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Minerals · a little, then salt

Chlorine

Fills, doesn’t build 5 min read

The short version

The plant takes chlorine up as chloride, the chloride in salt. A little is a nutrient: it works beside manganese at the start of photosynthesis and helps the leaf’s pores open and close. The crop nearly always has enough. The worry is too much. Chloride pulls water into a cell, and early in the season that makes soft, thin-walled leaves, so Steve’s programs hold it back and keep it out of the tank.

What it does

The plant takes chlorine up as chloride, the same chloride that’s in salt. A little is a nutrient. A lot is salt.

The leaf uses it in its engine. It works beside manganese at the very start of photosynthesis, where water is split and the oxygen comes from. It helps the two guard cells round each of the leaf’s pores swell and shrink, to open and close them. And it balances the charge that potassium carries in the sap.

Potassium. Chloride. 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 with chloride beside it, and water follows them in

  • 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 and the chloride move out, the water follows, and the gap closes

To open a leaf’s pore, the plant moves potassium into the two cells round it, and chloride often goes in beside it, balancing potassium’s charge. Water follows them in, the cells swell, and the pore opens. To close it, both move out again. An illustration, simplified.

The plant needs only a little, and it’s nearly always there. Rain near the coast, irrigation water, and fertilisers that carry chloride, such as potassium chloride (muriate of potash), usually bring more than a crop can use. Real shortages are rare: a few, mostly in cereals, on deep, leached sands far from the sea.

Water in, thin walls

Steve’s programs worry about too much, not too little. Chloride carries a single charge, so it travels with the minerals that carry a single charge the other way: potassium, and sodium. Like them, it pulls water into a cell, and a cell that fills with water fast stretches fast. Early in the season, that’s the quickest way to thin-walled, watery leaves: long, soft and floppy, and easy pickings for thrips and leaf disease all season. So his programs hold chloride back at the start, with nitrogen and potassium, while the young plant builds its frame with calcium and boron. Quality first, then speed.

It’s the same rule as the foundation: build the frame first, and fill it after. Water and chloride fill a cell. They don’t build one.

Water. Cell wall. Calcium. Boron. Chloride.

  • A young onion plant cut away below the ground: slender, upright blue-green leaves and short white roots.

    Built first

    young onions, chloride held back

    calcium and boron build small cells with strong walls, and the leaves stand firm

  • The same young onion, its leaves longer, paler and softer, several flopping over.

    Chloride early

    the same onions, chloride from the start

    chloride pulls water in, the cells stretch fast, and the leaves grow long, soft and floppy

Chloride pulls water into a cell, and a cell that fills fast stretches fast. Early in the season, that makes thin-walled, watery leaves. Build the frame first, with calcium and boron, and fill it after. An illustration, simplified.

Too much is salt

Chloride and sodium together are salt, and the Stress and salt page tells what salt does to a crop. Salty water is harder for the roots to draw up. What they do draw up builds up in the leaves, older leaves first, until their tips and edges burn. And chloride crowds nitrate out at the root, since the two come in through some of the same doors, so a crop on salty water can go short of nitrogen too. Tree crops are among the most sensitive: avocados, citrus, grapes and stone fruit.

Steve’s programs keep chloride out of every blend, in the spray tank and the fertigation. Onions are salt-sensitive, and his apple program says chloride leaves the fruit softer at the finish. So where a fertiliser brings its potassium as chloride, choose one that brings it as a sulphate or a nitrate.

Potassium. Chloride. Sulfur. Nitrogen.

  • Three small heaps of fertiliser on paper: pinkish-red granules, fine off-white granules, and small white round prills.

    Where the potassium comes from

    three potassium fertilisers

    the same potassium, with chloride, sulfur or nitrogen beside it: where chloride isn’t wanted, choose the sulphate or the nitrate

Every potassium fertiliser brings something with it: potassium chloride brings chloride, potassium sulphate brings sulfur, and potassium nitrate brings nitrogen. Where chloride isn’t wanted, choose the sulphate or the nitrate. An illustration, simplified.

How a shortage shows, and too much

A shortage is rare: wilting leaf tips and spotted leaves, mostly in cereals on leached sands.

Too much is common where water or soil is salty: tips and edges of the older leaves that yellow and then burn, and, early in the season, soft, stretched growth.

Chloride.

  • A young wheat plant cut away below the ground, its roots in pale sand: the tips of several leaves wilted and hanging, and small yellow spots along its leaves.

    Short

    rare: cereals on leached sands

    leaf tips wilt, and the leaves come out in spots

  • A young onion plant cut away below the ground: its outer, older leaves yellowing from the tips, the oldest burnt brown at the tip, its inner leaves green.

    Too much

    common, where the water or soil is salty

    it builds up in the older leaves first, until their tips yellow and then burn

A chloride shortage is rare: leaf tips that wilt and leaves that spot, mostly in cereals on leached sands. Too much is common where the water or soil is salty. It builds up in the older leaves first, until their tips yellow and then burn. An illustration, simplified.

Where it fits in a program

  • Most crops get all the chloride they need without anyone putting it on.
  • Test the water. On an irrigated farm, it’s often the biggest source of chloride.
  • Check where each fertiliser’s potassium comes from: chloride, sulphate or nitrate.
  • Hold it back early, while the crop builds its frame, and keep it out of salt-sensitive crops.