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Minerals · the lone wolf

Potas­sium

Earn the right Episode 4 5 min read

The short version

Potassium fills the house. It helps move water and sugar around the plant and into the fruit, so it’s a big part of size, sweetness and yield, and it’s often first on the order. Steve’s rule is to earn the right to use it: pour the foundation first, then bring the potassium in to fill it.

The lone wolf, Mr Potassium: he waits until the foundation’s poured.

What it does

Potassium runs the plant’s water and sugar. It opens and closes the pores in the leaves, keeps cells full and firm, and helps move sugar from the leaves into the fruit, tubers and grain. That’s why it shows up in size, sweetness and yield, and why it’s so often the first thing on the fertiliser order.

Each of those pores sits between two small cells. In most crops they’re bean-shaped. In wheat and other grasses they’re shaped more like a dumbbell. To open it, the plant moves potassium into the two cells, water follows it in, and they swell and bow apart. To close it, the potassium moves out, the water follows it, and they go slack and close the gap. A crop short of potassium is slow to close its pores, so in a hot, dry spell it loses more water than it should.

Potassium. Water.

  • The underside of a leaf, magnified: a jigsaw of pale green skin cells, and in the middle a pore, open between two plump, bean-shaped cells.

    Pore open

    a pore on a leaf’s underside, magnified

    potassium moves into the two cells, water follows, and they bow apart

  • The same pore, closed: its two cells slack and lying close together, the gap shut to a line.

    Pore closed

    the same pore

    the potassium moves out, the water follows it, and they close the gap

A leaf opens and closes its pores with potassium. In, and water follows it and the pore opens. Out, and the pore closes. A crop short of potassium is slow to close them. An illustration, simplified.

Earn the right

Steve’s picture is a house. Calcium, boron and silicon, the three amigos, are the concrete in the foundation. Potassium is the lone wolf, Mr Potassium: he fills the house once it’s built. Put him in the concrete mix and see how well the foundation sets. See the foundation.

It’s a talk he has with growers again and again. He asks what they put on around flowering, and one says some potassium sulphate and some calcium nitrate. What did you just do to the foundation? That grower said nobody had ever told them that, and they’d been doing it for thirty years.

potassium. calcium. magnesium.

  • 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.

    All three get in

    potassium, calcium and magnesium take turns at the root’s 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.

    A flood of potassium

    potassium takes most of the doors, and less calcium and magnesium get in

Potassium, calcium and magnesium share some of the doors into the root. Flood it with potassium, and the other two get in less. An illustration, simplified.

Here’s part of why. Potassium, calcium and magnesium compete to get into the root. Pour on a lot of potassium and less of the other two gets in, right when the young fruit is building its cells and needs calcium most. The foundation goes in short, and you don’t get that window back.

How it moves

Potassium moves freely around the plant. When it runs short, the plant pulls it out of the older leaves to feed the new growth and the fruit, so the old leaves show it first: yellow, then scorched brown edges. Fruit is a big draw. A heavy crop can strip the leaves of potassium late in the season, just as it’s filling.

Potassium, where it is in the plant. Potassium that has left.

  • A tall tomato plant on a single stem: at the top, the growing tip and a truss of yellow flowers; below it, a truss of small green fruitlets, then a truss of full-size green fruit, and at the bottom a truss turning red.

    Enough to go round

    a tomato plant

    potassium in every leaf, and some moving to the fruit

  • The same tomato plant later in the season, every truss heavy with fruit, its old lower leaves yellow at the edges and the lowest scorched brown along them.

    A heavy crop, late

    the same plant

    the fruit draws potassium out of the old leaves, and they yellow, then scorch at the edges

Potassium moves freely around the plant, and fruit is a big draw. Short of it, the plant pulls it out of the old leaves for the new growth and the fruit, so the old leaves show it first: yellow, then scorched brown at the edges. A heavy crop can strip the leaves late in the season, just as it’s filling. An illustration, simplified.

When it matters most, crop by crop

The stages where potassium 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

    Tuber bulking

  • Beans about 70 days

    Cell expansion and sizing

  • Carrots about 120 days

    Early bulking · Bulking · Maturity

  • Onions about 180 days

    Bulb filling and maturation

  • Poppies about 140 days

    Capsule fill · Ripening

  • Sweet corn about 90 days

    Kernel fill · Picking

  • Cotton about 180 days

    Boll fill · Boll opening to picking

  • Wheat about 170 days

    Grain fill · Ripening

  • Avocados a year

    Fruit growth · Autumn flush and oil

  • Apples a year

    Fruit sizing · Bulking to early ripening · Colour and ripening · Harvest

Where it fits in a program

Steve’s order: build the foundation first, through the cell division window. Then bring potassium in when the crop is sizing, bulking or filling, which is when it pays.

  • Pour the foundation first. Calcium, boron and silicon go in ahead of the cell division window.
  • Then fill the house. Put potassium where the crop is sizing and filling, not in the mix at flowering.
  • Watch the balance on your tests. Heavy potassium can bring on a calcium or magnesium shortage.
  • Don’t starve the crop to earn it. On a soil short of potassium, a base dressing at planting is usual practice, and the crop takes some up from early on. It just can’t stand in for the foundation.
  • The program builder checks the order for you: it flags any pass that puts potassium on before calcium and boron.

“Build the foundations, build the house, fill it with potassium. Now you’ve got yield and quality.”

— Steve, on earning the right