By Jeff Lowenfels
Updated: February 16, 2023 Published: February 16, 2023
I knew as soon as I saw last week’s column in the paper that I would be getting questions on whether it is OK to reuse soil, be it seed-starting soil or potting soil. Sure enough!
The short answer is that it is perfectly acceptable to reuse soil if the plants previously grown in it were healthy. We do it all the time in our household. You just have to take steps to ensure the soil has enough nutrients to support a new plant or plants.
If you flip through the internet researching this question, you will find plenty who think you need new soil, which is just wrong. You’ll also find those who justify their answer as a way to save their readers a few bucks. Of course, there is nothing wrong with saving money, but there is so much more to it. In fact, I actually prefer to use soil that has already supported plants.
There are several reasons I like used soil. First of all, the previous plants released carbon substances into the soil in order to activate and maintain the soil food web. This is a good thing. The more carbon you have the better it is for attracting the diverse set of microbes which will feed and protect plants grown in their soils. They produce nutrients in plant-useable form right at the root zone and they release metabolites that keep bad guys in check, break up phosphate molecules and more.
Secondly, if previous plants were harvested properly, the used soil will have good structure, meaning it has nice aggregates and will drain well and has lots of pore spaces to hold air and microbes avoiding their predators. “Proper harvesting” entails simply cutting the plant at the soil level rather than yanking it out of the soil which, naturally, disturbs things.
And, the roots of new plants will follow the tracks left by the old roots. The soil around them harbors those carbon exudates. They also serve as organic matter ready for decay by microbes.
There are some concerns with using old soil. The first is that if harbors some kind of disease, it will pass that on to the new plants. Obviously, if the plants previously grown did not do well or actually showed signs of a pathogen, then that soil should be tossed on a compost pile to be “corrected” by the composting process.
A second obvious problem is lack of nutrients because previous plants used them. This problem is easily overcome by several methods. First, use a fast-acting, liquid organic fertilizer before you plant new stuff in old soil. If you can apply it a couple of weeks before planting, so much the better. Keep the soil moist and activate the microbes in it. And/ or use a fertilizer that is full of active microbes like Down To Earth’s Bio-Live. Remember, whatever you use, it has to be organic
Of course, you can also add a bit of fresh organics to previously used soil. Compost is great as it has the full complement of necessary microbes to support your plants. You don’t need much — 1/4 inch applied on the surface of the existing soil.
New soil or old soil, it is time to start preparing for starting seeds, tubers and bulbs. Build those pots and containers, collect those seeds and get those lights up if you don’t have them going for your houseplants already.
Jeff’s Alaska Garden Calendar
Spring Conference: The Annual Spring Garden Conference will be back in person this year, as well as offering virtual opportunities! All speakers on March 8 will be virtual via Zoom. Sessions on Friday, March 10 and Saturday, March 11 will be held in person on the Alaska Pacific University campus.
Alaska Botanical Garden: Do check www.alaskabg.org as there are lots and lots of things going on which you won’t want to miss.
Forcing bulbs: Time to get yours out into the light.
FAQs
The short answer is that it is perfectly acceptable to reuse soil if the plants previously grown in it were healthy. We do it all the time in our household. You just have to take steps to ensure the soil has enough nutrients to support a new plant or plants.
Can I use old potting soil to start seeds? ›
Yes, potting soil can be reused, but it's essential to understand the conditions and precautions for doing so effectively. Potting soil can lose nutrients and structure over time as it supports plant growth, but with proper care, it can be rejuvenated for multiple planting seasons.
How many times can you reuse potting soil? ›
Like many aspects of gardening — it depends. If the potting soil has just been used one season and all the plants were completely healthy, sure. However, if you know your plants ended up hit with downy mildew, or other plant disease, you'll want to either toss the soil or spend some time sterilizing it.
How do you revitalize and reuse potting soil? ›
As old soil might be deficient in plant nutrients, you need to replenish it. Here are some ways you can add nutrients and organic matter to soil: Add a handful of a slow release fertilizer like manure. Mix in one part compost for every three or four parts of the old potting mix.
Can seed starter mix be reused? ›
I would suggest you take the mix out of the cells, dump it into a sealable plastic baggie and fill the baggie one half to three quarters full with water then seal the bad and knead or massage the bag to help the seed starting mix take in moisture. Once it is wet you can put it back into the cells and plant your seeds.
How to sterilize potting soil for reuse? ›
Be sure to remove any insects, place the soil in an oven-safe pan, and cover with foil. Bake at 175 to 200 degrees Fahrenheit for 30 minutes. You also can microwave 2 pounds of soil in microwavable containers at full power for 90 seconds. Now that your soil is sterile, you can use it again.
Can I plant seeds directly in potting soil? ›
Although potting soils may be used to start seeds, they tend to have a more coarse texture and may contain field soil, compost or composted manure along with vermiculite, peat moss or perlite. Some seed-starting or potting mixes may contain fertilizer as an additive.
How to tell if potting soil is bad? ›
You can determine if your potting soil has gone bad by checking for:
- Compaction. Compaction refers to soil that's heavy or tightly packed. ...
- Foul Odor. A foul, rotten eggs odor indicates that your potting soil bag has gone bad. ...
- Mold. Your soil is bad if it has mold growing on it. ...
- Insects.
Give the soil beneath a quick turn with a cultivator. If the soil level has dropped, mix in potting soil or garden mix to fill the planter back up. Turn compost or fertilizer into the existing soil to add back in some much-needed nutrients.
Can I mix old potting soil with new? ›
In some cases, revitalized potting soil can be used exclusively, but blending old soil with fresh in equal parts is a safe way to ensure productive density and nutritional balance.
Combine compost, topsoil, a bit of coarse sand, and something like vermiculite, perlite, or coco coir until you have a mix with a consistency that holds together when wet. For seed starting, you'll avoid using as much sand as you would when making soil for your garden.
Should I water starter seeds every day? ›
Many people think they need to water seedlings daily, but that's often more water than your plants need. Especially when plants are small, their root systems are not yet developed, and they will not take up very much water. Keep your soil moist, but try to avoid soaking it.
Can I use old potting soil still in a bag? ›
Yes, you can use potting soil that is two years old. However, the nutrients in the soil may have depleted over time, especially if the soil has been used before. To ensure good plant health, you might need to replenish the soil's nutrients by mixing in fresh compost or a slow-release organic fertilizer.
Can you use old opened potting soil? ›
With proper storage and care, opened potting soil can stay good for a long time. It's best to use opened potting soil within a year or two for optimal plant health.
Is potting soil the same as seed starting soil? ›
It is important to use seed starting mix when starting seeds indoors for all of these reasons. You can use just a regular potting mix, however, this makes it much easier and more likely to run into issues with drainage, moisture retention, pests, soil density, nutrient availability, and more.
