Mul­der’s chart

• Holds back: too much of one, and the crop runs short of the other.
• Needs more: more of one, and the plant needs more of the other.
• How sure: heavy is well proven, fine shows mostly at the extremes, and faint has little behind it.

In 1953 a Dutch scientist, D. Mulder, working with fruit trees, set out how the plant’s minerals work on each other. The ring chart that carries his name joins any two that do. More than seventy years on, his chart is still on fertiliser sheets and classroom walls.

It answers a question every grower runs into sooner or later: why is my crop short of something that’s right there in the soil? Often, something else is in the way. The chart is a map of what might be.

Look at it whole and it’s a tangle: eleven minerals, and more than two dozen lines. Read it a few lines at a time, and it starts to make sense. That’s what the rest of this page does, one step at a time.

• Potassium holds magnesium back: they share the root’s doors, so pile on potassium and less magnesium gets in.
• More nitrogen, and the plant needs more magnesium: more leaf, and every bit of green has magnesium at its heart.
• Molybdenum and nitrogen work as a pair: the plant can’t build nitrate into protein without a little molybdenum.

A solid line means too much of one holds the other back. The arrow points at the one that loses out. Where it works both ways there’s no arrow: magnesium holds potassium back too, if far less.

A dashed line means more of one, and the plant needs more of the other.

The two work differently, but they end in the same place: the crop runs short of one mineral because of what you did with another.

• In the soil: lime holds iron back, and manganese, zinc, boron and phosphorus with it.
• At the root’s doors: potassium and magnesium hold each other back, crowding in through the same doors.
• Inside the plant: heavy nitrogen grows more leaf than the copper can go round.
• In stock: molybdenum, with sulfur, locks copper up in a cow’s or a sheep’s gut, so they need more copper.

A line can play out in four places: in the soil, at the root’s doors, inside the plant, or in stock. Where it happens tells you what to do about it.

In the soil. Most of calcium’s lines are really about lime: it raises the pH, and as the pH climbs, the others get locked up in the soil. A crop can stand in plenty of iron and still go yellow. It isn’t only lime, either. “If you’re going to put nitrates in, you’re going to get hydroxyls out,” Steve says, and the pH right around the root climbs with them.

At the root’s doors. Minerals get into a root through doorways in its cells, and some share them. Potassium, calcium and magnesium share some of theirs, so flood the root with one and the other two get in less. Steve sees it in potatoes, where a big load of fertiliser goes in under the seed: “You’ve got antagonist relationships happening,” he says, “like potassium out-competing everything else.”

Inside the plant. Some lines play out after the minerals are in. Feed a cereal plenty of nitrogen on ground short of copper, and it holds what copper it has in the old leaves: the heads can come up empty.

In stock. One line isn’t about the crop at all. Molybdenum’s line to copper is about stock grazing pasture high in molybdenum: they can go short of copper.

• Lime holds iron back.
• Lime holds manganese back.
• Lime holds zinc back.
• Heavy liming leaves boron short.
• Lime and phosphorus hold each other back.
• Potassium and magnesium crowd each other.
• Heavy phosphorus leaves zinc short, where zinc is low.
• Heavy nitrogen leaves copper short, where copper is low.
• Molybdenum and nitrogen work as a pair.
• In stock, molybdenum locks copper up.

Here’s what the chart doesn’t tell you: most of its lines only bite at the edges. In 2017, a review went through 94 studies that tested pairs of minerals on a crop’s yield. Of the 117 results, 43 were pairs that helped each other: they did more together than the two did apart. Only 17 held each other back. Another 35 made no difference to each other. Of the rest, most showed no clear effect on yield, and in seven the yield fell.

And the lines that did bite mostly bit where one mineral was flooded, or the other was already short. Most were among the metals and the minerals that share the root’s doors. Put right what’s short, don’t flood the root with any one thing, and most of the chart stops mattering much.

That’s Steve’s order, in the chart’s terms. Build the foundation first, so nothing’s short while the cells are being made. Then bring in the potassium when the crop can use it. “Earn the right to use potassium,” he says. That way it fills the house, instead of crowding the door.

So on this page, the weight of a line says how sure we can be of it: heavy for the lines seen again and again in the field, fine for the ones that show mostly at the extremes, and faint for the ones with little behind them since 1953.

• Calcium and potassium hold each other back: potassium crowds calcium out at the root.
• Lime holds boron back: heavy liming can leave boron short.
• Potassium holds boron back, on the old chart, though trials since have found little to show it.

“Calcium, boron and silicon, which I call the three amigos,” Steve says. They’re the concrete in his foundation: calcium glues the cells together, boron ties their walls, and silicon sets them hard. Out on his own is the lone wolf, Mr Potassium.

On the chart, the three amigos aren’t drawn as a team. Silicon isn’t on it at all. Calcium’s line to boron is a warning about the soil, mostly through the pH. Yet in the cell wall the two are partners, building the same wall.

The line that runs into this set is potassium’s. Put it in the concrete mix while the cells are dividing, and less calcium reaches the young fruit just when it needs it most. That’s Steve’s question to growers: “Why don’t you throw some potassium in that mix and see how the foundation works?”

In short: calcium and boron build the cell walls together, though lime can leave boron short. Potassium crowds calcium at the root, so it waits until the foundation’s poured.

• Potassium and magnesium hold each other back, potassium far the harder.
• Calcium and potassium hold each other back, at the root’s doors.
• Calcium holds magnesium back, mostly where calcium is heavy, as after liming.

Potassium sizes the crop, calcium keeps it firm, and magnesium keeps the leaves green and working. All three come in through some of the same doors into the root, so they’re always jostling. The chart draws a line between every pair.

The usual way it goes wrong is heavy potassium, for size. Magnesium is often first to show it, yellow between the veins of the old leaves. In fruit, it’s calcium: the more potassium and magnesium an apple carries for its calcium, the worse its bitter pit.

So the three are balanced by their order more than their amounts: calcium in while the cells divide, potassium when the crop is sizing, and never a flood of any one at once. Or, as Steve puts it: “Build the foundations, build the house, fill it with potassium. Now you’ve got yield and quality.”

In short: potassium, calcium and magnesium share doors into the root. Heavy potassium shows first as magnesium short in the old leaves, and in fruit as bitter pit. Balance them by their order.

• Phosphorus holds zinc back: on a soil low in zinc, heavy phosphorus brings on little leaf.
• Phosphorus and iron hold each other back, in the soil and inside the plant.
• Phosphorus holds copper back, partly by sending away the root fungi that help bring it in.
• Zinc holds iron back where zinc runs high: they share a door into the root.
• Copper holds iron back where copper has built up, as in old orchards and vineyards.

Phosphorus goes in at planting, close to the seed, to get the roots away. The program builder puts it first in every crop that’s sown or planted.

But more lines go out from phosphorus than from almost anything else on the chart, and three run to trace minerals the young plant needs just as much: zinc, for the hormone that stretches new cells; iron, for the green; and copper.

The answer isn’t less phosphorus at planting. It’s enough, close to the seed, rather than a flood, and knowing whether your soil is short of zinc before you put it there. The root fungi matter here too: they help bring in phosphorus and zinc, and a heavy hand with phosphorus sends them away.

In short: phosphorus gets the roots away, but a flood of it holds back zinc, iron and copper, the trace minerals the young plant needs. Enough, close to the seed, and know your zinc.

• Molybdenum and nitrogen work as a pair: molybdenum runs nitrate’s first step.
• More nitrogen, and the plant needs more magnesium, to keep the extra leaf green.
• More nitrogen, and the plant needs more sulfur: the two go into protein together. Mulder left it off, so it’s added here.
• Sulfur holds molybdenum back: sulphate and molybdate come in through the same door. Added here too.

“Nitrogen is a liability until it becomes a protein.” That’s Steve. The plant builds it in on an assembly line, and the dashed lines out of nitrogen are the parts the line needs.

Push the nitrogen without them, and it doesn’t become protein. It sits loose in the leaves, as the soft, sappy growth that pests go for.

In one Australian canola trial on ground short of sulfur, nitrogen on its own did next to nothing. With sulfur, each kilogram of nitrogen grew about eight times as much grain.

There’s one hold-back in this set too, and it’s also an addition. The plant can hardly tell sulphate and molybdate apart, so plenty of sulphate means less molybdenum gets in, where it’s low to start with.

In short: molybdenum, sulfur and magnesium are parts nitrogen’s assembly line needs. Push nitrogen without them, and it sits loose instead of becoming protein.

• Nitrogen holds copper back: it grows more leaf than the copper can go round.
• Nitrogen holds boron back: lush growth can run ahead of the boron.
• Nitrogen holds potassium back: heavy ammonium can crowd potassium at the root.
• Potassium holds boron back, on the old chart, with little behind it.

When a crop’s pushed hard with nitrogen for a big yield, look at where its solid lines run. The strongest runs to copper. In cereals on ground short of copper, the pollen fails, and the heads come up empty.

In one wheat trial on ground short of copper, nitrogen on its own cut the yield. With copper as well, the same nitrogen gave a large rise in yield.

Boron and potassium are on this set’s lines too, more faintly. Boron is what lets a flower become a fruit.

So before you push nitrogen for a big crop, check the copper. In cereals it goes on before the boot, while the pollen forms: one of the crop’s critical stages.

In short: push nitrogen on ground short of copper, and the heads can come up empty. Check the copper before you push.

• Manganese holds iron back: on waterlogged or very acid ground, where manganese runs high, the leaves yellow as if short of iron.
• Copper holds iron back where copper has built up in the soil.
• Zinc holds iron back where zinc runs high, and the young leaves pale.
• Copper holds manganese back, on the old chart, with little behind it.
• More potassium, and the plant needs more manganese, on the old chart, with little behind it.

The leaf’s engine runs on trace metals. Manganese splits water at the very start of photosynthesis, iron is needed to build the chlorophyll and works in the engine, and copper and zinc each have a part in it. The number one thing for photosynthetic efficiency, Steve says, is “making sure your manganese levels are optimal in your plant.”

On the chart, these four share a set of doors into the root, and they jostle. The arrows from manganese, copper and zinc all point at iron: too much of any of them can hold it back. And lime, back in the soil, holds three of the four back at once.

One more line runs in, from potassium. Steve has his own name for manganese, “a potassium thermostat.” Ask him what he means by it at the live Q&A.

In short: manganese, iron, zinc and copper run the leaf’s engine and share its doors. On the chart, too much manganese, copper or zinc holds iron back. Lime holds three of them back.

• Potassium holds magnesium back, at the root’s doors.
• Calcium holds magnesium back, mostly where calcium is heavy.
• More nitrogen, and the plant needs more magnesium, for the extra leaf.
• More phosphorus, and the plant needs more magnesium, and back: the two work together wherever the plant spends energy.

Say a test comes back with magnesium low. Before you reach for more magnesium, look along its lines. Two solid lines run into it, from potassium and calcium: has either been flooded? Two dashed lines run in, from nitrogen and phosphorus: has growth been pushed hard? If one of those is the cause, more magnesium is only half the answer.

Then ask Steve’s question: “How is it shifting?” One test is a snapshot. A run of sap tests across the season is the moving picture, and that’s where you see a line at work: one mineral climbing while another slides, after something you changed. Time the tests to the critical stages, so there’s time to act.

And keep it in proportion. Mulder drew his chart from what he saw in fruit trees, and plenty of its lines have little behind them since. Chasing every line on a soil test is what Steve warns about: “You’re just balancing the chemistry. That’s not health.” The chart tells you what might be happening. The plant tells you whether it is. The Soil health page has more.