittMoss Commercial Blend, go for it! All of those are good options for GreenStalk Gardening and probably container gardening in general. If you choose Pro-Mix, be sure to add some fertilizer upon planting.

Of course, there are several other good mixes out there that we just didn't include in this test. Your local garden center can make excellent recommendations based on your area and climate.

Potting mix is a surprisingly controversial topic.Many people have their own blends that they swear by and won't use anything else. And we think that's great! Use what you love and works for you.

Leave us a comment below. Are you surprised with the result of our soil test? What potting mix do you use? What should we test next?

We'd love to hear from you!


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