have you ever wondered what kind of raised bed soil mix is the best?
i have been growing in raised beds from the very beginning of my garden journey. up until last year, i used native soil on my property to fill them. but when i needed to find soil outside my property for three new raised beds last season, 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.
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3 types of raised bed soil
when deciding which raised bed soil combinations i wanted to test, i chose two common mixes that many gardeners choose, as well as a 100% bagged option for those who can’t purchase ingredients in bulk. here are the three raised bed soil blends i tested:
i first chose mel’s mix from mel bartholomew’s book square foot gardening. it consists of 1/3 peat moss, 1/3 vermiculite, and 1/3 compost (using as many sources as possible). 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, and 20% organic matter. joe has many different suggestions on sources of organic matter, and 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. not everyone has access to bulk ingredients, so i wanted to choose an option that many people must use. in this bed, i purchased four bags of organic garden soil, two bags of organic garden soil “plus,” (with extra organic fertilizers), two 40-lb. bags of topsoil, and three cups of worm castings. everything in this bed, you could buy at your garden center.
method of testing raised bed soil mix
in these three mixes, 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.
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 ingredients were the most expensive to purchase.
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. 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.
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 gets very hard as it dries out.) although i kept constant drip irrigation on the beds during the dry summer, 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.
- mel’s mix: 7 lbs/plant
- perfect soil recipe: 5.3 lbs/plant
- bagged soil: 1.34 lbs/plant
- 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.
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 planting them in the bagged soil bed about a month later.
whatever difference i
witnessed 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’ve 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.
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:
- 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.
- 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.
- bagged soil mix: ph 7.5 — this is very 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.
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.
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.
the bagged mix is a mystery to me because this brand was recommended by a trusted gardening friend, 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:
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 because it was all bagged if the soil was missing some of the micro-organisms native in soil. perhaps this was why it eventually corrected itself with the broccoli. third — and perhaps most obvious — it was just 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.
after having tested these three blends of raised bed soil mix, what will i do with future beds?
- native soil is my favorite. the year that i had the best growth was when i used soil from another part of our property. it worked well and it’s free.
- 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. this would be my second choice if native soil isn’t an option.
- if i use bagged soil again, i will add more topsoil — a lot more. i think more topsoil would help balance out the forest material in the bagged soil and perhaps buffer the ph issues.
- i think, 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!
- 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.
i loved doing this test and i’ll continue to test in my garden. i’ll have to see what soil i’ll use in my new beds, but there will be plenty of opportunities to test new things!
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?