The Best Soil for Microgreens

I’ve personally tried growing microgreens in everything I could get my hands on, and some things definitely work better (seed starting mix, potting soil) than others (peat moss, coffee grounds). Microgreens are incredibly forgiving of what you grow them in. But there is a growing medium that I’ve had the most success with when growing microgreens:

The best soil for microgreens is a mixture of 20% perlite and 80% organic materials (including peat moss or coconut coir for water retention, and pasteurized compost for nutrients.)

This is often sold as “seed starting mix, or potting soil”.

Often limestone or other additives are included to balance the pH and increase wettability, but they’re not critical.

There’s a lot to consider when choosing (or mixing your own) soil for microgreens. Keep it in the back of your head that almost anything will work, and you can have a lot of fun with it. Unlike a lot of plants that need sandier low-nutrient soil (like cacti and succulents), or rich water retentive soil (like tropical houseplants), microgreens will grow in almost anything. 

Here are a few tips and tricks I picked up along the way while doing research for choosing the best microgreen soil. In no particular order:

Learn Each Soil Component and its Impact

• Perlite is basically rock popcorn. Rock, often obsidian, is heated and gases are released which causes the rock to expand and blister, forming a highly permeable glass-like substance that has become a staple of the gardening world. It’s a great additive to lighten up soils and improve water, drainage, and aeration characteristics of a soil’s structure. Most potting mixes that are great for microgreens contain some perlite.
• Vermiculite: Similar to Perlite, but made from laminated rocks (rocks that form in layers). It has similar properties to Perlite, but vermiculite gives soil less aeration, and holds a bit more water. A lot of potting mixes have both. Vermiculite is a great addition to microgreen soils.
• Compost: Most organic matter can be composted, but some things compost faster than others, and result in compost with different nutrient profiles. Compost is commonly made from herbivore manure, and provides a significant boost to nutrients.
• Sphagnum/Peat Moss: Peat moss is a great way to lighten up a soil and add aeration, drainage, and acidify the pH. Peat moss is made up of partially decomposed plant matter. It takes a long time to form, so it’s a semi-renewable resource. There are arguments on both sides of whether or not it’s a renewable resource, something to be mindful of and worth doing a bit of deeper digging.
• Sand: sand is important is soil because it provides a long-term supply of minerals and ions for plants to absorb. You can think of the organic matter as the fast food of the soil world, whereas rock fragments and minerals are the foundation that’s necessary for the long term. Microgreens don’t grow for very long, so sand and mineral content is less important.
• Clay: clay isn’t usually added or found in significant quantities in bagged soil that’s commonly used for microgreens. If you’re growing outdoors in a clay-rich soil, consider adding in organic matter, perlite and vermiculite to increase drainage and start building the texture of the soil. Tilling in the roots and remaining stems from harvested microgreens will begin to build a healthy thriving soil.
• Gases: It’s easy to just think about the solid ingredients in soil, but what’s in-between the particles matters too. Roots need oxygen, without it anaerobic conditions develop and bacteria and other microorganisms can dominate. Good drainage, and good watering practices make sure your roots are able to exchange gases (breathe) within the soil properly.
• Water: Soil needs to have water in it for plant growth. Microgreens, especially the thinner stemmed, and smaller seed varieties like Amaranth, or Broccoli & other Brassicas are very sensitive to water in the soil. If it dries out, the microgreens will begin to soften and wilt, if left too long they’ll die.
• Microbiome: A healthy living soil, especially outdoors, will have a diverse and thriving biome full of worms, insects, fungi, bacteria and other critters. While they’re a huge benefit outdoors, and they keep each other in check, it’s usually a good idea to control the soil life indoors. Indoors, if one species gets out of check, it can quickly dominate and cause problems. Fruit fly infestations, mold overtaking your trays, fungus gnats. Use soil that’s as close to sterile as you can, so your microgreens have a head-start on everything else.

Consider Nutrient Content

You might be surprised but microgreens can get away with 100% water and growing in a sterile hydroponic medium. On the other end of the spectrum most microgreens will grow well in a compost-rich potting soil. So you can get away with a huge range of nutrients.

But just because you can get away with less nutrients, doesn’t mean you need to.

A lot of microgreen growers find growing in soil to be more natural, and find that the flavor is fuller with soil. More nutrients also make the microgreens grow quicker. 

If you do decide to go the hydroponic route using a growing mat, you’ll be on the low end of the nutrient spectrum. There are many hydroponic nutrients available out there. They bring a lot of their own energy and nutrients with them in their seeds. I would err on the side of less concentrated for the young seeds. And you can always experiment with adding a (carefully measured) splash of fulvic or humic acid to boost the nutrient profile.

Think about water retention

Microgreen roots need to breathe, but they also can’t dry out. When you’re choosing a soil you’ll want to consider how much water it can hold. This is influenced by what the soil make-up is.

Organic materials generally hold more moisture, like peat moss, sphagnum, coconut coir, and other plant-derived materials. 

Perlite and Vermiculite increase drainage (which reduces water retention). 

Water retention is also impacted by how much soil you have. If you’re growing with 1” or soil vs 2”, the 1” tray will dry out quicker. But you can always adjust your water schedule to accommodate this. Ultimately it’s a trade-off between soil cost, watering more often, and how much soil you’re comfortable using for each tray (and composting afterwards). 

Consider the Particle Sizes

Some soil are coarser than others! And some clump.

Clumpy soils can be a problem for even seed germination. If microgreen seeds fall in-between the clumps, they’ll get more moisture, and less light, and germinate differently than the ones on top. It will also make harvesting harder if the plants are growing at different heights. 

You can press the clumps down and break them up in the tray, or you can process the soil a little bit ahead of time. Think about Sifting your Soil.

Sifting is the process of forcing your soil through a mesh or screen. Usually 3/8” or ½”, you can buy them pre-made, or make your own with chicken wire or whatever you can source locally. It’s a way to remove debris and improve your soil consistency. 

Roots Breathe: Make Sure your Soil has Aeration

When you’re choosing a soil to grow in, remember that roots don’t just need water and nutrients, they need air too! You can drown microgreens if their roots can’t breathe.

Roots need oxygen. One of the main functions of roots is to exchange gases with soil. Different plants have different tolerances for aeration (think of seaweed), but they all need some oxygen. If microgreen roots can’t breathe, they’ll begin to drown and die.

Low aeration soil, especially if it’s waterlogged, is also an ideal environment for microbial growth. Mold, bacteria, and other pathogens thrive in anaerobic (low oxygen) environments. Don’t give them a chance. 

Make sure it drains

Hand-in-hand with aeration is drainage. Soil that has poor drainage isn’t a huge problem for microgreens as long as you don’t over-water. But you do need SOME drainage. Almost any soil you buy at the store in bags will have enough drainage for microgreens.

If you’re mixing your own soils, avoid soils that are really high in sand, or in clays. Drainage isn’t really a problem if you’re growing indoors, but if you’re growing microgreens outdoors in your yard, in a field, or in a greenhouse, you might be dealing with more compact soils with drainage problems.

Tailor the Soil to the Container

A container that doesn’t have drainage might benefit from a different soil than one that does. If you’re growing directly in solid greenhouse trays consider using a soil with higher drainage (more vermiculite or perlite) than if you’re growing in a perforated tray.

Also if you’re growing in a thinner container, something 1” thick or less, you’ll want a finer grained soil. Some soils are clumpy and have chunks of twigs. You’ll want to sift them through a mesh or chicken wire to break up the clumps and get the debris out, or use a soil that comes finer from the supplier. 

Use Soilless Mixes if Insects are a Problem

Soilless mixes often look like soil, but they lack the microbiome (the diverse microorganism life that’s typical of soil). They can be a good answer if you’ve been having mold problems, or if you’re having trouble tracking down soil that doesn’t come with insect eggs and larva in it. 

In some areas, for whatever reason, the garden supply places have big problems with insects. Look for bags that are stored indoors, because even if the soil is pretty sterile at the source, if it’s stored outdoors, and the bags inevitably get a little bit damaged and punctured, critters will make their way in.

With soilless mixes, there’s little to no food for insects to thrive on, so they’re less likely to move in.

Look at What’s being Used by Others

I took a bit of an inventory of what’s being used across the web and here’s what I was able to find:

Microveggy

This blog recommends making up your own blend using:

3 parts of peat moss/coco-coir + 1 part of sand (optional) + 1 part of perlite + 0.5-1 part of compost- Source

Curtis Stone Urban Farmer

He uses Sunshine Potting Mix #3 (Links to Sungro Website, the supplier)

Which after doing a bit of digging is around Is mostly sphagnum peat moss, with 20% Vermiculite, and a few other additives. It’s low on drainage, with fine particle size, and high water retention. It’s not organic. 

Bootstrap Farmer

Bootstrap farmer makes the following recommendation for microgreens:

“Grow medium can be compost, a soil mix or even just a 50/50 blend of perlite & vermiculite.” – Source

Homemicrogreens

This guy tested coconut coir-based mix, pure coconut coir, and a peat moss based potting soil. 

The results? All three worked great. 

So what’s the takeaway here? Almost anything will work.

Mold Problems?

If you have mold problems you’ll want to take a good look at your soil. Some soil, especially if it’s been stored outside, in the rain, in standing water can pick up a good population of mold.

The key to preventing mold, is starting with clean soil, and then making the conditions unfavorable for mold growth. Make sure the soil isn’t waterlogged, and that humidity isn’t too high in the area where the microgreens are growing. Fans, dehumidifiers, lowering seeding density, and watering less frequently are all ways to decrease humidity.

Temperature is another factor in microbe growth. If your temperatures are too warm, fungi and other microorganisms will grow at an accelerated rate, and can overtake your microgreens and lead to damping-off, visible mold, and poor quality product that spoils quickly.

Mold grows from spores that make their way into the soil. But even the most sterile soil can grow mold, because the spores are everywhere, and travel through the air. They’re so tiny the naked eye can’t detect them a lot of the time.

Compare the benefits of Soil vs Hydroponic Grow Mats

Soil usually grow more robustly and with less fuss than hydroponics, but it’s a lot messier. The choice isn’t just whether one grows better than the other.

Hydroponics can be done at different price points, some grow mats cost over $1 per tray, but you can also get away with burlap at one quarter of that price. The big difference is how much moisture the substrate can hold, and the texture. Larger seeds seem to do better on burlap than smaller ones.

Smaller seeds can fall through the texture of a loose hydroponic grow medium. They can also fall deeper into soil, so a fine hydroponic pad, made from wood fiber, hemp, or jute can be a good choice. 

Conclusion

Wow, you actually made it all the way down here!

If you want the quick answer: you can grow microgreens in almost any soil. Pick up a potting mix from your local garden supply shop and you probably won’t be disappointed.

If you’re concerned about fruit flies, fungus gnats, and mold problems, consider ordering soil online. You can check out the soil I’d recommend starting with here:

Microgreen Soil Buyer’s Guide 2020 USA & Canada (Links to Article with specific Recommendations)