Introduction

Every microgreen farmer prefers one type of substrate over all others. If you grow in a spare bedroom, you might prefer a reusable stainless steel mesh to keep your house clean. If you grow in a dry climate, you might prefer a coco coir-based soil because it has better water retention. If you're a beginning farmer, you might prefer hemp fiber mats. They're precut to fit microgreen trays and you can compost them after harvesting. No prep and no cleanup. My preference is a peat moss-based soil, like Promix BX. The point of this article is not to argue about the best microgreen growing medium. The point is that there are a myriad of options; some are reusable, and others are not. It can be tempting to reuse a single-use substrate, like Promix, Coco Loco, or hemp fiber mats. I’ll explain why you shouldn’t reuse your microgreen soil or any single-use substrate, and how it could backfire if you do.

As a resource, here is a short list of different single-use and reusable grow mediums.

• Peat Moss-Based Soils (i.e., Promix BX, HP, or MP | Coast of Maine)
• Coco Coir-Based Soils (i.e., Coco Loco)
• Natural Fiber Mats (i.e., Hemp | ReNu Terra Mats | Bamboo | Jute | Coco Coir)
• Reusable Mats (i.e., Stainless Steel Mesh or Silicone Mats | Polyester Mats)
• No Substrate (i.e., Mesh Bottom Trays)

The first three bullets are categories of single-use growing mediums. The last two bullets are categories of reusable growing mediums.

The Cost of Single-Use Soil

Before I explain why you shouldn’t reuse your single-use substrate, let’s look at the direct cost of using soil in a microgreen farm. My farm, Piedmont Microgreens, plants 400+ trays per week (TPW). Planting 400 TPW means we use a pallet of soil, or 30 compressed bales, every five weeks. We pay $1,600 for a pallet of Promix BX, which means we spend roughly $16,640 a year on soil. This doesn’t include the cost of the labor to fill, compost, and clean trays of soil. If we could stand to save half of that $16,640 by reusing our single-use substrate once, why wouldn’t we try?

One & Done

Here are a few reasons why you shouldn’t reuse your single-use substrates. All these reasons apply to reusing a reusable substrate without cleaning and sanitizing it first, except #2.

• Mitigate Pests & Disease: The biggest benefit of tossing your soil after each use is it prevents most pests and diseases from establishing in your farm. Why? Your soil is the most hospitable place in your farm. As a result, pests like fungus gnats will preferentially lay their eggs in your soil. If you discard your soil after each grow cycle, most pests and diseases won't have time to establish and reproduce in it. For example, fungus gnats, one of the most common farm pests, have a lifecycle of four weeks. Very few microgreen varieties have a grow time exceeding four weeks. If you plant a second round of microgreens in that tray, it could have gnat eggs in it. Those eggs now have enough time to complete their life cycle.
• Nutrient Depletion: Microgreens will extract nutrients from all natural substrates, like peat moss, coco coir, or hemp fibers. It might not be much, but it is there. Any bit of nutrients the microgreens can derive from the substrate will boost the plant’s yield, color, flavor, and nutrient content. If you plant a second round of microgreens in that tray, the second batch will be disadvantaged compared to the original batch. If you first yielded six ounces of microgreens, you might only yield four ounces in the second round.
• Competition for Space: More harmful than nutrient depletion is the fight for space. Plants need space to properly develop their canopy and root system. Imagine you first plant a batch of tendril peas in a tray of soil. At a planting density of 300g, which we use at Piedmont Microgreens, the soil is fully covered in seeds. After growing and harvesting that tray of tendril peas, you now have a robust root system. It completely envelops the mass of soil. If you plant a second round of microgreens in that tray, the seeds will struggle to find the soil and root. The seeds are competing with the prior batch for space, even if the prior batch is dead. Germination rates and yields will decline, and the plants' health will suffer.
• Late Bloomers & Regrowth: Seed batches with high germination rates can always have stragglers that germinate later than expected. Many microgreen varieties can also regrow after being harvested. Double harvesting is not recommended because the regrowth is slow and uneven. The regrown microgreens are also less vibrant, less nutritious, and less flavorful. If you plant a second batch of microgreens in a used tray of soil, your new crop will have late bloomers and regrowth mixed in. If the first and second crops are different varieties, you’ll need to separate out the old crop at harvest. Even if the second crop is the same as the first, the microgreens will be subpar. This will taint the second harvest.

#2 doesn’t apply to reusable substrates because most reusable substrates are inert. Plants cannot extract nutrients from materials like stainless steel, silicone, or polyester.

Summary

Despite the temptation to grow a second batch of microgreens in a used tray of soil, it will only bite you in the ass. What money you think you’re saving by using less soil, you’re more than losing when disease runs rampant, seeds don’t germinate, and yields decline. As they say, “don’t step over dollars to pick up pennies.”

Related Articles

Can I Regrow My Microgreens After Harvesting?

Damping Off: How to Prevent & Manage A Common Seedling Disease

We use Promix BX, a peat moss-based soil, at my farm, Piedmont Microgreens. We grow 400+ trays per week, which means we use a pallet of soil every six weeks. A pallet of soil costs us $1,600, which means we spend $_ on soil each year. If we could reuse our soil, I’d recoup $_ a year to invest in better equipment, pay off debt, or raise my employees’ pay. Those seems like compelling reasons, but I still refuse to reuse my soil. Why?