Raised Bed Soil Experiment: Early Results After Five Months
How much does the type of soil in a raised bed affect plant growth? That is the question behind a three-year experiment comparing eight different growing media. The first season is only about halfway complete, but after roughly five months, some clear differences are already appearing.
The experiment includes eight raised beds, each filled with a different soil or soil mix. Each bed contains the same types of plants—tomatoes, peppers, zinnias, salvia, marigolds and petunias—and all of the beds are irrigated and managed as consistently as possible. When fertilizer or water is applied, every bed receives the same treatment.
Growing medium Composition Early results
100% soil Soil of unknown origin, probably silt loam Best overall growth; large tomato, salvia and
zinnia plants; pepper producing
Miracle-Gro raised bed soil Commercial soilless bagged raised-bed mix Very good; strong growth from the beginning
½ soil + ½ garden mix More than half actual soil plus garden mix Very good; strong recovery and vigorous salvia and zinnias
Garden mix ¼ soil, ¼ compost, ¼ pine fines, ¼ coarse Moderate; initially stunted but recovered
sand
Pro-Mix ⅓ soil, ⅓ sand, ⅓ compost Moderate; initially yellow and stunted, then improved
Black Kow manure Bagged composted manure Initially poor and yellow; plants have begun recovering
“Topsoil” Bagged finely shredded pine bark Initially very poor; improved with supplemental nitrogen
Leaf compost Shredded, composted leaves Weakest early performance; plants remain small and some leaves are yellow
Soil Comes Out on Top—For Now
The strongest early performer is the bed containing 100% soil. The tomato, zinnias, salvia and pepper are all growing well, with the salvia particularly vigorous. However, there is an important qualification: the source and exact composition of the soil are unknown. It appears to be a soil with substantial clay content, possibly a silt loam.
That may explain some of its early success. Clay soils can hold water and nutrients, giving plants access to resources during the growing season. The potential drawback is that a soil with little organic matter can compact over time, reducing the pore space that plant roots need for air and water movement. Also organic matter is important to plants trying to get nutrients from the clay soil.
The second strongest performer so far is the commercial Miracle-Gro bagged raised bed soil. Plants grew well almost immediately, although the growing medium includes a significant amount of fertilizer. It will be interesting to see whether that early advantage continues into the second and third years.
Some Amendments Don't Work Well as Standalone Growing Media
Several of the other beds demonstrate an important distinction between a soil amendment and a complete growing medium. “Topsoil” as sold in big box stores the southeast US, for example, may consist largely of finely shredded pine bark. Leaf compost, manure and mixes such as Pro-Mix and garden mix can also be useful amendments, but, as the results so far show, they may not provide the combination of nutrients, water-holding capacity and structure needed when used alone.
The topsoil, Black Kow and leaf compost beds all produced small, yellow plants early in the season. Supplemental nitrogen helped the plants recover, but the leaf compost bed continues to lag behind the others.
The experiment also revealed that some materials are settling. The Black Kow bed has dropped noticeably as the organic material decomposes. Similar settling is occurring in the leaf compost and other organic-heavy beds. That change in volume will be another important factor to watch during the three-year experiment.
The Importance of Organic Matter
The strongest long-term mix may ultimately be one that combines actual soil with organic materials. The bed containing half soil and half garden mix is already among the better performers. The added organic matter can help maintain pore space and keep the growing medium from becoming excessively compacted.
For now, the results are preliminary. The experiment is designed to run for three years, and a growing medium that performs well during its first season may behave very differently after repeated watering, settling and decomposition.
At roughly five months into the first growing season, the early ranking is clear: 100% soil is performing best, followed by Miracle-Gro raised bed soil and the half-soil/half-garden-mix combination. The next two years will show whether those early results hold up—or whether the advantages of organic matter and improved soil structure begin to change the standings.
Related Videos:
How to Mix Soil for a Raised Bed: A Simple Guide to the Right Ingredients
Choosing the Right Soil for a Raised Bed Garden
A Season-Long Look at Five Mulches: What Worked, What Didn’t
Related Resources:
Soil Health in Raised Beds
Raised bed gardens
The experiment includes eight raised beds, each filled with a different soil or soil mix. Each bed contains the same types of plants—tomatoes, peppers, zinnias, salvia, marigolds and petunias—and all of the beds are irrigated and managed as consistently as possible. When fertilizer or water is applied, every bed receives the same treatment.
Growing medium Composition Early results
100% soil Soil of unknown origin, probably silt loam Best overall growth; large tomato, salvia and
zinnia plants; pepper producing
Miracle-Gro raised bed soil Commercial soilless bagged raised-bed mix Very good; strong growth from the beginning
½ soil + ½ garden mix More than half actual soil plus garden mix Very good; strong recovery and vigorous salvia and zinnias
Garden mix ¼ soil, ¼ compost, ¼ pine fines, ¼ coarse Moderate; initially stunted but recovered
sand
Pro-Mix ⅓ soil, ⅓ sand, ⅓ compost Moderate; initially yellow and stunted, then improved
Black Kow manure Bagged composted manure Initially poor and yellow; plants have begun recovering
“Topsoil” Bagged finely shredded pine bark Initially very poor; improved with supplemental nitrogen
Leaf compost Shredded, composted leaves Weakest early performance; plants remain small and some leaves are yellow
Soil Comes Out on Top—For Now
The strongest early performer is the bed containing 100% soil. The tomato, zinnias, salvia and pepper are all growing well, with the salvia particularly vigorous. However, there is an important qualification: the source and exact composition of the soil are unknown. It appears to be a soil with substantial clay content, possibly a silt loam.
That may explain some of its early success. Clay soils can hold water and nutrients, giving plants access to resources during the growing season. The potential drawback is that a soil with little organic matter can compact over time, reducing the pore space that plant roots need for air and water movement. Also organic matter is important to plants trying to get nutrients from the clay soil.
The second strongest performer so far is the commercial Miracle-Gro bagged raised bed soil. Plants grew well almost immediately, although the growing medium includes a significant amount of fertilizer. It will be interesting to see whether that early advantage continues into the second and third years.
Some Amendments Don't Work Well as Standalone Growing Media
Several of the other beds demonstrate an important distinction between a soil amendment and a complete growing medium. “Topsoil” as sold in big box stores the southeast US, for example, may consist largely of finely shredded pine bark. Leaf compost, manure and mixes such as Pro-Mix and garden mix can also be useful amendments, but, as the results so far show, they may not provide the combination of nutrients, water-holding capacity and structure needed when used alone.
The topsoil, Black Kow and leaf compost beds all produced small, yellow plants early in the season. Supplemental nitrogen helped the plants recover, but the leaf compost bed continues to lag behind the others.
The experiment also revealed that some materials are settling. The Black Kow bed has dropped noticeably as the organic material decomposes. Similar settling is occurring in the leaf compost and other organic-heavy beds. That change in volume will be another important factor to watch during the three-year experiment.
The Importance of Organic Matter
The strongest long-term mix may ultimately be one that combines actual soil with organic materials. The bed containing half soil and half garden mix is already among the better performers. The added organic matter can help maintain pore space and keep the growing medium from becoming excessively compacted.
For now, the results are preliminary. The experiment is designed to run for three years, and a growing medium that performs well during its first season may behave very differently after repeated watering, settling and decomposition.
At roughly five months into the first growing season, the early ranking is clear: 100% soil is performing best, followed by Miracle-Gro raised bed soil and the half-soil/half-garden-mix combination. The next two years will show whether those early results hold up—or whether the advantages of organic matter and improved soil structure begin to change the standings.
Related Videos:
How to Mix Soil for a Raised Bed: A Simple Guide to the Right Ingredients
Choosing the Right Soil for a Raised Bed Garden
A Season-Long Look at Five Mulches: What Worked, What Didn’t
Related Resources:
Soil Health in Raised Beds
Raised bed gardens