Categories
BLOG

can potting soil be reused

Gardener’s Guide to Reuse Potting Soil

A new growing season is just around the corner and you’re beginning to think about all the materials you’ll need to get your plants off to a good start. There is no ingredient more important than the soil that you are using as it provides the plants the necessary nutrients for healthy growth. But good soil doesn’t grow on trees so to speak. It can take a lot of hard work and money to acquire a growing medium that will best support your plants, from hauling bags of compost to purchasing expensive seed starting mix. No matter where you get your soil or what mix you use, you’ll definitely think twice before tossing it out. But is it okay to use your soil for more than one season? In this article we’ll explore the what, why and how of reusing potting soil and whether or not it is the best option for your plants. Also, remember that the best fabric pots are washable and reusable as well.

What is a good soil composition?

If you purchase a high quality seed-starting mix or sterile growing medium you will find that it has three main ingredients.

1. A light material like peat moss

But what is peat moss…?

This is to keep the soil lighter unlike dirt from your yard which can be mostly clay or sand. Sometimes a brand will substitute coir for peat moss. Coir is a fibrous material extracted from between the inner and outer shells of a coconut. It has become more popular as a potting soil ingredient since it has better water retention than peat moss and resaturates much easier as well. It is also sterile and organic and is considered a sustainable resource, making it a top choice for growers and gardeners who are environmentally minded. But before you jump straight to coir and leave the peat moss behind, it has its drawbacks as well. The added water retention can result in salt build up, it is prone to compacting and it is much more expensive than peat moss.

2. Soil Nutrients

In a store bought potting mix nutrients will often come in the form of perlite or vermiculite. They are the white, puffy granules you find mixed into the bag of seed starter mix. They are minerals that have been exposed to high heat and essentially puff up like popcorn.
An organic alternative is worm castings or vermiculture . Worm castings are the product of earthworms. When the organic matter moves through the earthworm’s system a product is excreted that is full of beneficial bacteria and rich in minerals that are immediately available to the plant’s roots. The use of worm castings offers a lot of benefits including correcting pH levels of the soil, adding nutrients, providing heavy metal and chemical filtration and providing added moisture retention. You can purchase them or you can find plenty of resources online to help you make your own.

3. Compost

Many gardeners choose to make their own compost of two parts carbon-rich materials like dry leaves or egg shells and one part nitrogen-rich materials like grass clippings or food scraps. With the right mix of organic materials, moisture, air and time you can develop a nutrient rich product right at home. However, if you’re lacking time, space or materials you can always purchase a bagged compost like the Plant-Ton Organic Plant Food from Amazon or another seller. This mix is full of ingredients that will add a ton of nutrients to any soil mix to grow healthier plants. So whether you build your own mix at home or purchase a manufactured mix make sure it includes the three ingredients for a successful potting soil. Keep in mind that when growing in a fabric pot like the Fabric Burner Pots you can use soil that is a bit denser than those used in plastic, clay or metal pots since water drainage is not an issue.

What are the risks of reusing potting soil?

So once you have your perfect soil mix put together and you’ve used it for a season with excellent results, why wouldn’t you want to continue to use it in upcoming seasons? For starters, several things could have changed in your soil since you first mixed it up that will make it a less suitable environment for future plants. Over a growing season soil will collect organic matter like roots and leaves, nutrients will be removed from the soil, insects and nematodes will take up residence along with pathogens like bacteria and viruses and weeds will grow and drop their seeds. All of these things can lead to problems in the second growing season if soil is not treated properly before reuse and in some cases even proper care isn’t enough to restore soil to its original state.

Reusing soil with roots

If you had healthy plants in the previous season, chances are when you remove them from their container there will be a large system of roots at the base. Most of the time a good portion of these roots will cling to the material in the container leaving behind a stringy mess in your potting soil. Roots aren’t the only thing that gets left behind, especially in plants grown outdoors. You may find leaves, grass, nut shells or feathers and other dried up organic matter that has fallen from trees or blown in from the wind. While organic matter like leaves can be beneficial when composted or placed on top of the soil to reduce water evaporation, they are not useful when they are simply taking up room in your otherwise clean potting soil.

Depleted nutrients

The reason plants grow in soil is because it offers a perfect environment for roots to absorb not only water, but nutrients as well. But if your plants are restricted to a pot the nutrients in that soil can be depleted quickly. Once those nutrients are used up the plant does not have the option of sending out roots further in search of additional minerals like they would in an open space like a garden. Instead it is up to the grower to provide additional nutrients to make up for those lost.

It is also important to know that different plants require different amounts of each mineral that is available in the soil. Some plants such as tomatoes use a lot of phosphorus (P) while others like flowers need significant amounts of potassium (K). Therefore if you plant tomatoes in the soil the first season they will deplete the phosphorus in that container’s soil. If you try and plant tomatoes in that same soil the following year you will end up with lack-luster plants that are unable to produce due to a lack of soil nutrients.

So when we get the question “Can I reuse potting soil from a dead plant?“, the answer will vary depending on how long the plant has been dead, how long the soil has been in use prior to the plant dying and what you are growing. A good idea is to add some nutrients and changing the type of plant that you are growing.

Weeds

Weeds and weed seeds can also be a major issue, especially for container plants that are grown outdoors. Seeds can be blown in by the wind, dropped in by animals or already exist in the soil and are waiting for the right growing conditions to germinate. They are so small it is very difficult or at times impossible to see. The seeds can become burried deep in the soil. When you turn the soil to plant the following season’s plants, these seeds will be brought to the surface where they can germinate. Not only do they become a nuisance requiring extra time for weeding but they can take nutrients away from your plants and get in the way of their stretching root systems.

Insects

While some beneficial insects may not pose and issue, the majority of insects that overwinter in your soil are harmful to plants and plant roots. Grubs will burrow into the soil, overwinter and then feed on plants and plant roots in the spring. Organisms like plant-parasitic nematodes, a microscopic round worm, hide in soil and can destroy roots or burrow inside host plants eventually causing plant damage and death. Some insects will lay their eggs in the potting soil and when temperatures warm in the spring season they will hatch to feed on the growth of the new plants in the container.

Pathogens

Some of the worst issues are those you can’t see and that includes plant pathogens like bacteria, viruses and soil fungi. Not only are they easy to contract if given the right conditions but they can be very difficult to get rid of. There are so many varieties of plant pathogens and they each have different symptoms. Sometimes the symptoms of plant pathogens mimic those of nutrient deficiencies making them even more difficult to identify. Tomatoes are a plant that is especially prone to contracting viruses and bacteria. They are so inclined to falling victim to certain bacteria and viruses that many growers will get rid of the soil they grew tomatoes in altogether. For this reason it is considered a hard and fast rule that you never replant tomatoes in the same soil they were planted in the year before, whether you are using a container or are growing in a backyard garden.

How to get your potting soil ready for the second season

What to do with potting soil at the end of the season?

Despite the possible complications with reusing potting mix, there are ways around these issues if you are willing to put in a little time and energy. In the long run it can be very well worth it in order to reduce costs associated with purchasing new potting soil or materials to make it every season. This is especially true if you have a large container garden.

So what is the proper way to recondition your soil so it is ready to start growing healthy, sturdy plants for a second or third season?

The best practices for container gardening recommend that you empty out your pots at the end of the growing season. Much like you would clean up all the organic matter from your backyard garden, this gives you an opportunity to sift through the soil and remove any stowaway insects, weeds or debris. It also offers you the opportunity to clean and dry your pots thoroughly to ensure that there are no residual bacterias, funguses or viruses from the previous season. Make sure you use organic soaps or other plant-friendly substances that will both clean and disinfect. Rinsing with water may remove other materials like dirt and insect eggs but it will not sterilize the pots.

You can choose to use a large tub, or several, to store the potting soil from the previous season over the winter. If you have grown tomatoes, store their soil separately so you know which soil should not be used for tomatoes the following season.

So, “How to store potting soil over winter?“… Before storing the soil make sure to sift through and check for insect larva, grubs and eggs. This may prevent you from having an insect infestation on new plants next season.

Another option to ensure that weed seeds, insect eggs or larva and pathogens have been destroyed is to bake the soil in the sun. This can be done by placing the soil in dark plastic bags or tubs and leaving them in the sun until they are very warm. Similarly to composting, the high temperatures act to kill the pests and seeds so they won’t present a problem in the coming season.

Of course, despite the efforts you make to sterilize the pots, burn the seeds, and sift out insects and plant roots, there is still a chance that one or more of these issues may present in the coming year’s crop. Although you can take certain steps to mitigate the risks there is always the possibility of a problem from last season carrying over to the next if you reuse potting soil.

How to sterilize potting soil

One sure-fire way to destroy pathogens, insects and seeds is through the process of pasteurization. While this technique is mainly only available to commercial growers it greatly reduces the risk of carrying over organisms from one plant to the next when reusing soil. The process consists of raising the temperature of the soil to 180 degrees fahrenheit and maintaining it for 30 minutes. If the temperature reaches 212 degrees fahrenheit most organisms are killed and the soil is considered sterile. Home growers have the option of baking their soil in their ovens at this temperature for 30 minutes, however, this can often leave an earthen odor behind. If pasteurization is not an option it is usually best to err on the side caution and refrain from reusing soil that you know has a plant pathogen that could be passed on to your other plants.

Just as important as cleaning your soil the previous season, it is just as crucial to prepare it prior to planting the following season. To prepare soil for planting you will want to add in enough elements to bring the nutrient content back up to keep your plants in good shape all season. So what should you add to get your potting soil ready to plant? You should plan to add up to 1/3 new potting soil mix that you either make yourself or that you have purchased, as well as 1/4 new compost. Keep in mind that when using fabric pots like the Classic Spring Pot the medium can be a bit heavier than a soil you would put in a plastic pot as water retention is not as great an issue. If you feel the potting mix is too heavy, you can also add fillers like coir or perlite to fluff the soil up.

Once you’ve added in plenty of nutrients and made sure there are no leftover organisms or organic matter like leaves and roots, you are ready to plant your new crop. As an added safeguard against weed seeds you can add a layer of compost or dried leaves to the top of the soil. This added layer will block out sunlight and keep the seeds from germinating.

Still not sure…?

Still not sure it is in your plant’s best interest to reuse soil as your growing medium for a second season? There are plenty of other ways to reuse and recycle potting soil. It can be turned into established gardens and flower beds or used to help fill out raised beds whose soil has settled. You can use it to fill in holes or gullies in your yard that are collecting rain water or to grade the ground away from the foundation of your house. Lastly you can add it to your compost pile or bin. During the composting process the heat will destroy the living organisms and the process of decomposition will aid in returning nutrients to the potting soil so it can be used in future growing seasons.
Reusing potting soil can be a great way to cut down on the hassle and cost of purchasing new materials season after season. As long as you take the necessary precautions to ensure that your potting soil is clean and sterile and that an adequate amount of nutrients have been added back in, your soil can be used for many seasons. Spring Pots hold their own shape for easy filling and are also collapsible for simple winter storage. Click here to see some of the Spring Pot designs that make cleaning and refilling your pots easy and convenient.

Learn how and why it is important to reuse potting soil for more than one season. Pick the right soil composition and soil texture for your garden.

Can You Re-Use Potting Soil From Your Containers?

Helpful Products from Gardens Alive!

Q. Mike: My wife likes to plant flowers and vegetables in large pots. Come winter, we empty the pots. The used potting soil usually contains an extensive root structure and often is beginning to sprout weeds. What is the best way to store and recondition the potting soil for reuse the following year? Thanks!

    —Dr. Mitch; The Children’s Hospital of Philadelphia

Both my wife and I thoroughly enjoy your programme for the sound, useful advice and the humorous and boisterous tone in which it is delivered. Would you please settle a long-standing disagreement between us: Can one re-use potting soil after having grown plants/flowers in it for one season? The husband says: “Yes! You bet-your-potting-soil!” The dear wife (who, admittedly has spent 100 hours gardening for every hour the husband has dabbled) claims that such soil is spent after one growing season and may now harbour insects and/or disease. Who is right? Please do let us know…

    —Michael (the husband) in Spokane, Washington

Q. An excellent question, and one that deserves an intelligent discussion. But instead of that, I’ll tell you what I do with mine.

First, let’s review the basics of container contents. Because you are going to trap these poor plants in a finite space, as opposed to the great outdoors where they can send their roots out much further in search of help, you have to supply them with a light growing mediumthat drains exceptionally well. That means no garden soil in the mix. Instead, the ideal medium for containers is three-quarters soil-free mix and one-quarter compost.

“Soil-free mix” is the term I use for high-quality potting soil; it may also be called professional mix, seed-starting mix, sterile growing medium or some other synonym. It’s generally composed of milled peat moss (with a little lime to adjust the pH), perlite and/or vermiculite (naturally occurring minerals that are ‘popped’ in big ovens) and some compost or “composted forest products”. Some packagers substitute coir (shredded coconut fiber) for the peat; and some companies add nutrients to the mix (which is bad if the nutrients are man-made chemicals, but wonderful if they’re natural things like worm castings).

Mix one of those mixes up with some high-quality compost, and you’ll have a growing medium that retains moisture and drains well, contains a nice amount of organic matter, and is light enough for you to move the containers around fairly easily.

So—what do you then do with this wonderful stuff at the end of its first season? Because the soil will expand and contract greatly over a harsh winter, those who grow where the ground freezes hard should empty out plastic, ceramic and clay pots to protect them from cracking. Or you can just bring the whole schmageggie inside to a place that will remain above freezing. (If you do empty them out in the fall, remove any roots or weeds and add them to your compost pile. If you store the pots full, plan to remove this debris when you freshen up the mix the following Spring.)

In my opinion and physical reality, the only hard-core issue of re-use here is The Tomato Rule. Potting soil that was used to grow tomatoes should not be used to grow tomatoes the following two years. BUT that soil can be used to grow flowers, bush beans, peppers, salad greens—whatever you want, as long as it’s not tamatas. Conversely (like the sneakers), soil that hasn’t ever been used for tomatoes (or that hasn’t seen their roots for a few seasons) can be used to grow this year’s love apples.

One way to achieve this noble end is to have two big galvanized or hard plastic trash cans, label one with a T and one without, and use these to store your soils over winter. Don’t worry about otherwise mixing the soil from different pots; I actually prefer to combine mine to mitigate any potential nutrient imbalances and such.

The following season, buy some fresh soil-free mix and use it to freshen up every pot that gets filled with old soil. How much? Up to a third new mix if your old soil is really old or if it seems to be bulking up on you; less if your old stuff is still light and fluffy. Always add fresh compost to the tune of one-quarter of the container.

Now the risks. Insect carry-over is fairly remote, as is the risk of keeping a disease alive other than the soil-borne wilts that attack tomatoes. Weeds could be an issue, especially if you don’t mulch the tops of your containers with shredded leaves (which I highly recommend as the leaves also retain moisture, a very important consideration for pots in direct sun or during an especially hot dry summer).

But those weeds (and any tricky diseases) will still be much less of an issue than in outdoor gardens, and the weeds can be even further avoided by layering the new season’s compost a couple of inches thick on top of the old soil-free mix instead of mixing it in.

And if, like me, you garden in ground and in containers, it’s a wonderful idea to give one or two of your containers a completely fresh set of clothes every few years and mix their old potting soil into your garden, where its mix of lightweight ingredients will be welcomed by the roots of your plants—especially if those poor rooties have to try and fight their way through the misery of clay.

Read these Previous Questions of the Week on CONTAINER GROWING BASICS and GROWING TOMATOES IN CONTAINERS for more info on those important topics.

Q. Mike: My wife likes to plant flowers and vegetables in large pots. Come winter, we empty the pots. The used potting soil usually contains an extensive root structure and often is beginning to sprout weeds. What is the best way to store and recondition the potting soil for reuse the following year? Thanks! —Dr. Mitch; The Children’s Hospital of Philadelphia Both my wife and I thoroughly enjoy your programme for the sound, useful advice and the humorous and boisterous tone in which it is delivered. Would you please settle a long-standing disagreement between us: Can one re-use potting soil after having grown plants/flowers in it for one season? The husband says: Yes! You bet-your-potting-soil! The dear wife (who, admittedly has spent 100 hours gardening for every hour the husband has dabbled) claims that such soil is spent after one growing season and may now harbour insects and/or disease. Who is right? Please do let us know… —Michael (the husband) in Spokane, Washington Q. An excellent question, and one that deserves an intelligent discussion. But instead of that, I’ll tell you what I do with mine. First, let’s review the basics of container contents. Because you are going to trap these poor plants in a finite space, as opposed to the great outdoors where they can send their roots out much further in search of help, you have to supply them with a light growing mediumthat drains exceptionally well. That means no garden soil in the mix. Instead, the ideal medium for containers is three-quarters soil-free mix and one-quarter compost. Soil-free mix is the term I use for high-quality potting soil; it may also be called professional mix, seed-starting mix, sterile growing medium or some other synonym. It’s generally composed of milled peat moss (with a little lime to adjust the pH), perlite and/or vermiculite (naturally occurring minerals that are ‘popped’ in big ovens) and some compost or quote composted forest products. Some packagers substitute coir (shredded coconut fiber) for the peat; and some companies add nutrients to the mix (which is bad if the nutrients are man-made chemicals, but wonderful if they’re natural things like worm castings). Mix one of those mixes up with some high-quality compost, and you’ll have a growing medium that retains moisture and drains well, contains a nice amount of organic matter, and is light enough for you to move the containers around fairly easily. So—what do you then do with this wonderful stuff at the end of its first season? Because the soil will expand and contract greatly over a harsh winter, those who grow where the ground freezes hard should empty out plastic, ceramic and clay pots to protect them from cracking. Or you can just bring the whole schmageggie inside to a place that will remain above freezing. (If you do empty them out in the fall, remove any roots or weeds and add them to your compost pile. If you store the pots full, plan to remove this debris when you freshen up the mix the following Spring.) In my opinion and physical reality, the only hard-core issue of re-use here is The Tomato Rule. Potting soil that was used to grow tomatoes should not be used to grow tomatoes the following two years. BUT that soil can be used to grow flowers, bush beans, peppers, salad greens—whatever you want, as long as it’s not tamatas. Conversely (like the sneakers), soil that hasn’t ever been used for tomatoes (or that hasn’t seen their roots for a few seasons) can be used to grow this year’s love apples. One way to achieve this noble end is to have two big galvanized or hard plastic trash cans, label one with a T and one without, and use these to store your soils over winter. Don’t worry about otherwise mixing the soil from different pots; I actually prefer to combine mine to mitigate any potential nutrient imbalances and such. The following season, buy some fresh soil-free mix and use it to freshen up every pot that gets filled with old soil. How much? Up to a third new mix if your old soil is really old or if it seems to be bulking up on you; less if your old stuff is still light and fluffy. Always add fresh compost to the tune of one-quarter of the container. Now the risks. Insect carry-over is fairly remote, as is the risk of keeping a disease alive other than the soil-borne wilts that attack tomatoes. Weeds could be an issue, especially if you don’t mulch the tops of your containers with shredded leaves (which I highly recommend as the leaves also retain moisture, a very important consideration for pots in direct sun or during an especially hot dry summer). But those weeds (and any tricky diseases) will still be much less of an issue than in outdoor gardens, and the weeds can be even further avoided by layering the new season’s compost a couple of inches thick on top of the old soil-free mix instead of mixing it in. And if, like me, you garden in ground and in containers, it’s a wonderful idea to give one or two of your containers a completely fresh set of clothes every few years and mix their old potting soil into your garden, where its mix of lightweight ingredients will be welcomed by the roots of your plants—especially if those poor rooties have to try and fight their way through the misery of clay. Read these Previous Questions of the Week on CONTAINER GROWING BASICS and GROWING TOMATOES IN CONTAINERS for more info on those important topics. Ask Mike A Question    Mike’s YBYG Archives    Find YBYG Show