As any gardener knows, not all dirt is created equal. Wondering about the content of potting soil, and whether or not it’s safe to use potting soil in the ground? You’re not alone. Here are all the answers to safe soil use.
What is in Potting Soil?
Potting soil isn’t just dirt conveniently packaged in a bag. It’s a mixture of organic materials that are designed specifically for growing plants in containers.
There is going to be a mixture of sterile soil along with a blend of moss, bark, compost, and possibly a little sand too.
Small pieces of mica, or vermiculite, is another ingredient in potting soil to help with aeration and drainage. Pieces of vermiculite look like small pebbles, made up of thin layers. It’s another natural compound, and not a problem if it ends up in your outside soil.
A common question or concern about potting soil is about those tiny white “foam” balls that are found in many mixes. Contrary to popular belief, that is not styrofoam and is completely natural.
That white material is called perlite, and it is made from expanded volcanic rock. It is very light and porous, and is added to potting soil to help with drainage. If you are going to use potting soil in the ground, this isn’t something you need to worry about either.
For more info, check out my post about using perlite or vermiculite when growing vegetables.
Benefits of Potting Soil
There are a number of reasons you would want to use potting soil for your houseplants, or for any container plants that are outside.
Potting soil is a lightweight growing medium and allows for better drainage and root development than plain outside dirt, especially if you are starting new plants. All of the added organic material makes it very light, ideal for fine roots to grow without getting smothered.
Quality potting soil is also a more sterile product, and should be free from weed seeds, insects and any other plant pathogens. That means a healthier environment for your plants.
Potting Soil in the Garden
Back to the original question, there is no particular risk or danger in using potting soil in outside garden areas. To revitalize a garden bed, it’s topsoil you want rather than potting soil.
Topsoil is a lot more like “dirt in a bag,” and is intended for widespread use in the garden. Though potting soil may be a superior product, it’s also priced much higher than plain topsoil.
If you are transplanting some indoor plants with potting soil in their containers, to an outside bed, you don’t have to worry about the little bit of potting mix from the ball of soil around the roots. Definitely not going to do any harm to the environment.
Other Options for Soil Amendments
If you are planning on using potting soil in the ground as a way of improving your outdoor gardening space, there are a few other natural products that would work better.
Manure and compost are the most common, and they add a whole host of additional organic material and nutrients to your soil.
Compost is something you can make on your own, with a simple outside bin and your kitchen scraps and yard waste. Unless you have space for a few farm animals, manure is best purchased from a garden center or your local farmer.
For simple drainage, or to lighten up a clay-heavy soil, you can add sand, peat moss, shredded bark or even fine gravel to aerate the soil.
Garden beds that need a boost of nutrients can benefit from a number of other products that target specific plant requirements. Here is a quick overview of the best choices:
- Seaweed emulsions – potassium
- Fish emulsions – nitrogen, potassium, phosphorus
- Potash – reduce the acidity (pH)
- Coffee grounds – nitrogen
- Blood meal – nitrogen
- Bone meal – nitrogen, calcium
- Dolomite lime – calcium
- Epsom salt – magnesium
Not sure which of these nutrients you need? Before you go fertilizer shopping, get a simple soil test kit and find out. These are easy to use and can tell you if your existing soil is deficient in the usual elements (nitrogen, potassium or phosphorus).
Just collect a bit of soil, add water and the reagent in the kit to see the color change. You can use a similar type of kit to test for pH too.
Can You Use Garden Soil in a Pot?
You may also be wondering about the other side of the coin: using your outside soil in your houseplant pots. Given the differences we’ve already discussed above, it’s not ideal.
Soil from outside is going to most definitely have weed seeds in it, as well as possible insects or other problems that are best left outdoors.
Repotting an established plant in outdoor dirt is not a huge concern because the plant should be strong enough to handle any additional pathogens.
If you are starting some seeds or rooting some fresh cuttings, that’s a different story. To give your tender new plants a fighting chance, it would be a smart idea to sterilize the soil from outside before you use it.
All you need to do to properly sterilize your outdoor soil is a deep baking tray, some foil, a thermometer and your oven. Collect your dirt, and dampen it just enough so that it will hold its shape if you ball it in your hands.
Fill the pan, no more than 4 inches deep, and tightly cover with foil. Poke a thermometer through the foil into the soil so you can monitor the temperature. You don’t want to overheat or burn it.
Set the oven to 180F and when the dirt itself is registering 180F, start timing to 30 minutes. Watch the thermometer to make sure it doesn’t get too hot. Be aware that this isn’t going to smell very good, and plan on keeping a window open while your soil is cooking.
When done, remove from the oven and let it naturally cool until it reaches room temperature. Now you can use this as potting soil, knowing that it won’t do your new plants any harm.
You are now an expert on soil, and should easily be able to decide if you want to use potting soil in the ground or stick with topsoil for your outdoor gardening.