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Minerals · the green in the leaf

Magne­sium

Old leaves first Episode 3 4 min read

The short version

Magnesium sits at the centre of every chlorophyll molecule: it’s the green in the leaf that catches the sun. It also helps load sugars into the sap. The plant moves it around, so a shortage shows on the older leaves first, yellow between green veins.

Short of magnesium, the plant moves it out of the old leaves to feed the new, and the old ones yellow between green veins.

What it does

Magnesium sits at the centre of every chlorophyll molecule, the green in the leaf that catches the sun. No magnesium, no chlorophyll, and the leaf’s engine has nothing to catch the light with. It also switches on many of the enzymes, the plant’s tiny tools, that make and move sugar, and it helps load the sugar into the sap for the trip to the fruit, roots and tubers. Short of magnesium, sugar gets stuck in the leaves.

The roots feel it first. The sugar that should go down to feed them stays in the leaves, and the roots grow less, often before the leaves show a thing.

Sugar. Magnesium.

  • A young beetroot, cut away below the ground: glossy green leaves on red stalks, a round red-purple root, and a wide fan of fine roots in red-brown soil.

    Enough magnesium

    a young beetroot

    the sugar loads into the sap and goes down to feed the roots

  • The same beetroot, its outer leaves yellowing between their veins, its root smaller and its fine roots fewer and shorter.

    Short of magnesium

    the same beetroot

    the sugar stays in the leaves, and the roots grow less

Magnesium helps load the leaves’ sugar into the sap. Short of it, the sugar stays in the leaves and the roots go short, often well before the leaves pale, as they have here. An illustration, simplified.

Old leaves first

Magnesium moves around inside the plant. When it runs short, the plant takes it out of its older leaves to keep the new growth green. So the old leaves show it first: yellow between the veins, while the veins themselves stay green. In some crops the yellow turns reddish or purple at the edges.

the mineral, where it is in the plant.

  • A young bean plant: its oldest leaves, at the bottom, are yellow between green veins, and the leaves above are green.

    Short of magnesium

    the old leaves go yellow between green veins: magnesium moves to the new growth

  • The same young bean plant: its oldest leaves are green, and its youngest leaves, at the top, are pale and mottled between green veins.

    Short of manganese

    the new leaves go pale and mottled: manganese mostly stays where it is

Magnesium and manganese look alike on a leaf, but they show at opposite ends of the plant: magnesium on the old leaves, manganese on the new. An illustration, simplified.

That’s the first clue to telling it from manganese. The names are close and the look is similar, but they show at opposite ends of the plant: magnesium on the old leaves, because it moves, and manganese mostly on the new ones, because it barely moves. The Mobile and immobile minerals page has the rest of the list.

Worse in bright sun

Short of magnesium, a leaf still catches the sun, but it can’t use all of it. The sugar it’s made can’t get away, so it slows down, and the light it can’t use damages it instead. So a shortage yellows and scorches faster on leaves out in full sun than on leaves in the shade, and a crop in strong sun needs more magnesium.

Light. Sugar.

  • An old bean leaf of three leaflets, yellow between green veins, with brown, scorched patches toward the edges.

    In full sun

    an old leaf, short of magnesium

    it can’t use all the light it catches, and the light it can’t use damages it

  • Another old bean leaf from the same plant, grown in the shade: still green, only faintly paler between the veins.

    In the shade

    another from the same plant

    less light to do harm: it stays green longer

Short of magnesium, a leaf can’t use all the light it catches: its sugar can’t get away, so it slows down, and the light it can’t use damages it. So the shortage yellows and scorches faster on leaves out in full sun than on leaves in the shade. An illustration, simplified.

What crowds it out

Magnesium competes with potassium, calcium and ammonium to get into the root. Heavy potassium, in particular, can bring on a magnesium shortage in a crop growing in soil that has plenty. That’s one more reason Steve’s rule pays: earn the right to use potassium.

Magnesium also runs short on sandy, acid soils in high-rainfall country, where it washes out, and in cold, wet soils early in the season, while the roots are slow.

Magnesium. Rain. Water.

  • Three young wheat seedlings cut away below the ground, their fine roots reaching down through pale, loose sand.

    Washed out

    wheat seedlings, in sandy soil

    in high rainfall, a sandy, acid soil loses its magnesium down past the roots

  • The same seedlings early in the season, the sand dark and soaked, water lying on its surface, and their roots short and few.

    Cold and wet

    the same seedlings, early in the season

    the roots are slow, and the crop runs short

Magnesium runs short on sandy, acid soils in high-rainfall country, where it washes out, and in cold, wet soils early in the season, while the roots are slow. An illustration, simplified.

When it matters most, crop by crop

The stages where magnesium is one of the minerals that matter most, in the program builder’s seasons. Each bar is a whole season, from sowing or planting to harvest, or a tree’s year. Build yours in the program builder.

  • Potatoes about 120 days

    Sprout development · Vegetative growth · Tuber bulking · Maturation

  • Beans about 70 days

    Bud development · Maturity

  • Carrots about 120 days

    Leaf growth · Bulking

  • Onions about 180 days

    Canopy development

  • Poppies about 140 days

    Rosette · Capsule fill

  • Sweet corn about 90 days

    Kernel fill

  • Cotton about 180 days

    Early growth to first square · Boll fill

  • Wheat about 170 days

    Grain fill

  • Avocados a year

    Fruit growth · Autumn flush and oil

  • Apples a year

    Fruit sizing · Bulking to early ripening

Where it fits in a program

  • Test the old leaves as well as the new. Magnesium falling in the old leaves is the early warning.
  • Keep potassium in its place: foundation first, and don’t flood the root with potassium.
  • Read magnesium alongside potassium and calcium on your tests. Too much of one, especially potassium, can hold the others back.
  • A magnesium foliar can tide the crop over while you sort out the cause: it’s a top-up, not a cure. Rates come from your tests and the label.