“Good soil is the base of beautiful, productive, and healthy gardens and containers” is something you hear and read all the time. It is something I include in most presentations and when answering gardening questions. Having healthy soil is important for gardening, but what should you buy or add to your soil to make it the best for plants?
A lot of gardening is learned by doing, but knowing the basics of soil and amendments can help you get started faster. 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. The lower layers with more sand, clay, and lime are also sold as topsoil after the first layer is gone. 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.
If you’re an avid composter, you may have wondered if you can use your home-made compost in place of store-bought potting mix for your container plants. While compost offers some benefits in containers, there are good reasons why potting soil is specially formulated for growing plants in pots In this article, we’ll look at the pros and cons of using compost as a potting mix substitute and offer some best practices on how to incorporate compost into containers safely
Why Potting Mixes Are Specially Formulated
Potting mixes are blended for several important reasons
-
Drainage – Container plants need a very porous lightweight medium that drains rapidly after watering. Dense garden soil compacts over time and prevents drainage. Potting mixes contain ingredients like perlite vermiculite, coco coir, bark chips, and sand to create lots of air pockets and prevent compaction.
-
Moisture retention – While drainage is important, potting mixes also need to retain some moisture so roots don’t dry out too quickly. Ingredients like peat moss and coco coir help achieve this balance.
-
Low fertility – Potting mixes are formulated to provide only minimal nutrients, as it’s easier to control fertilizer in containers. Garden soil and compost may provide excess nutrients, leading to problems.
-
pH buffering – Mix components like peat moss and lime help buffer pH in containers, where it can swing up and down rapidly. Garden soil and compost lack these buffers.
-
Sterile – Bagged potting mixes are pasteurized to kill pathogens, pests, and weed seeds that can plague containers. Compost and soil are not sterile.
Benefits of Adding Compost to Potting Mix
While straight compost may not make the ideal container medium, adding some compost to potting mix offers the following benefits:
-
Nutrition – Compost provides a gentle nutrient charge from organic matter and microorganisms that boost plant growth.
-
Moisture retention – The organic matter in compost holds moisture well when blended into potting mix.
-
Soil structure – Compost improves the texture of potting mix, creating nice clumping and aggregation for better root growth.
-
Suppression of diseases – Beneficial microbes in compost help protect against root diseases like pythium root rot.
-
Sustainability – Using homemade compost reduces reliance on peat-based mixes which aren’t renewable.
Best Practices For Adding Compost to Containers
Follow these guidelines to safely incorporate compost into your container potting mix:
-
Use well-aged, finely sifted compost to reduce potential for weed seeds or pathogens. Screen compost through 1/4″ hardware cloth.
-
Limit compost to no more than 30% of the total potting mix by volume. For example, mix 3 parts potting soil with 1 part compost. Too much can compact, retain excess water, or create nutrient imbalances.
-
Consider pasteurizing your finished mix by heating to 180°F for 30 minutes to kill weed seeds, fungi, and insects if desired.
-
Test the pH and nutrient content of the amended mix before planting. Adjust as needed to maintain a neutral to slightly acidic pH.
-
Consider reducing fertilizer levels to account for nutrients supplied by compost. Use a dilute liquid organic fertilizer and monitor plants closely.
-
Increase the perlite or vermiculite levels in the mix to counteract compost’s density and improve drainage.
-
Use within 1-2 months, before compost has time to compact and degrade further. Don’t store amended mixes long term.
Other Options for Using Compost in Containers
If you don’t want to blend compost directly into your potting mix, here are two other great ways to gain its benefits in containers:
Top dressing
Spread a 1/4 – 1/2 inch layer of compost over the surface of potting mix when first planting containers. This allows the compost to slowly filter down into the mix with each watering. Reapply topdressing annually.
Compost tea
Brew a diluted compost tea using a ratio of 1 part compost to 10 parts water. Use this nutrient-rich “tea” to water container plants every 2-4 weeks as an organic liquid fertilizer. The microbes will help suppress disease as well.
The Bottom Line
With some precautions, adding compost to potting mix can provide nutrition and good moisture retention for container plants. But compost should not be used to completely replace a properly formulated, commercial potting soil. Test amended mixes carefully and adjust fertilizer and drainage components as needed to achieve the right balance for thriving container plants.
What is Potting Mix?
You’ll find a variety of bags labeled as planting mix, potting mix, container mix and more. Check the label to see what the bag contains and recommendations for its use.
For now, let’s focus on potting mixes. These usually contain inorganic and organic materials and may also include sand and mineral soil. They may or may not be sterilized to kill weed seeds and pests. If it doesn’t say sterilized it probably isn’t.
Many potting mixes are labeled as “Soilless”. They have peat moss, sphagnum moss, compost to keep the soil moist, and vermiculite or perlite to help water drain away. They don’t have any mineral soils like sand or clay in them. They are lightweight and blended to hold moisture while draining well.
Some potting mixes are modified to accommodate the needs of certain plants. For better air flow, orchid mixes often have more bark. Cacti and succulent mixes have more sand or perlite for better drainage. African Violet potting mix has more organic matter to make a moist, rich growing medium.
Organic potting mixes are also available. Many gardeners prefer to know the ingredients are free of pesticides and other contaminants. Check for the word organic and OMRI on the label.
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 might have a good amount of organic matter, but it could also mean that it was dug up 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 have a lot of clay that doesn’t drain well, gritty samples have a lot 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. When you get the test results, the soil you tested might have been sold, and you can now get new topsoil or subsoil from a different place.
The most practical solution is to know your topsoil supplier. Talk to your family and friends about where they buy topsoil and whether they are happy with the product. And ask the vendor you select for information about the soil they sell. Find out where the soil came from and if it was changed or screened to get rid of big rocks and other junk. Also, find out if the subsoil was changed to make 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. The soil in the bags might not be the same from one bag to the next, just like when you buy topsoil in bulk.
Can you grow in only compost?
FAQ
Can I use only compost as potting soil?
Is compost good for potted plants?
Can you plant directly into compost?
How much compost should I add to potting soil?
Can compost be used as potting soil?
Compost can be used as potting soil under certain conditions, but it has to be done right to achieve success. Gardeners need to understand the differences between these two substances before planting directly in compost. In general, it is not a good idea to plant straight in compost.
Is compost a potting mix?
Compost is a soil amendment; it is not soil or a suitable alternative like potting mixes. First, compost is light, and can’t support the root structure of plants. Furthermore, compost drains easily, leaving plants needy for water. How do I Select a Potting Mix? There are a ton of products available on the market.
Can you plant a plant in compost?
While it may seem like a good idea at first, it’s not advised to plant in compost. Compost is a soil amendment; it is not soil or a suitable alternative like potting mixes. First, compost is light, and can’t support the root structure of plants. Furthermore, compost drains easily, leaving plants needy for water.
Can you use compost as fertilizer?
Use compost to improve soil for planting vegetable gardens, landscape areas and lawns. Adding compost to your soil improves its ability to accept and store water. It also helps aerate soil. Adding compost increases the activity of soil organisms. They help to provide nutrients to your plants over time. Compost may not replace fertilizer.