Sodium · The wiki · Learning hub

What it does

Sodium is salt’s other half. A few crops can use a little. Beet and spinach take some in place of part of their potassium. Some plants need a trace of it to run their engine. Australia’s own bladder saltbush was the first flowering plant found to need sodium. And stock need it, which is why they’re given salt licks.

In most crops, though, sodium is a stress. Steve’s programs treat it as one: a salt stressor that blocks potassium and calcium at the root, and builds up in the leaves until their tips scorch. They keep it low in the sap, and his onion program keeps potassium far ahead of it.

Potassium’s look-alike

To a root, sodium looks a lot like potassium: both are small, and both carry the same single charge. The root has doors made for potassium, and the more sodium there is around it, the more sodium gets into those doors, and the less potassium gets in. So a crop can run short of potassium in a soil that’s full of it, if the water round its roots is salty.

That’s why Steve’s onion program reads the two together in a sap test. It isn’t only how much potassium the plant has. It’s how far ahead of sodium it is.

Inside the plant, sodium does potassium’s work badly. A cell holding sodium in potassium’s place takes on more water to balance the salt. It swells, and its walls stretch thin. Some cells burst and leak, and the leaf goes soft, easily damaged and quick to break down. So when sodium runs ahead of potassium, the crop pays for it in every cell, not only at the root.

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. — To a root, sodium looks a lot like potassium. The more sodium in the water round the root, the more of it gets into potassium’s doors, and the less potassium gets in. So a crop can run short of potassium in a soil that’s full of it. An illustration, simplified.

Sodic soils

In the soil, sodium does a different harm. Clay holds calcium, magnesium, potassium and sodium on its surface. Calcium draws the clay’s fine particles together into crumbs. Sodium pushes them apart. When too much of what the clay holds is sodium, the soil falls apart as soon as it’s wet. The crumbs slump into mud, the water in a puddle turns milky, and the surface seals. When it dries, it sets hard and crusts. Water runs off rather than soaking in, and roots and seedlings can’t get through. That’s a sodic soil, and there’s a lot of it in Australia’s cropping country.

Calcium is the cure. Gypsum, which is calcium sulphate, puts calcium on the clay in sodium’s place. Rain or irrigation then washes the sodium down, below the roots, and the crumb comes back. That needs water that can drain away, and it can take more than one season. It’s one of the plainest cases of why calcium’s place on the clay matters.

Three young wheat seedlings cut away below the ground, their fine roots reaching down through loose, crumbly red-brown soil. — Calcium draws a clay’s fine particles together into crumbs, and sodium pushes them apart. With too much sodium on the clay, the soil seals when it’s wet and sets hard when it dries. Gypsum puts calcium back in sodium’s place, and rain or irrigation washes the sodium down. An illustration, simplified.

Calcium draws a clay’s fine particles together into crumbs, and sodium pushes them apart. With too much sodium on the clay, the soil seals when it’s wet and sets hard when it dries. Gypsum puts calcium back in sodium’s place, and rain or irrigation washes the sodium down. An illustration, simplified.

How it shows

In the crop, the tips and edges of the older leaves scorch, and the plant shows the signs of a potassium shortage, even where the soil test says there’s plenty.

In the soil, a puddle or a jar of water goes milky over a clod, the surface crusts and seals, and the ground sets hard when it dries.

A tomato plant, every truss heavy with fruit, its old lower leaves yellow at the edges and the lowest scorched brown along them. — How sodium shows. In the crop, the older leaves scorch at their tips and edges, with the look of a potassium shortage, even where the soil test says there’s plenty. In the soil, water goes milky over a clod, and the surface crusts and seals, then sets hard as it dries. An illustration, simplified.

A glass of water with crumbs of red-brown soil on a wire mesh inside, a pale milky cloud spreading from them and hanging in the water. — How sodium shows. In the crop, the older leaves scorch at their tips and edges, with the look of a potassium shortage, even where the soil test says there’s plenty. In the soil, water goes milky over a clod, and the surface crusts and seals, then sets hard as it dries. An illustration, simplified.

Where it fits in a program

Test the water. How much sodium irrigation water carries, against its calcium and magnesium, shows how likely it is to make the soil sodic. Ask the lab for the sodium adsorption ratio (SAR) and the total salt (EC).
On a sodic soil, put on gypsum, with enough water to wash the sodium below the roots.
Keep potassium well ahead of sodium. The further ahead potassium stays, the better the plant copes with the salt.
A soil test shows how much of what the clay holds is sodium. A sap test shows what got into the plant.