You hear and read it all the time “Quality soil is the foundation of beautiful, productive and healthy gardens and containers”. It is something I include in most presentations and when answering gardening questions. Healthy soil is key to gardening success but what should you buy or add to your soil to create ideal growing conditions for your plants?
Much of gardening comes from experience but understanding the basics of soil and amendments can help shorten the learning curve. Let’s compare topsoil, potting mixes, and compost; three common planting media. Understanding their differences and best uses will help you invest wisely and achieve the gardening results you desire.
Topsoil is used to create new or raised planting beds and improve existing garden soil. True topsoil is the uppermost portion of the ground beneath our feet. It contains organic matter that gives it the typical darker color. Its structure makes it easier to cultivate and better able to absorb water. The higher level of biological activity, insects and microorganisms, reduces compaction and increases soil health.
Unfortunately, this upper layer is the first to be harvested by companies that sell topsoil. Once the first layer disappears the lower levels that contain more sand, clay and lime are also sold as topsoil. These lower depths, called subsoil, tend to be compacted or fast draining and difficult to till. This makes them less ideal for gardening, although some can be amended to improve plant growth.
As an avid gardener and composter, I often get asked if compost can be used as a substitute for potting soil. It’s a great question!
Compost offers many benefits – it’s nutrient-rich, full of beneficial microbes, and helps soil retain moisture. So it would seem to make an ideal potting medium, right?
Well, not exactly While compost has a lot going for it, potting soil is specially formulated to provide the right blend of drainage, moisture retention, and light weight for container gardening
In this article, I’ll compare compost and potting soil and offer tips on how to use compost safely and effectively in containers.
The Benefits of Compost
First let’s look at why compost CAN potentially work as a potting mix ingredient. Here are some of the main benefits it provides
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Nutrients: Compost contains nitrogen, phosphorus, potassium and many micronutrients that plants need. This acts as a mild organic fertilizer.
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Microbial life: The bacteria, fungi and other microbes in compost help suppress disease and make nutrients available to plant roots.
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Moisture retention: Compost’s organic matter helps increase moisture retention in fast-draining potting mixes.
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Soil structure: Compost provides organic matter that improves soil texture and creates healthier root growth.
So with all these pros, why not just plant directly in pure compost?
The Drawbacks of Straight Compost
While compost can certainly be added to potting mix, using it on its own has some drawbacks:
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Lack of support: Compost is very lightweight and doesn’t provide adequate structural support for large root systems.
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Poor drainage: Straight compost compacts easily and tends to get waterlogged, which deprives roots of oxygen.
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Nutrient issues: Immature compost can tie up nitrogen, while mature compost may provide excess nutrients.
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Weeds/diseases: Compost can contain weed seeds, pathogens, or pest larvae that can plague containers.
For these reasons, potting soil is specially blended to avoid these pitfalls.
Why Potting Mixes Are Formulated Specifically
Here’s what makes a good potting mix different than straight compost:
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Ingredients for drainage: Perlite, vermiculite, bark, coconut coir, and sand keep soil loose.
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Peat or coir for moisture: These help retain water so roots don’t dry out as quickly.
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pH buffering: Ingredients like lime or peat control pH fluctuations in containers.
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Low fertility: Excess nutrients can accumulate quickly in pots. A mild starter charge is ideal.
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Sterile: Commercial mixes are pasteurized to kill weed seeds, insects, and diseases.
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Lightweight: Reduced soil weight prevents compaction and provides better drainage.
Blending materials like this creates an ideal growing medium for containers.
Best Practices for Adding Compost
The key is finding the right balance by amending potting mix with compost, not replacing it entirely. Here are some tips:
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Use finished, screened compost to remove large pieces and weed seeds.
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Limit compost to 25-30% of total mix. Too much causes compaction or sogginess.
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Consider sterilizing the amended mix with heat to kill pathogens if needed.
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Test pH and nutrient levels of amended mix and adjust as needed.
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Reduce fertilizer to account for compost nutrients. Use a mild organic liquid feed.
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Add more perlite or vermiculite to improve drainage.
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Use compost-amended mixes within 1-2 months before they decompose further.
Follow these guidelines and your plants will reap the benefits of compost while avoiding the pitfalls of straight compost as potting soil.
Other Ways to Use Compost in Containers
If amending potting mix seems too complicated, here are two easier ways to gain benefits of compost for containers:
Topdressing
Spread a 1/4 to 1/2 inch layer of compost over the soil surface when first planting. This allows compost to slowly filter into soil each time you water.
Compost Tea
Brew a mild compost tea by steeping compost in water, then use the “tea” to fertilize plants every 2-4 weeks. The nutrients and microbes actively boost plant health.
Mature, high-quality compost can be an excellent addition to potting soil at a 25-30% mix ratio. But potting soil should still make up the bulk of the medium to provide proper drainage and moisture control.
With some care taken during preparation and amendments, compost and potting soil can work hand-in-hand to grow thriving container plants.
How Do I Select a Compost?
Just like buying topsoil, ask the compost vendor for details about the ingredients, processing, and quality control used when creating their product. Review any available test data and ask about potential of pesticides, weed seeds, and invasive plants or pests. Some states have strict standards for composting. Contact your local Extension Service to find out if your state has standards to ensure compost vendors are producing a quality product.
Take a look at the compost. Finished quality compost is dark and crumbly and has a mild earthy smell. It should be moist not overly dry or wet. Avoid extremely hot compost that is still in the midst of the composting process. Finished compost may be warm and give off some steam.
Check the label when purchasing bagged compost. More detail is now being provided since consumers are requesting assurances the product they buy is organic. Look for words on the label such as organic, free of pesticides and contaminants and OMRI listed. The Organic Materials Review Institute (OMRI) is a private, nonprofit organization that determines if a product meets the organic standards of the United States Department of Agriculture (USDA) National Organic Program. The OMRI listing on the bag means the compost is approved for organic use according to USDA rules.
How Do I Select a Topsoil?
Most states do not have regulations on the makeup of topsoil being sold. That means you need to be an informed and discerning consumer. A visual inspection is a good place to start but this alone is not enough. Dark soil may indicate a desirable level of organic matter but it may also signify the soil was dredged from acidic wetlands. These types of soil do not work well in a typical garden setting. In addition, some vendors add wood ash to subsoil to darken the color without improving the soil.
After a visual inspection do the feel test. Grab a handful of moist soil and gently squeeze. It should crumble into smaller pieces. Then wet a handful and run it through your forefinger and thumb to create a ribbon. Sticky samples contain a high percent of slow-draining clay, gritty soil has a higher percent of sand that drains quickly and silt feels like flour. A soil that contains some of all three is better for gardening.
Some topsoil is harvested from farmland being developed for businesses and housing, forests being cleared or abandoned fields. This soil may contain lots of weed seeds, invasive plants, and jumping worms or herbicide residue. No one wants to spend money on unwanted surprises that result in more work or problems to manage.
The best option is to ask the topsoil vendor for soil test information. Most do not test their soil before selling. You may decide to take a sample and send it to the University Extension or a State certified soil testing lab for analysis. Ask for the basic testing and if you are growing edibles consider having them check for heavy metals as well.
This approach may not be practical. Once you get the test results the soil you tested may have been sold and new topsoil from a new location or subsoil is now available.
The most practical solution is to know your topsoil supplier. Ask friends and family for recommendations on where they purchase topsoil and if they have been satisfied with the product. And ask the vendor you select for information about the soil they sell. Get information on where the soil was collected, was it amended and screened to remove large stones and debris and did they amend subsoil to create planting mixes. And take a whiff. Don’t buy soil with a chemical smell or off-odor.
When buying bagged soil you will need to depend upon the sales staff. Ask the same questions and see if they have a broken bag you can examine. Just like buying bulk topsoil the soil in the bags may not be consistent from one bag to the next.
Do’s and Don’ts to making your own potting soil – Cheapest way to make your own soil
FAQ
Can I use only compost as potting soil?
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YouTube · LSU AgCenterhttps://www.youtube.comUse compost to replenish your potting soil – YouTubeOct 18, 2019 — and mix it in and it’s a great way to not only amend this old potting soil for drainage. and workability. but we’re also putting a shot of nutrients…
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Milorganitehttps://www.milorganite.comThe Difference Between Topsoil, Compost & Potting Mix – MilorganiteJan 4, 2020 — Use compost to improve garden soil, topdress your lawn, as a component in potting mixes or for mulching gardens and houseplants. Growing directly in …
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Minnesota Pollution Control Agencyhttps://www.pca.state.mn.usHow do I use my compost?Potted plants, flower gardens, and new trees and shrubs all require specific ratios of compost to soil. These soil mixes range from 5 to 50 percent compost. The…
Can you use compost as potting mix in pots?
It is best dug into the existing soil and not recommended for use in pots and containers because it is continually breaking down and as it does this, it …
Is compost good for potted plants?
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Improves Soil Structure:Compost adds organic matter to the soil, which helps to improve its structure, making it looser and more porous. This allows for better drainage and aeration, preventing root rot and promoting healthy root growth.
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Enhances Water Retention:Compost helps the soil retain more moisture, which is particularly important in pots where soil can dry out quickly. This can reduce the need for frequent watering.
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Provides Nutrients:Compost is a rich source of nutrients for plants, providing them with a slow-release form of fertilizer. It also introduces beneficial microbes to the soil, which help to break down nutrients and make them available to plants.
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Reduces Soil Compaction:Compost can help to prevent soil from becoming compacted, which is a common problem in containers. Compaction can restrict root growth and reduce drainage.
Can you put mature compost in a potting mix?
Just like in garden soil, mixing mature compost into your usual potting mix for container improves its water-holding capacity. Keep in mind though, that in the confined space of a container, too much compost can lead to root burn from the excess phosphorus in the soil.
Is compost good for potting soil?
Compost is full of essential nutrients that plants need for healthy growth. It also helps to aerate the soil and improve drainage. Coco peat or vermiculite helps to retain moisture, making it ideal for planting in pots or containers. To make your potting soil, mix equal parts of each ingredient.
Can you add compost to potting soil?
Adding compost can boost growth when potting new houseplants or transferring plants to larger pots. Mix with potting soil and/or peat moss for better absorption. Steeping your compost in a liquid emulsion is one way to concentrate the nutrients because it makes them easier to absorb.