A Year-long Test Reveals the Best Soil Mix for Raised Beds
Comparing Three Blended Soils for Raised Beds: A Detailed Analysis
Growing vegetables in raised beds has been a part of the gardening journey for many enthusiasts. In the early years, using native soil is a common choice, but when it comes to acquiring soil for new raised beds, the question arises: What is the optimal raised bed soil mix?
One gardener decided to find the answer by conducting an experiment, testing three different raised bed soil mixes over the course of a year. Here are the results of his year-long test, detailed in the Beginner’s Garden Podcast.
Three Raised Bed Soil Mixes
In selecting the raised bed soil mixes to test, the gardener chose two common combinations that many gardeners use, including topsoil and compost in various proportions. For the third bed, he used organic bagged soil, bought from Home Depot, to compare performance with bulk-purchased ingredients.
The three raised bed soil blends the gardener tested were:
- Mel's Mix: Consisting of one third peat moss, one third vermiculite, and one third compost, as recommended in Mel Bartholomew's book Square Foot Gardening. The use of diverse compost sources is key in this mix, with contributions from bulk compost, worm castings, and the gardener's own chicken manure.
- Perfect Soil Recipe: With 50% topsoil, 30% compost, and 20% other organic matter, as recommended by Joe Lamp'l. The gardener used various organic matter sources, including homemade compost, chicken manure, and worm castings.
- Bagged Soil for Raised Beds: A blend of organic garden soil and topsoil, with added fertilizers, purchased from Home Depot.
Method of Testing
In each of the three raised beds, the gardener planted Roma tomatoes, growing them from seed indoors and planting all nine seedlings at the same time.
First Impressions
Upon initial planting, each raised bed presented unique characteristics. Mel’s Mix offered excellent workability, draining well and not becoming waterlogged or experiencing drainage issues in heavy rain. No weeds were present in the first month, and the tomato plants started out very healthy.
The bed with the "Perfect Soil Recipe" did not perform as well as Mel’s Mix, primarily due to excessive clay content in the topsoil sourced from a river bottom. This resulted in poor drainage, compacted soil, and smaller, struggling tomato plants.
The bagged soil mix was easy to put together and similar to the light, fluffy nature of Mel's Mix when planted. However, the tomato plants barely grew, and this continued to be an issue for this bed.
After One Month
The tomatoes in Mel's Mix thrived, with strong growth and healthy appearance. Weeds began to develop, but they were manageable.
The tomatoes in the Perfect Soil Recipe bed caught up with the Mel's Mix plants towards the end of the season, likely due to improved drainage as the rain let up.
The tomatoes in the bagged soil mix were not healthy, with many leaves turning yellow and the plants appearing stunted. Although they did not die, their growth remained poor.
Total Yield Comparison Between Raised Bed Soil Mixes
The three raised beds were weighed meticulously for the total yield of Roma tomatoes throughout the growing season. The results showed that the beds with Mel's Mix and the Perfect Soil Recipe yielded 5.3 and 7 pounds per plant, respectively, compared to 1.34 pounds per plant in the bagged soil mix.
This shows that the plants in Mel's Mix performed the best, while the bagged soil mix was disappointing. It is important to note that overall tomato yields were lower in that particular year due to excessive rainfall and the gardener's battles with early blight.
Fall broccoli plants, grown following the tomatoes, revealed slightly different results, with the bagged soil mix producing larger broccoli heads than the other two mixes. This difference mainly occurred because bagged soil mix was planted later and thus benefited from the return of soil life.
Soil Test Results from Three Raised Bed Soil Mixes
Soil tests conducted by the local cooperative extension service revealed the following differences between the three raised bed soil mixes:
- Mel's Mix: pH 5.6, with high levels of phosphorus, potassium, and zinc, but all other micronutrients tested were also high. The high acidity is not surprising due to the naturally acidic quality of peat moss.
- Perfect Soil Recipe: pH 6.8, which is near ideal for most vegetables. The blend had high levels of phosphorus, potassium, and zinc, with all other micronutrients also high. Notably, the levels of calcium were almost double that of Mel's Mix.
- Bagged Soil Mix: pH 7.5, which is high for most vegetables, especially tomatoes. This mix had high levels of phosphorus, potassium, and zinc, but some micronutrients were erratic. The mix also had a high amount of sulfate and a very high level of calcium.
Biggest Lessons
Based on the test results, the gardener drew several conclusions for future raised bed gardening:
- Although the Mel's Mix blend proved more successful, its high cost makes it impractical for large-scale raised bed gardens. If only a small raised bed is being built, it may be worth the investment.
- The Perfect Soil Recipe, with a more neutral pH and richer nutrients, has better longevity.
- To maximize soil fertility, it's essential to incorporate various compost and organic matter sources.
The Best Raised Bed Soil Blend
Using what he learned from the test, the gardener developed his preferred raised bed soil blend:
- A layer of tree limbs, logs, sticks, fallen leaves, and garden debris at the bottom of the bed
- Approximately a 50/50 blend of compost (bulk and homemade) and topsoil (bulk and bagged)
The inclusion of organic matter not only reduces the amount of soil needed but also provides a nutrient-rich base for the raised bed. The combination of compost and topsoil forms the basis of the soil, while the incorporation of organic matter helps maintain its longevity.
The key to the mix is healthy, living organic matter. Learn more about this in the article, “What's Wrong with my Raised Bed Soil?”
Calculating Soil for Raised Beds
If you're unsure about the soil volume required for a raised bed, this handy online soil calculator can help you determine the right quantity. For a specific calculator on how much peat moss, vermiculite, and compost to purchase, when going with Mel's Mix, this soil mixture calculator can help you find the right proportions.
Share Your Experiences
Share your experiences with different raised bed soil mixes in the comments below. We would love to hear from you.
Resources
- Introduction to Square Foot Gardening
- The Smart Gardener: Essential Knowledge for Making Your Garden Grow, by Joe Lamp'l
- Updated Raised Bed Soil Recipes
- Soil Testing for Gardeners
- For those interested in raised bed gardening, growing vegetables, and improving their gardening lifestyle, the Beginner’s Garden Podcast offers an in-depth look at the best raised bed soil mixes, as discovered through a one-year experiment conducted by a gardener.
- In the course of the experiment, three raised bed soil mixes were tested – Mel's Mix, the Perfect Soil Recipe, and a Bagged Soil Mix – with the former consisting of a 1:1:1 ratio of peat moss, vermiculite, and compost, respectively.
- The gardener's findings revealed that Mel's Mix delivered the best results, with larger yields and healthier plants compared to the other two mixes, which experienced varying levels of success based on their specific compositions.
- To help home-and-garden enthusiasts achieve their raised bed gardening goals, the gardener also shared a preferred raised bed soil blend, which includes a layer of organic matter at the bottom of the bed and a 50/50 blend of compost and topsoil for a nutrient-rich base.
