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Minerals · under stress

Sele­nium

Sulfur’s cousin 5 min read

The short version

Every working cell makes sparks, and under stress it makes them faster. The plant puts them out with antioxidants it builds from sulfur, and a little selenium helps keep that shield up, so a crop holds its green longer through heat, drought and cold. The plant takes selenium in through sulfur’s doors. A crop can only carry into food what it finds in the soil, and much of Australia’s farmland is low in it.

What it does

Every working cell makes sparks. They’re a by-product of its own engine, of photosynthesis and of breathing, and in a healthy plant they’re put out as fast as they’re made. Under stress they’re made faster: in heat, drought, cold and salt, after a frost, or with hard sun on a wilting leaf. Then they start to burn. Membranes leak, chlorophyll bleaches, and the leaf ages before its time.

The plant puts the sparks out with antioxidants. One of the main ones is glutathione. It’s a small molecule the plant builds from sulfur and keeps in every cell, to catch the sparks and pass them on harmlessly. Vitamin C works alongside it.

Selenium helps keep that shield up. A little of it lifts the enzymes that put out the sparks and recycle the antioxidants, glutathione among them, so a plant with some selenium holds its green longer under stress. It isn’t on the textbook list of what a plant can’t grow without. But once a crop is under stress, it’s one of the traces that makes a difference.

Light and heat. A spark, from a cell’s engine. A spark, put out. A little selenium. A bleached cell.

  • A young broccoli plant on a hot day, cut away below the ground: broad blue-green leaves, a little limp, and a small green head.

    With a little selenium

    young broccoli, on a hot day

    the leaf puts its sparks out as fast as it makes them, and holds its green

  • The same broccoli plant, its outer leaves bleached in pale patches and yellowing at the edges.

    Short of selenium

    the same plant, the same heat

    the sparks burn, and the outer leaves bleach and age before their time

Every working cell makes sparks, and heat and hard sun make more. The plant puts them out with antioxidants, glutathione among them, which it builds from sulfur. A little selenium helps keep that shield up, so the crop holds its green longer under stress. An illustration, simplified.

It follows sulfur

Selenium is sulfur’s close cousin, and the plant treats the two alike. In most farm soils it comes as selenate, which goes in through the same doors as sulphate. The plant builds it into the same amino acids, in sulfur’s place. In wet or acid soil it’s more often selenite, which goes in mainly through phosphate’s doors. So the crops that gather the most sulfur gather the most selenium too: the brassicas, like broccoli and canola, and the onion family. And a soil heavy with sulphate can hold selenium back, crowding it out at the door.

That’s also why too much does harm. Built into a protein in sulfur’s place, too much selenium bends it out of shape. The line between the little that helps and too much is narrow.

Sulfur, as sulphate. Selenium. A door into the root, made for sulphate.

  • A young root growing down through moist soil, magnified: pale and slightly translucent, fuzzed with fine root hairs, with a smooth rounded tip, among soft red-brown soil crumbs.

    A little sulphate

    a young root, magnified

    selenium comes in through sulphate’s own doors

  • A young root growing down through moist soil, magnified: pale and slightly translucent, fuzzed with fine root hairs, with a smooth rounded tip, among soft red-brown soil crumbs.

    Heavy with sulphate

    the same root, the same selenium

    sulphate crowds the doors, and less selenium gets in

Selenium is sulfur’s close cousin, and in most farm soils the plant takes it in through the same doors as sulphate. So a soil heavy with sulphate can hold selenium back, crowding it out at the door. An illustration, simplified.

The stock and people who eat it

Animals and people can’t do without selenium. It’s in the enzymes that guard their cells, just as glutathione guards the plant’s, and in their thyroid. Short of it, lambs and calves can get white muscle disease, and stock do poorly.

A crop can only carry into food what it finds in the soil, and large parts of Australia’s farmland are low in selenium, mostly the higher-rainfall country of the south-east and south-west. Graziers there give their stock selenium, or put it in the fertiliser on their pasture. Finland went further. Since 1984 it has added selenium to its fertilisers, and the selenium in its people’s food rose with it.

That’s part of what this way of farming means by food quality. What’s in the crop is what’s in the people and animals who eat it.

Selenium. Fertiliser granules.

  • A pasture of grasses and clover, cut away below the ground to show its fine roots in red-brown soil, with a ewe grazing and her lamb beside her.

    Low in selenium

    a pasture on low-selenium soil

    a crop can only carry into food what it finds in the soil

  • The same pasture and sheep.

    Selenium on the pasture

    the same pasture, selenium in its fertiliser

    what’s in the pasture is what’s in the stock that graze it

A crop can only carry into food what it finds in the soil, and large parts of Australia’s farmland are low in selenium. Stock need it, so graziers there give it to their stock, or put it in the fertiliser on their pasture. What’s in the crop is what’s in the animals who eat it. An illustration, simplified.

How a shortage shows, and too much

A plant short of selenium doesn’t show it in its leaves. It shows in how the crop stands up to stress, and in the stock that graze it.

Too much does show. In most crops it stunts the plant and yellows its leaves, much like a sulfur shortage, as selenium takes sulfur’s place.

Where it fits in a program

  • It’s a trace. Use a little, and keep to the label. The line to too much is narrow, in the plant and in the stock that eat it.
  • Read it with sulfur. They mostly share the same doors, and the plant builds glutathione from sulfur.
  • The brassicas and the onion family gather it most. A soil heavy with sulphate holds it back.
  • On pasture, a feed or blood test tells you whether the stock are short, and your vet can tell you how best to give it.