3 Raised Bed Soil Mixes Compared

Updated October 2021

Have you ever wondered what kind of raised bed soil mix is the best?

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I have been growing in raised beds from the very beginning of my garden journey. In the early years, I used native soil from my property to fill them.

But when I needed to find soil outside my property for three new raised beds, I decided to conduct an experiment. I tested three different raised bed soil mixes and today I’m sharing the results of my year-long test.

(Click below to listen to my discussion on the Beginner’s Garden Podcast, or continue reading.)

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3 Types of Raised Bed Soil Combinations

When deciding which raised bed soil combinations I wanted to test, I chose two common mixes that many gardeners use that include topsoil and/or compost I bought in bulk from my local landscaping company.

In the third bed I used organic bagged soil only, purchased from Home Depot. Because many gardeners can’t purchase ingredients in bulk, I wanted to test this option to see how it compared.

Here are the three raised bed soil blends I tested:

Mel’s Mix

I first chose Mel’s Mix from Mel Bartholomew’s book Square Foot Gardening. This mix consists of:

• 1/3 peat moss
• 1/3 vermiculite
• 1/3 compost

The compost is the key here. In Square Foot Gardening, Mel recommends using compost from as many sources as possible. (Just an anecdotal observation here… those to whom I’ve spoken over the years who have had poor results using Mel’s Mix seem to have had poor quality compost or compost from just one source.)

To achieve as diverse of compost blend as I could, I used bulk compost (compost I bought from a local landscaping company), worm castings, and chicken manure from my own chickens.

Perfect Raised Bed Soil Recipe

The second mix I tested is known as the “Perfect Soil Recipe” as recommended by Joe Lamp’l. It consisted of:

• 50% topsoil
• 30% compost
• 20% other organic matter.

Joe has many different suggestions on sources of organic matter. I used homemade compost, chicken manure, and worm castings.

Bagged Soil for Raised Beds

In my third bed, I chose a blend of bagged soil. I purchased:

• four bags (8 total cubic feet) of organic garden soil
• two bags (2 total cubic feet) of organic garden soil “plus” (with extra organic fertilizers)
• two 40-lb. bags of topsoil
• three cups of worm castings.

Most of this I got at Home Depot, but I’ve also seen it in other garden centers.

Method of Testing Raised Bed Soil Mix

In these three raised beds, I planted Roma tomatoes — three per 3’x6′ bed. Each tomato plant I had grown from seed indoors and I planted all nine of them at the same time.

First Impressions

Mel’s Mix

Planting in Mel’s Mix was a dream. It was light and fluffy and wonderful to work with. No matter how much rain we got (and we had a LOT of rain) it never got waterlogged nor had drainage problems. I had no weeds in the first month and my tomatoes started out incredibly healthy.

But the drawback of Mel’s Mix? The ingredients were the most expensive to purchase.

Perfect Soil Recipe

The bed with the “perfect soil recipe” didn’t start out as great as Mel’s Mix. But, I have to consider a few extraneous factors.

First, we had a record spring rainfall year in 2019, and the topsoil I purchased for the “perfect soil recipe” seemed to contain a lot more clay than I have seen with other topsoil mixes.

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In hindsight, I realized that this particular topsoil is sourced from a river bottom. So while I’m sure it contained high amounts of nutrients, it was pretty dense. It certainly had a different color and texture compared to bagged topsoil mixes (like the one I used in the bed with the bagged soil).

Because of this (from my observations), this bed didn’t drain well and seemed to compact after the heavy rains. Initially, my tomato plants struggled and were smaller than those in Mel’s Mix.

I also had more weeds in this soil than in the Mel’s Mix, though that was probably due to the higher percentage of my homemade compost.

Bagged Soil Mix

The bagged soil mix was extremely easy to put together and, like Mel’s Mix, was a dream to plant in. It did not compact or have drainage issues in the heavy rain. But, the tomato plants from the very beginning barely grew, something that continued to be an issue for this bed.

After 1 Month

The tomatoes in Mel’s Mix grew vibrantly and were strong with tender leaves. Weeds did start to develop, but they weren’t too bad.

The tomatoes didn’t grow quite as well in the “perfect soil recipe,” but they did grow consistently.

The biggest shock was with the tomatoes in the bagged soil mix. These were not healthy at all. Many of the leaves were very yellow and the plants itself appeared severely stunted. I had planned to keep everything the same in all 3 beds. However, these looked so unhealthy that I started watering them with fish emulsion to try to save them because I feared I would lose them.

Rest of the Season

The tomatoes in Mel’s Mix continued to be strong and vibrant. I expected them to slow in growth due to the high concentration of peat moss.

When I’ve used potting soil with a lot of peat moss in the past, it tends to repel water as it dries out. Surprisingly, this did not happen. I did keep consistent drip irrigation on the beds during the dry summer, and this mix never did compact or dry out as I would have expected.

The tomatoes in the Perfect Soil Recipe bed ended up almost catching up with the Mel’s Mix plants. I think this was probably because the rain let up and the plants were no longer drowning.

While the tomatoes in Mel’s Mix and the “perfect soil recipe” ended up performing well, I can’t say the same about the tomatoes in the bagged soil mix. Although they didn’t die as I had feared, they never rebounded and never looked healthy.

Total Yield Comparison Between Raised Bed Soil Mixes

The biggest indicator of the success of any raised bed soil mix is the total yield, and this I measured meticulously throughout the garden season.

As mentioned previously, I grew Roma tomatoes in all of these beds. As a determinant variety that bears all at once, I could weigh the harvest within a short window of time before pulling them up and planting a second crop to test.

Below are the yield results from these Roma tomatoes in each mix, compared with Romas I also planted in my in-ground garden. I must point out, in this particular year, my tomato yields were lower overall, probably due to the excessive rainfall and my battle with early blight. Still, you can see the marked difference between the beds.

1. Mel’s mix: 7 lbs/plant
2. Perfect soil recipe: 5.3 lbs/plant
3. Bagged soil: 1.34 lbs/plant
4. In-ground Roma tomatoes: 2.59 lbs/plant

You can see that my plants in Mel’s Mix did much better than any other soil, and the bagged soil mix was quite disappointing.

Also of note: the Roma tomatoes in the ground suffered more from early blight, and I also planted them slightly closer to one another. Still, this result remains consistent for my garden — tomatoes in raised beds generally out-perform their counterparts in the ground soil.

Fall Broccoli in Different Raised Bed Mixes

As you see above, my first experiment tested the performance of Roma tomatoes in the summer. For my second test, I planted broccoli. The results of that test differed slightly.

In September, I planted broccoli I started from seed indoors both in the Mel’s Mix and the Perfect Soil Recipe beds. I didn’t have enough seedlings to plant them in the bagged soil bed. I ended up buying transplants from a garden center and planted them in the bagged soil bed about a month later.

Whatever difference I saw between Mel’s Mix and the Perfect Soil Recipe in the early growth of Roma tomatoes disappeared in the fall-planted broccoli. Both beds performed extremely well and produced lush, vibrant broccoli — the largest I’d ever grown.

I did have a peculiar fall, however. A record-October heatwave prevented this lush broccoli from forming heads, and an early freeze in November damaged the plants severely. Because of this, I barely harvested any heads from this group.

As a contrast, the smaller plants in the bagged soil mix (because I planted them a month later), withstood the cold snap and went on to produce full, large broccoli heads. Though the plants themselves never grew at the rate that those in the first two mixes did, they produced well.

Soil Test Results from 3 Raised Bed Soil Mixes

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At this point, my only conclusions could be drawn from observing the growth of two crops — tomatoes and broccoli. But what would a soil test reveal about the differences between these three raised bed soil mixes?

Prior to the broccoli planting, I sent off soil samples to my local cooperative extension service. Here were the highlights of the differences between the soil test results:

1. Mel’s Mix: Ph 5.6 (not surprising because peat moss is naturally acidic). It tested “above optimum” on phosphorus, potassium, and zinc, and all other micronutrients tested.
2. Perfect Soil Recipe: Ph 6.8 which about perfect for most vegetables, but perhaps a little on the high side for tomatoes. This recipe showed above optimum levels on phosphorus, potassium, and zinc, and all other micronutrients. Of special note, these levels were also above Mel’s Mix in each. Plus, it had almost double the calcium of Mel’s Mix.
3. Bagged Soil Mix: Ph 7.5 — this is high for most vegetables, especially tomatoes. It was also above optimum on phosphorus, potassium, and zinc, but the levels were erratic on some of the micronutrients. I also noted a very high amount of sulfate compared to the other two and very high on calcium.

Speculations in Differences Between Raised Bed Soil Mixes

I’m not a soil scientist, and I can’t tell you all the “whys” behind what I found from my test. But between the soil test and my own observations, I have some thoughts about the differences in the soil mixes.

Conclusions on Mel’s Mix

I think that Mel’s Mix performed well because our spring was so wet and the peat moss didn’t have a chance to dry out. Instead, the drainage capacity of the peat moss proved helpful in our rainy spring.

Conclusions on the Perfect Soil Recipe

I believe the Perfect Soil Recipe started slowly because of the amount of rain and because of the high amount of clay in the topsoil I purchased. If I had obtained topsoil with less clay content, I imagine this blend would have rivaled Mel’s Mix or even surpassed it.

Conclusions on Bagged Soil Blend

The bagged mix blend is a mystery to me because this brand was recommended by several trusted gardening friends, and most of those I’ve asked have had a much better experience with bagged soil. This is also why I am not disclosing the brand — I do not believe my experience is necessarily indicative of all of this brand’s products.

So, if so many people get great results with bagged soil (even organic bagged soil like mine), why were my results so terrible? I have a few theories.

Why bagged soil didn’t perform well in my raised bed:

It seemed like these bags of soil had a lot of tree bark and perhaps it hadn’t broken down enough. Instead, as it continued to break down it tied up nitrogen my tomatoes desperately needed.

I also wonder if the soil was missing some of the micro-organisms and soil life that is present in both native soil and compost. Perhaps this was why it eventually corrected itself with the broccoli — the soil life had a chance to return.

Third — and perhaps most obvious — was the high Ph. Broccoli can tolerate higher pH than tomatoes can.

And although I can’t tell you which of my theories is correct — maybe all three played a role — I can tell you, I learned several lessons from this experience with bagged soil.

Biggest Lessons

After having tested these three blends of raised bed soil mix, what will I do with future beds?

1. Although Mel’s Mix performed better than I expected, it’s not my choice because of the expense. If I were to only build one small raised bed, it would prove worth it, perhaps. But as I expand my raised bed gardens, I just can’t justify the cost of Mel’s Mix when the “perfect soil recipe” (with better topsoil) has the potential to perform just as well if not better.
2. The Perfect Soil Recipe seems to have better longevity because of the amount of nutrients found in the soil, and it also had a more neutral Ph.
3. No matter what blend I choose, it’s important to add diversity. I would include as many sources of compost and organic material as possible. The greater variety we can have with our organic material the better!

The Best Raised Bed Soil Blend

I loved doing this test in 2019 and since then I’ve continued to test in my garden. I took what I learned and adapted to what has been my go-to raised bed blend since.

What’s the formula?

1. Tree limbs, logs, sticks, fallen leaves, and/or garden debris at the bottom of the bed
2. Add roughly a 50/50 blend of compost (bulk and/or homemade) and topsoil (bulk and/or bagged).

The tree limbs and sticks not only help to lessen the amount of soil I need to obtain for a raised bed, but as they break down they create rich, nutritious organic matter.

Although this may be seen as a negative thing, as it breaks down, the soil level will drop. This “forces” me to have to add more compost each season, which continues to increase the longevity of the soil fertility.

The 50/50 blend of compost and topsoil form the basis of the raised bed soil. Sometimes I mix it; sometimes I layer topsoil and then compost on top. Both have worked equally well for me.

The key, I believe, is rich organic matter that is alive. You can learn more about that in What’s Wrong with my Raised Bed Soil?

How much soil you need for a raised bed?

One more thing you may be asking is how much soil will you need in your raised bed, regardless of which mix you choose? This handy online soil calculator will get you started.

For a specific calculator on how much peat moss, vermiculite, and compost to purchase if you go with Mel’s Mix, this soil mixture calculator will help you know how much of each to obtain.

What about you? What blends of raised bed soil mix have worked well (or not so well) for you?

Source: https://gardencourte.com
Categories: Recipe