One of the most common questions we receive is:
"Can I reuse microgreen soil after harvesting?"
Our answer has always been, "No, it's best to use new soil to reduce the possibility of disease. Plus, the new microgreens will grow better on fresh soil."
The disease issue is no joke, but the real reason we say no is that we thought the microgreens would grow like crap.
We Ran A Test Reusing Microgreen Soil
In this article, we present test results where we replanted three trays after harvesting broccoli microgreens.
We even shot a video of the test showing you what we did and present the results and discussed it in the Microgreens Podcast too!
Spoiler Alert: We were shocked!
But first, let's go over the reasons that reusing microgreen soil might not be a good idea.
Listen to an Audio Version of the Article
We don't just read the article word for word in the audio version; it's a stand on its own piece of content that includes more details on the topic. These can include more tips, opinions, details, data, and information on this and related topics.
The Microgreens Podcast Episode 009
Reasons We Thought Reusing Microgreen Soil Wouldn't Work
We have lots of reasons why it shouldn't work.
All That Junk
Look at a tray after you finish harvesting it.
The tray is full of stems, broken microgreens, and roots.
How can the new seeds be able to get good seed to soil contact with all that junk on the soil?
Once the tray has been replanted, won't the decaying microgreens from the first harvest cause disease issues like damping-off disease?
The images above are trays that damping-off disease has ravaged.
No one wants that.
Not all seeds germinate simultaneously, so seeds that didn't grow the first time might once the tray is replanted.
Of course, this isn't an issue if you're planting the same variety of seed or don't care what does grow. But there are times that mixed microgreens wouldn't be welcomed.
Lack of Nutrition
Will the soil have enough nutrients remaining in it to grow a decent tray of microgreens? The most common microgreen soils are basically inert (peat moss and coconut coir), so did the soil give up all of its nutrients during the first planting?
Need Answers to Those Questions
Those are the questions we need to answer.
The biggest concern is a disease. Not only the damping-off disease shown in the photos above, but also bacterial growth such as E-coli, and other little buggers that can make us sick. Though rare, why increase the chances.
Our guess, however, is that there won't be good seed to soil contact, and germination will be low on the second planting.
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Our Test Reusing Microgreen Soil
In one tray, we planted Triton Radish seeds on top of the microgreen stubble and grew them using the weighed method.
With the second tray, we planted Wasabi Mustard seeds on top of the microgreen stubble and grew them using the dome method.
Lastly, we dumped out the soil, roots, and stubble and removed as much as possible. Then we put the used microgreen soil back into the tray and sowed Waltham Broccoli seeds.
Reusing Microgreen Soil the Video
We created a video of the process, discuss the problems, and present you with the results.
We hope you watch the video. If not, below is a summary of the procedure and results.
Our recommendation is at the bottom of this article.
Reusing Microgreen Soil For New Plantings
First, we clean up the stems and remaining leaves from the harvest.
For trays that remain in-situ, we clipped the microgreen stems close to the soil level and brushed off any remaining stems and leaves. Click on any of the images below to expand them for better viewing.
If you want to reuse microgreen soil by breaking up the root mass, dump out the roots and soil into a large tray and separate the plant matter.
Plant Trays Exactly As If Using New Soil
Plant the trays exactly like you're using new soil. For trays where roots are still in the trays, wet the upper third of the soil profile, spread seeds as evenly as possible on the tray, and mist the seeds to settle them into the plant matter and soil.
When reusing microgreen soil that has been broken up, use something to compact and smooth the soil surface. Then wet the upper third of the soil profile and spread the seeds evenly. Mist the seeds once they have been sown.
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Testing Blackout Methods
We tested both the weighted and domed blackout methods.
Upside down Home Microgreen tray lids serve as covers to the broccoli and radish trays. These two trays are stacked with five-pounds of weight on top of them.
A Home Microgreens Watering Tray turned upside down serves as a dome by placing it on top of the mustard planting tray.
Teas towels placed over the trays exclude any light that may reach the seeds. This is shown in the video.
Later, the weight is moved to the top of the domed tray to help keep it on the tray after the tea towels kept knocking the dome off the tray.
Results of Reusing Microgreen Soil for New Plantings
I am still shocked by the results!
In no way did I expect the microgreens to grow this well.
Take a look at how much the radishes have lifted the upper mustard tray, including the weight.
All three trays of microgreens are ready to be placed under lights after the 72-hour mark.
A 1020 shallow tray acts as a watering tray for all three of the microgreen trays. The light system is two inexpensive LED shop lights with a color temperature of 5,000K.
Below are photos of each tray right after being removed from the blackout period.
Truthfully, I expected some issues, more so than the few broccoli micros growing in the Wasabi Mustard tray.
At this point, water is added to the watering tray, any water remaining in the tray after 20-minutes is dumped out.
Five days after reusing microgreen soil to plant new seeds, the Triton radishes are ready to be harvested (middle tray).
Both the broccoli and radishes are dead on schedule. Both trays are full and growing well.
The radishes are ready to be harvested and are removed from the tray.
The Wasabi Mustard, grown using the dome method, is not doing as well as the other two trays.
It's not uncommon for Wasabi Mustard to be thin and narrow at 5-days. But the other microgreens are growing very even and uniform, while the mustard has staggered growth.
Also, many mustard seeds didn't germinate.
A few broccoli microgreens (not visible in the photo) grew from the previous crop and are mixed in with the mustard seedlings. However, we're not going to worry about a few stray broccoli plants.
Reusing Microgreen Soil Test Conclusions
The results are beyond our expectations.
Yet, it appears that using the weighted blackout method is the best way to grow a new batch of micros on used microgreen soil.
With the domed method, there isn't good seed to soil contact using a closely cropped tray. The previous crop stubble most likely raised the seed up off the soil and allowed the seeds to dry out.
We should have grown a tray of mustard using soil where we removed the root mass. Like with the broccoli in this test.
Maybe we will do that in the future and add the results to this article.
Our Soil Has Nutrients
The reused microgreen soil is the Home Microgreens Blend. It's a coconut coir-based soil with aged wood compost and other natural composted materials and minerals.
The results show that the soil has enough nutrients to grow at least two trays of microgreens.
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Our Recommendations For Reusing Microgreen Soil
Unfortunately, conclusions aren't recommendations.
Sort of like, it's incredible to ski out of an avalanche, but it's not recommended.
Or, it's thrilling to jump off a 35-foot waterfall into the pool of water at the bottom. Still again, it's not recommended (especially if you can't swim like me).
Now maybe those two examples are a bit riskier than growing microgreens in previously used soil, but you get the idea.
We do not recommend reusing microgreen soil.
We showed you it can be done, so if you want to give it a try, that's all on you.
Our Reasoning Why You Shouldn't Reuse Microgreen Soil
Safety is the number one reason.
We believe that eating microgreens grown in soil is safe (especially if bottom watered). Still the microgreens should be washed before you consume them.
It has been reported that the seeds are the most significant cause bacterial exposure. This is why some governments require sterilization of seeds, especially seeds grown as sprouts, not so much as microgreens.
As you know, bacteria reproduce exponentially, so the longer they exist in a favorable environment, the more there will be. The more there are, the more likely of them becoming a problem.
This is why we preach bottom watering. Keep the plants and soil surface dry once they start growing. Because moisture and wetness is the problem is also the reason we dislike using microgreen mats.
So reusing the soil increases the possibility of bacterial and fungal growth.
Reason Two We Don't Recommend Reusing Microgreen Soil
It takes too much time to clip the microgreens close to the soil surface or break up the root mats and sift them out of the soil.
Soil for the Home Microgreen Tray costs about a dollar. I'm busy; there's always something else to do or something else I should be doing. So taking the time to prepare a tray for re-planting is too much.
In the video, you saw how long it took to prepare the trays, and most of that time, the video was sped up 800-times.
For $1, I'll compost the old soil and fill my next tray with fresh soil.
It's faster, easier, and my garden loves the added compost.
It's Up To You - We've Shown You Can Reuse Microgreen Soil
Don't think my recommendations are a judgment or a pronouncement.
Recommendations are my opinions on what I would or will do.
You are your own person; search out other sources if you like and make your own decision.
In this article, to my surprise, we show that microgreens will grow well when you reuse the soil. I knew that the seeds would grow, just didn't expect them to grow so densely and be so uniform in size.
So I messed up and told a lot of people it can't be done. When I should've only said I wouldn't do it.
What Do You Think? Is It Worth The Risk To Reuse Soil?
Drop a comment below and let us know if you reuse microgreen soil and how you go about it. We are here to learn from each other.