Selenium is a trace element the body needs in small amounts and cannot manufacture. It sits at the active site of the enzymes that manage oxidative stress — the glutathione peroxidases and thioredoxin reductases — which is why it turns up in discussions of thyroid function and immune response.
Intake varies enormously by geography, and for an unglamorous reason: selenium in food traces back to selenium in the soil where it grew, and soil selenium varies by orders of magnitude between regions. Parts of China, Scandinavia and New Zealand sit at the low end. It is a lottery decided by geology.
That lottery is what makes controlled-environment growing interesting here. If selenium arrives from a solution you specify rather than soil you inherited, the geology stops being the deciding factor.
The kale study: same element, two routes, 37× apart
Researchers grew kale microgreens in a soilless system inside an environmentally controlled vertical farm, applying sodium selenate two ways — mixed into the nutrient solution, or sprayed onto the leaves — at 0, 10, 20 and 40 µM, in a randomised design with triplicates, harvested after 14 days [1].
The difference between the two delivery routes is the headline result:
| Delivery route | Selenium accumulated |
|---|---|
| Nutrient solution | up to 893.3 µg Se/kg dry matter |
| Foliar spray | up to 24 µg Se/kg dry matter |
Roughly a 37-fold gap. Spraying the leaves barely worked; feeding the roots worked substantially. For a seedling with a short cycle and a small leaf area, the root pathway is simply the one that moves material.
And fresh yield was unaffected by either method at any dose tested. That is not a given — plenty of interventions that change composition do so by stressing the plant, and you pay for it in biomass. Here the plants took up selenium without visible cost.
The dose they recommended was not the highest one
The authors identified 20 µM in the nutrient solution as the optimum for fresh consumption — not 40 µM, which the plants also tolerated [1]. The reasoning is dietary rather than agronomic: the target is a meaningful contribution to selenium intake from a realistic serving, not the largest number the tissue can hold.
This matters more for selenium than for most minerals, because selenium has a genuinely narrow window between adequate and excessive. It is one of the few micronutrients where the recommended intake and the upper tolerable limit are close enough that "more" becomes a real question rather than a rhetorical one. A biofortification protocol that optimises for a big headline number is optimising for the wrong thing.
Species differ by a factor of five
A second study treated kale, kohlrabi and wheat microgreens with a selenite/selenate mixture at 20 µmol/L and measured what accumulated [2]:
- Kale — 133 µg Se/g dry weight
- Kohlrabi — 127 µg Se/g
- Wheat — 28 µg Se/g
The two brassicas landed close together; wheat accumulated roughly a fifth as much under identical treatment. There is a mechanism behind that pattern. Selenium is chemically similar to sulphur and travels the plant's sulphur transport and assimilation pathways. Brassicas run those pathways hard — it is the same machinery that produces the glucosinolates behind sulforaphane and the family's characteristic pungency. A crop built to move sulphur moves selenium alongside it.
The researchers also checked whether accumulation was damaging the plants, which is the question a responsible study asks next. It was not: polyphenol content, antioxidant capacity, chlorophyll and carotenoid levels showed no sign of oxidative injury at that concentration [2]. The plants were not limping to the finish carrying a load of selenium.
How to read these numbers without misleading yourself
Watch the weight basis. Both studies report dry weight. Fresh microgreens are largely water, so the per-serving figure is far smaller than the headline. A number quoted per gram of dry matter and a number quoted per gram of fresh weight can differ by roughly an order of magnitude, and mixing them up is the most common error in this literature.
Watch the chemical form. The two studies used selenate, and selenite/selenate mixed. Plants take these up by different routes and convert them differently, so results are not interchangeable between forms.
Watch the species. A kale result is a kale result.
What this is, and what it is not. LUYA grows food, not medicine. Nothing here is medical advice, and enriched microgreens are not a treatment for any deficiency. If you think you are short of a nutrient, that is a conversation with a clinician and a blood test — not a growing tray.
For the wider frame, see the biofortification overview. The iodine work applies the same approach to a very different deficiency and reports fresh-weight figures, which makes the comparison instructive. Kale microgreens covers the crop itself.
Sources
Every figure in this article comes from one of the papers below, each read in full before being cited. Where a study measured one species under one set of conditions, we say so rather than generalising it to every crop. Follow the DOI to check any number yourself.
- Selenium biofortification of kale microgreens in a soilless cultivation system. Scientia Horticulturae 323 (2024) 112522. doi.org/10.1016/j.scienta.2023.112522
- Selenium biofortification of kale, kohlrabi and wheat microgreens. Plant Physiology and Biochemistry 206 (2024) 108283. doi.org/10.1016/j.plaphy.2023.108283
