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Minerals · the smallest job

Molyb­denum

Grams, not kilograms Episode 5 3 min read

The short version

A crop needs less molybdenum than almost any other nutrient: grams, not kilograms. But without it the plant can’t use nitrate: molybdenum runs the first step of turning nitrate into protein. Legumes need it to fix nitrogen in their root nodules, too. And unlike most trace elements, it’s the acid soils that lock it up.

A legume’s root nodules make its nitrogen, and they need molybdenum to do it. Split one open: pink inside means it’s working.

What it does

A crop needs less molybdenum than almost any other nutrient: grams, not kilograms. But it does a job nothing else can. Molybdenum is the working part of the enzyme (one of the plant’s tiny tools) that takes the very first step with nitrate, starting it on its way to protein. Without it, nitrate piles up in the leaves unused, and the crop looks hungry for nitrogen while it’s full of it. It’s nitrate’s first station on nitrogen’s assembly line.

Nitrogen. Built into protein. Molybdenum. Molybdenum missing. Can’t go on.

  • A broad blue-green cabbage leaf lying flat, its pale midrib running from the stalk to the tip.

    With molybdenum

    a cabbage leaf

    molybdenum takes nitrate’s first step, and the nitrogen goes on to protein

  • The same cabbage leaf, pale yellowish-green all over, its edges scorched brown and curled up.

    Short of molybdenum

    the same leaf

    nitrate piles up unused: the crop looks hungry for nitrogen while it’s full of it

Molybdenum takes the very first step with nitrate, starting it on its way to protein. Without it, nitrate piles up in the leaves unused, and the crop looks hungry for nitrogen while it’s full of it. An illustration, simplified.

Legumes and their nodules

Legumes need it twice over. The bacteria in their root nodules, which take nitrogen from the air and hand it to the plant, can’t do it without molybdenum. Short of it, a clover, bean or lucerne crop can’t feed itself, however good its nodules look from the outside.

Nitrogen in the soil’s air. Nitrogen the plant can use. Molybdenum. Stopped.

  • A young bean plant’s roots, dug up and washed: a pale taproot and fine side roots, with small, round, pale brown nodules, two clusters near the top and a few single ones lower down.

    A legume’s roots

    clover, beans, lucerne

    dig up a few plants carefully, and find the nodules

  • A bean root nodule cut in half, close up: pink to red inside.

    Working

    one nodule, cut open, magnified

    pink to red inside: it’s fixing nitrogen for the plant

  • The same nodule cut in half: white to grey-green inside.

    Not working

    another, cut open

    white, grey or green inside on a well-grown plant: it isn’t fixing, and molybdenum is one reason to check

Cut a few nodules open: the working ones are pink inside. Short of molybdenum, a legume can’t fix its own nitrogen, however good its nodules look from the outside. An illustration, simplified.

That gives you a check you can do in the paddock. Dig up a few plants carefully and cut some nodules open. A healthy, working nodule is pink to red inside. Check around flowering, once the plants are well grown, because young nodules are white inside until they start working. White, grey or green inside, on a well-grown plant, means it isn’t fixing. Late in seed fill, nodules age and turn green, and that’s normal. Molybdenum is one possible reason; the wrong bacteria, waterlogging and acid soil are others.

Acid soils lock it up

Molybdenum is the odd one out. Most trace elements get harder for the plant to get as the pH rises. Molybdenum gets harder to get as the pH falls. On acid soils, liming often frees up enough on its own.

How a shortage shows

Plants look pale and hungry for nitrogen, often with scorched or cupped leaf edges. In cauliflower and broccoli it causes “whiptail”: narrow, twisted, ribbon-like leaves. In legumes it looks like plain nitrogen hunger, because the nodules have stopped doing their job.

Molybdenum. Held in acid soil. Lime.

  • A young cauliflower, cut away below the ground: its outer leaves cupped and scorched at the edges, its young leaves narrow, twisted strips along their midribs, and no curd.

    Acid soil

    a young cauliflower

    the soil holds its molybdenum back, and the new leaves come out as whips

  • The same young cauliflower, healthy: broad blue-green leaves cupping a small white curd, and a stout taproot in red-brown soil.

    Limed

    the same cauliflower

    lime often frees enough molybdenum, and the new leaves come out broad

Most trace minerals get harder to get as the pH rises. Molybdenum is the odd one out: acid soil holds it back, and lime often frees enough on its own. Short of it, a cauliflower’s new leaves come out narrow and twisted: whiptail. An illustration, simplified.

When it matters most, crop by crop

The stages where molybdenum 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.

  • Beans about 70 days

    Germination · Flowering

  • Onions about 180 days

    Bulb filling and maturation

  • Apples a year

    Tight cluster to first pink

Where it fits in a program

  • On acid soils, check the pH first.
  • For legumes, molybdenum usually goes on the seed or in the fertiliser, and a foliar spray works too. The amounts are tiny, so keep to the label.
  • Don’t overdo it on pasture. Too much molybdenum in the feed can cause copper problems in cattle and sheep.
  • A leaf test and a look inside the nodules beat a guess.