The best growing mediums for microgreens have high moisture, air and nutrient availability. While most backyard soils are rich in minerals, more often than not, backyard soils cannot be used for growing microgreens because they only fulfill two out of the three criteria.
Getting the right kind of growing medium is the most critical aspect of growing microgreens because it buffers the microgreens against changes in humidity and watering frequency, allowing you to have a set and forget microgreens growth.
Potting soil is the most readily available growing medium. It is specifically formulated for growing in containers, which means all potting soil have extremely high moisture rentention, aeration, and nutrients availability. However, most potting soil is designed for mature plants. Using peat moss as a base, potting soil is typically mixed with perilite for aeration and is fertilized to 0.5 - 0.5 - 0.5.
Perilite aerates the medium, it doesn't have any moisture retention properties. Moreover, microgreens have a high demand for phosphorus, but the fertilizer ratio (N - P - K) are equally balanced. Nitrogen is the primary source of protein for plants, needed to make enzymes and structural proteins. Phosphorus is a crucial part of DNA and ezymatic reactions.
The biggest problem with potting soil is the cost. Potting soil costs to 3 to 4 times more expensive than making a superior medium from separate components. And if you purchase potting soil specifically geared towards seedlings, it's even more expensive than regular potting soil.
Peat moss is the primary ingredient in all potting soil mixes. Peat moss is either produced from dried up peat plants, or is mined from peat bogs.
Sustainability wise, peat bogs are a finite resource. Peat moss is primarily produced in Canada, where there is an abundance of peat bogs. If you’re looking for a more environmentally friendly solution, bags that are specifically labelled sphagnum moss are more likely to be not mined. The advantage of peat moss versus soil is that it doesn't contain foreign seeds, which makes weeding non-existent.
Peat moss is composed mostly of decayed organic matter. Unlike compost, peat moss is not full of readily available nitrogen or minerals.
Peat moss can sustain a plant's growth for the first two weeks without additional nutrient input. The re-use of peat moss as a growing medium is encouranged because peat moss is used for its superior areation and water retention properites. Peat moss is impossible to compact, which makes it easy to mix additional nutrients and fertilzer.
when it’s sold, peat moss is usually compressed and dehydrated. The biggest mistake that beginners make when using peat moss is to water the peat moss after seeding. Peat moss repels water when dry and floats, which makes it extremely difficult to water without mechanically mixing the water.
Coconut coir is a great alternative to peat moss. It is made from leftover dehydrated and compressed coconut fibers. Coconut coir is cheap and easy to transport, which makes it easy to buy online.
When hydrated, coconut coir expands to 20 to 30 times its original size. And like peat moss, coconut coir retains water very well while providing good aeration.
However, just like peat moss, coconut coir is also devoid of readily available nutrients such as Nitrogen, which makes it necessary to add fertilizer or minerals.
Mulch should be the last resort for your growing medium choice. This is because it doesn’t hold water very well. Plus, if you buy it from commercial sources, there is a possibility that wood mulch originated from treated wood, which may contains heavy metals or carcinogenic compounds.
It is still possible to grow in mulch, given that you add additional nutrients and minerals to the medium or water.
Peat moss alone is a good enough base for aeration and moisture retention. However, additives such as vermiculite and perilite further improves the aeration and moisture retention of the growing meidum.
Aeration is important for a plant's growth. Plants absorb oxygen from their roots, which is inhibited by compaction and overwatering. A water-loggeed soil is will asphyxiate your plants. It's very easy to overwater plants growing in a container, even with drainage holes.
Perlite and vermiculite are suited for growing microgreens. Perlite is an extremely porous white volcanic rock. Their porosity acts as an air reserve and create disruptions in the growing medium, which decreases compaction and increases air flow.
Vermiculite are a synthetic material that expands when water is added. The expansion of these tubes also breaks up the soil and creates pockets of air. But more importantly, vermiculite is extremely water absorbant. This makes vermiculite better for microgreens because it provides aeration and moisture. Microgreens need a constant supply of water because their roots are not as deep as mature plants.
Fortunately, these additives are cheap. A small bag of perlite or vermiculite costs about 10 dollars. Mixed at a 10% ratio by weight, the cost to make your own peat moss + vermiculite mixture is still less than half the cost of potting soil.
Peat moss has plenty of Si, Al, Fe, Ca, Mg, Na and P micro elements present (1). Peat moss doesn't have a lot of nitrogen present, which means that adding fertilizer is necessary to support growth beyond one week.
If you are re-using the growing medium, then you should also consider re-plenishing the nitrogen content every six to eight weeks.
The biggest questions among microgreen growers is, should you use organic or inorganic fertilizer?
There is nothing wrong with inorganic fertilizer. As long as you use it properly, inorganic fertilizers will not have health or environmental consequences. The main issue with inorganic fertilizers are fertilizer runoff, which doesn't apply to microgreens.
The main advantage of inorganic fertilizer is that you know exactly the type and concentration of nitrogen you're using, which typically consists of ammonium or urea. These two forms of nitrogen are more readily utilized by the plants versus organic fertilizers.
The final concentration of Nitrogen in the growing medium should not exceed 0.5% by weight or you will risk burning the plant. All fertilizers are labeled with three numbers. For example, if the fertilizer is 10-10-10, the concentration of nitrogen is 10% by weight. To get the amount of fertilizer to use, divide the weight of the growing medium by 10. Then divide the result by two.
Organic fertilizers are not always good. If the fertilizer is labelled “bio solids”, this means that it’s made from human poop from the sewage, which is known to contain heavy metals. Fertlizer made from manure also has a high chance of containing foreign seeds.
The biggest issue with organic fertilizers is that the readily available form of nitrogen is highly variable. Second, it takes weeks for microbes in the soil or growing medium to convert the nitrogen into readily available form of nitrogen.
If you are keen on using organic fertilizers, compost and worm castings are your best options. These two types of fertilizers are expensive. Moreover, they are bulky because you have to mix these fertilers to at least 10% by weight. But in terms of growth and environmentally friendliness, compost and worm castings are the best available options.
Selenium enrichment allows the microgreens to be enriched in selenium. Selenium is an element that helps fight cancer. Consuming selenium enriched microgreens provides a more bioavailable form of selenium versus selenium supplmentation, which is the selling point for selenium enrichment.
To enrich the medium, the easiest way is to dissolve sodium selenite to the water at a concentration of 10 micrograms per ml. Else, you can simply add 30 - 60 miligrams per square foot of growing space.
Of all the methods of growing microgreens, the hydroponic method is the most superior method for large scale growing. This should only be considered if you’re growing commercially.
The best way to grow microgreens with hydroponics is called the Nutrient Film Technique. With this method, the microgreens are supported on inert medium such as rock wool. Since the nutrients and minerals are dissolved in the water, which is already highly oxygenated, the only variables you should consider are the flow rate and concentration of nutrients.
I am the author of this website and owner of growyourmicrogreens.com. I am an hobbyist gardener and a passionate scientist. I was trained as a scientist in the Molecular Genetics program in University of Toronto, where I received my Masters of Science and published a journal article.