Soil: The case for growing microgreens hydroponically in homes
We are often asked why we designed Microbed as a hydroponic growing system when soil is the default setting of gardening.
The purpose of this article is not to demonise soil, which is the very basis of food production outdoors. Soil is extraordinary, and a single teaspoon of healthy forest soil contains more microorganisms than there are people.
Unfortunately, the soil we buy in bags is not that soil, instead it is an engineered composite carrying its name while bearing little resemblance to it.
Applied indoors, for microgreens, soil is a technology that requires reconsideration because proper soil management requires more space, time and expertise than most people realise.
For those of us without these factors, the problems with growing microgreens with soil indoors, as part of a kitchen garden system, are multiple:
- Part 1: Soil is a living ecosystem that attracts insects to lay their eggs
- Part 2: Reused soil carries pathogens that cause damping-off. Potting soil is single-use only for microgreens, though it can be reused for other plants
- Part 3: Studies have found that bagged soil can contain forever chemicals like PFAS
- Part 4: A key component of bagged soil is peat moss, which a non-renewable resource that has an extremely high carbon footprint
In Part 5, we explore whether the growing medium affects the nutritional value of microgreens.
Part 1: Soil is a living ecosystem that attracts insects
Indoor plants (whether ornamental or edible) grown in soil attract insects, primarily due to the presence of organic matter and moisture. For example, fungus gnats (small black flies) are attracted to moist indoor potting soil, where they lay eggs that hatch into larvae that feed on decaying organic matter.
In nature, predators exist to check the population of fungus gnats, but indoors, managing the insect population attracted by soil can become a frustrating and time consuming task.
There are many strategies to combat insects like fungus gnats, but the fact remains that introducing soil into indoor spaces means importing an ecosystem that operates on its own terms.
For those who want a simpler, lower maintenance set up, hydroponic growing does not present these same challenges.
Cover your Microbed while the seeds are germinating to create blackout conditions for the first few days. Subsequently, when the cover is removed to expose the sprouts to light, change the water every 2 days to keep the system clean and prevent mosquitoes and other insects from breeding.
Part 2: Soil carries pathogens that cause damping-off
Most bagged soil is pasteurised, but it is important to check the label and use the soil promptly after opening the bag. Once exposed to air and handling, pasteurised soil can be reintroduced to environmental pathogens over time.
The main problem is that soil used to grow microgreens cannot be reused. This is because soilborne pathogens (which include Pythium, Rhizoctonia, Fusarium, and Phytophthora) cause damping-off: a condition where seedlings collapse and rot at the base shortly after germination. These pathogens persist as spores in soil and are activated by the warm, moist conditions that microgreen trays produce (Lamichhane et al., 2017).
This means that soil used for one microgreen crop cannot be reused. It can be repurposed for other plants or composted but if you only grow microgreens, it is effectively single-use, a consideration worth weighing for growers who chose this method with sustainability in mind.
Hydroponic growing addresses this because without soil, there is no reservoir for spores to persist between cycles.
Sowing your Microbed at a lower density improves airflow between seedlings, reducing the surface moisture that triggers mould. Change the water every two days to further prevent the stagnant conditions that pathogens depend on to establish. This ensures a growing environment that is less hospitable to the organisms that cause damping-off.
Part 3: Where does your soil come from, exactly?
Another one of the problems of growing microgreens with soil comes from understanding where your soil comes from. Unfortunately, a bag of soil doesn't tell a complete story:
- What kind of land were its components removed from?
- What was the land used for before? What kind of residues might remain in the soil from previous use?
- Is it only suitable for growing ornamental plants, or also safe for growing food?
As nutrition geeks, we are careful about what goes into our bodies, yet the reality of commercial potting mixes means that the starting point of the growing system is often taken on faith. This does not mean every bag of soil is unsafe for growing food, but that the consumer is asked to trust a highly complex supply chain with very little visibility.
Exactly what your soil contains might not matter for ornamental plants, but for growing food inches from your chopping board, soil is far too complex a product to take for granted.
Studies (Sivaram et al., 2022) have found forever chemicals like PFAS in commercially available composts, garden soils, and potting mixes, which matters because PFAS can persist in the environment and potentially enter food systems.
Most bagged potting mixes are not clumps of soil taken directly from the ground in the way we might think. Instead, they are engineered blends designed to retain water and have a certain consistency. Typical components include:
- Peat moss: a sponge-like base that holds water well, but has an extremely high carbon footprint
- Perlite: a lightweight, expanded volcanic mineral that improves air pockets and drainage
- Sand: adds weight and structure, and helps drainage depending on particle size
- Coco coir (sometimes called coco peat): a fibrous byproduct of coconut processing, used as a peat alternative
- Organic matter (often composted materials): adds nutrients and microbial activity, but quality varies
Of these components, peat is the most controversial:
Part 4: The high carbon footprint of peat moss
Peatlands are one of the earth's most carbon dense ecosystems, covering just 3% of the earth's surface, but storing roughly twice as much carbon as all the world's forests combined. When peat is drained or extracted, the stored carbon is exposed to oxygen and released as greenhouse gases. This is why peatland degradation is treated as a significant emissions source in climate literature.
The scale of this is significant: emissions from drained peatlands are estimated at 1.9 gigatonnes of CO₂ equivalent annually. To put that in perspective, fires in Indonesian peat swamp forests in 2015 alone emitted nearly 16 million tonnes of CO₂ a day, more than the entire United States economy produces in the same period.
Horticulture is one of the drivers of peat extraction, with the growing media industry relying heavily on peat.
For growers motivated by sustainability, hydroponic growing sidesteps this issue. Since no peat-based media is required, the system's footprint is largely limited to the water used and the transportation of seeds. Growing your own food to reduce your environmental impact, only to use a peat based substrate, is unfortunately like switching off the lights to save electricity and then leaving the tap running.
Part 5: Does the growing medium affect the nutritional value of microgreens?
Because microgreens are usually harvested in around a week, their root systems are not fully developed at the point of harvest. During this phase, the plant is mobilising nutrient reserves stored within the seed itself to fuel rapid growth, meaning microgreens draw primarily on what the seed contains, rather than what the growing medium provides.
This is why the choice of substrate matters less for microgreens than it would for a longer cycle crop, and why hydroponics is well suited to the task of growing microgreens.
That said, research does suggest that the growing medium still has a measurable effect. For example, a study on broccoli microgreens found that soil grown microgreens possessed higher vitamin C concentrations than hydroponically grown ones (Tan et al., 2020). In figure C below taken from the study, CH and FH refer to microgreen samples grown hydroponically, while FS refers to samples grown in soil.
Conclusion: A better fit for indoor kitchen gardening
The honest conclusion is that soil has a nutritional edge in many measures, while hydroponics offers control and consistency.
Hydroponics does not pretend to replicate soil, but it removes most of the problems with growing indoors. For us at least, the ease of being able to grow microgreens on our kitchen countertop outweighs the partial sacrifice of nutrient content.
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