Soil Mix Conundrum - Peat Moss vs Coconut Coir
If you grow microgreens in a soil mix, you have two choices, peat moss or coconut coir.
Neither grows microgreens very well by itself (more on this later). This is why we use soil mixes. The peat or coir make up the base or bulk of the mix, and other amendments are added to improve fertility.
There are quite a few environmental, social, political, cost, renewability, and sustainability issues with peat moss and coconut coir. Those issues will be addressed in other articles because they are essential to understand, and deserve a whole article for discussion.
But for the sake of arguing over those matters right now, let's assume that all things are equal and look at the growing propensity of these two soil materials.
Peat Moss vs Coconut Coir - Growing Microgreens
As mentioned, neither grows microgreens very well by itself. They are, for all intents and purposes, nutritionally inert for plants.
Though we haven't tried to grow microgreens in pure peat moss (yet), we have attempted to grow microgreens in pure coconut coir. It didn't turn out that well.
You can read and see photos of that attempt in this article: The Best Soil for Microgreens.
The title of that article seems to be the end all be all for the best soil. But that isn't the case, because we haven't tested all soil mixes, and we also want to experiment with organic fertilizers in the cheaper, but inert soil types.
Audio Presentation of the Peat vs Coir / Love & Hate Debate
In the podcast we talk about the problems we have when we use peat moss based soil mixes. This is the Microgreen Podcast Episode 003.
What About Growing Microgreens Without Soil?
We tried that too. So far, our testing has shown that soil outperforms mats by a long shot.
You can see the results of that testing by clicking this link.
Plus, some microgreen seeds can't be grown hydroponically, or on mats.
This shouldn't be a surprise, because where do seeds grow best in nature? In soil, not on cloth, hemp, cotton, some unique fabric, or water.
However, we actually do have some new (to us) bamboo mats to try. At Home Microgreens, we try to keep an open mind, and if testing shows our previous results to be wrong, we will change our opinion.
What is Soil Mix?
Since peat moss and coconut coir are nutrient-poor, other amendments are added to improve fertility and drainage.
Each soil manufacturer has their own blend of amendments. The amendments are usually perlite and composted or aged wood products, composted manure or another nitrogen-rich product, and mineral additives.
The peat moss and coconut coir provide a relatively inexpensive bulk product that is sterile and inert.
Pin the Image Below to Your Pinterest Microgreens Board
How Peat Moss and Coconut Coir Contribute to Soil Mixes
Both are a natural inert organic bulky material perfect for soilless potting mixes. Each holds 8 to 10 times their weight in water.
Holding water is vital for any potting mix, especially those used in containers.
They also resist breaking down (rotting) and keep their volume. Even though both can be compressed, once loosened, they remain relatively fluffy.
Since both peat moss and coconut coir can be compressed into bales, it is easier and cheaper for the product to be shipped.
Peat moss is easily mined for vast deposits, and coconut coir is a waste product that needs to be transported away from the processing plant anyway. Best to transport it to a facility that can use it rather than used as landfill material.
Since the material is easily obtained and the volume can be shrunk for transport, these materials become less expensive to use as the base for soil blends.
The price of each depends on your location relative to the source of the material. About 80% of the world's peat moss comes from Russia and Canada.
While 90% of coconut coir comes from India and Sri Lanka.
It's a Curse and a Blessing, aka Love / Hate Relationship
Here comes the Love-hate aspect of the peat moss vs coconut coir debate - at least for us.
Let's face it microgreens have it tough. They really do. We overseed them in shallow trays, grow them in a warm area on some sort of open metal rack, and often (at least commercially) have fans blowing on them.
All those plants are looking for water, and the shallow trays quickly become root-bound. The small volume of soil in those trays needs to hold water but still allow for oxygen exchange.
Due to their ability to hold water, peat moss and coconut coir seem to be a perfect media for growing microgreens.
But let's take a look at the video below to see if this is completely true.
By the way, the peat-based and coir-based soil mixes in the video are produced by the same company and, for the most part, contain the same amendments. They are very similar, except one uses peat and the other coconut coir.
Video - Wetting Microgreen Soil Mixes
Did You See That?
What a mess.
Besides the muddy beginnings for peat moss, the problem is much more profound.
Here Are The Reasons We Hate Peat Moss
1. Who Has the Time?
When I'm planting microgreens, I don't care if it's one tray or ten; I want it done.
Yes, you can sprinkle on the seeds before all of the water as soaked down into the peat moss. But you sure can't spread-out the seeds if you clump them together by mistake.
That sticky peat moss mud on my finger isn't the best substrate for pushing seeds around and spreading them out.
We bring this up because you will see how much time it takes for water to soak into an organic peat-based soil mix manufactured by a national and well-liked brand in a soon to be released article.
It's ridiculous. If it wasn't a national brand, we wouldn't publish the article. But we don't want anyone buying this soil and have to put up with it.
We'll link the article here when it's published.
2. What Takes Time Going Down, Takes Time Coming Up
We believe entirely in bottom watering microgreens once they have started growing.
So if it takes that much time for water to soak into the peat moss, imagine how long it will take to wick up from the bottom.
In the video, water had gravity to help it soak in, but it sat there. So think about the problems water will have fighting gravity to wick upward when we bottom water a microgreens tray.
Now, we'll have to give peat moss some benefit of the doubt.
Once wet, peat moss more readily takes on water. So if it stays damp, water will absorb and move through the soil column quicker.
But what if your microgreen trays do dry out?
Yeah, you'll be forever rewetting that peat moss.
3. Too Much of a Good Thing
According to our research, peat moss and coconut coir hold about the same amount of water per unit weight. Now we're not sure if an average tray of peat moss and coco coir weigh the same or not.
We also don't know what the experts mean by "hold water" Does this mean the weight of water actually absorbed by the media? Or does it include the water held between the media particles?
Big difference. We think what they mean is how much water the organic fibers absorb.
But we do know that the pores in peat moss are much smaller and the capillary forces are higher, and the cohesive attraction and adhesive forces are more significant.
So a tray containing a peat moss mix will retain more water than a tray full of a coconut coir mix. We know this because we've watered hundred of trays, and water more readily drains from those full of a coir mix.
In other words, peat moss doesn't drain.
You'd think this is what we are after, more water retention. However, we do need pore space for the soil to also hold air. We need the water to wet the media and drain through, so air can reenter the soil media.
Coconut coir has longer, coarser strands than peat moss does. Water will run down these strands and drain. The larger spaces between the fibers reduce capillary forces and leave more room for air.
Too Much Moisture Is Bad During the Blackout Period
Remember what we recommend when seeding a tray of microgreens.
Don't wet all of the soil.
We only want the upper surface (top 1/8- to 1/4-inch) to be wet.
We either tightly cover the soil or put a dome over the soil during the blackout, so a little bit of moisture will go a long way.
Only after the plants are under the lights do we bottom water.
Peat moss has a tendency to hold too much water against the seeds and takes up water slower when we do bottom water.
Those Are Our Reasons to Love One and Hate the Other
In our opinion, the peat moss vs coconut coir debate is over.
We hate growing microgreens in peat-based soil mixes. Sometimes, such as during the CoVid19 shutdown, we had no choice. We had to use what was available and adapt to using peat-based blends.
It took us longer to plant our trays of microgreens, they did not grow as well, we had to change our methods to stop fungus and lengthen the grow times to get the same yield.
It was cheaper on the wallet, but that was about it.
It isn't that you can't grow microgreens in peat moss. Thousands do.
But it's easier to grow microgreens in coconut coir based mixes.
We love growing microgreens in coir mixes because the process is faster, easier, and the yields are larger using our methods.
Our Preferred Soil Mix
Here's our preferred soil mix. This soil is included in the Home Microgreens Kits, and we also use it to grow trays of retail microgreens.
What's Your Experience?
What do you use to grow your microgreens?
Have you had good luck with peat moss? With coconut coir? Maybe mats?
Let us know in the comment section. We can learn from each other.