Trace elements: why a few grams do so much

A tonne of plant dry matter contains around 15 kg of nitrogen but only 0.1 g of molybdenum. Yet this tenth of a gram decides whether the nitrogen becomes protein or stays in the leaf as nitrate. The reason: trace elements work as catalysts. Here we explain what that means, without a chemistry degree.

What a catalyst is

Think of the plant as a factory. It builds sugar, protein and cell walls from light, water, air and nutrients. The work on the assembly lines is done by enzymes. These are tools made of protein, and each one carries out exactly one work step.

Many of these tools carry a single metal atom at their centre. That is the actual blade of the tool. And this is exactly where the trace elements sit: iron, manganese, zinc, molybdenum.

The key point: a catalyst is not used up by the work. A pair of scissors cuts a thousand times and the blade stays. It is the same with the metal atom in the enzyme. After every work step it is free again for the next.

How fast this can go is shown by a zinc enzyme, carbonic anhydrase. It converts up to a million molecules of carbon dioxide per second, always with the same zinc atom.

Little mass, big effect

This is why the plant needs nitrogen in kilograms and molybdenum in grams. Nitrogen is in every protein and is built in there permanently. Molybdenum only converts. For every million nitrogen atoms there is a single molybdenum atom.

In my opinion this is why trace elements are so often forgotten. What is measured in grams is quickly seen as minor. But if the tool is missing, the whole assembly line stops, no matter how much building material lies next to it.

Five trace elements, five workstations

Iron passes electrons along in photosynthesis and respiration, like an electrical conductor. The plant also needs it to form chlorophyll. If iron is missing, the youngest leaves turn yellow while the veins stay green. Very high iron values in the leaf sap, on the other hand, can point to waterlogging. To the iron foliar fertiliser.

Manganese sits at the heart of photosynthesis. A cluster of four manganese atoms splits water there, producing the oxygen we breathe. In dry weather the soil locks up manganese. That is why often the whole field shows manganese deficiency except the tramline: the compacted soil releases more manganese there. To the manganese foliar fertiliser.

Zinc is the busy one. It sits in over 200 enzymes and is needed, among other things, for the growth hormone auxin. If zinc is missing, leaves stay small and the distances between them short. To the zinc foliar fertiliser.

Boron is the exception, and we say so deliberately: boron is not a building block of an enzyme. Over 90 % of the boron sits in the cell wall, where it cross-links its building blocks. The plant also needs boron for sugar transport. Because boron hardly moves in the plant, it has to be supplied continuously. More in the post on boron deficiency.

Molybdenum shows most clearly how a catalyst works. That is why we look at it more closely.

Molybdenum: one atom against the nitrate backlog

The plant takes up most of its nitrogen as nitrate. It becomes usable only when the plant converts it: nitrate to nitrite, nitrite to ammonium, ammonium to amino acids and protein. The first step is done by nitrate reductase. Its heart is a molybdenum atom.

If molybdenum is missing, this first step stands still. The nitrogen piles up as nitrate in the leaf. This has several negative consequences for the plant:

• More susceptible to disease: Free, unprocessed nitrogen in the leaf sap is food for fungi and bacteria. At the same time the plant lacks the protein for stable cell walls and for its own defence. Crops with a lot of nitrate in the leaf are therefore attacked more often by mildew and rust.
• More susceptible to drought: A lot of free nitrogen drives soft, watery tissue with thin cell walls. Such plants lose water faster in heat and wilt earlier.
• Attracts sucking insects: Aphids, thrips and spider mites get their nitrogen from the plant sap. Where there is a lot of free nitrogen in the sap, they find more food and multiply faster.

A low nitrate value in the leaf sap is therefore a good sign: the nitrogen has arrived where it belongs, in the protein.

Our own data shows how clear this is. We analysed all leaf sap analyses from 2019 to 2026, across all crops, with duplicate samples removed: 1090 samples. We split them into two groups, depending on whether molybdenum was below or above 0.05 ppm. That is the lower limit of the optimum.

With molybdenum deficiency, nitrate had a median of 68 ppm; with enough molybdenum it was 32 ppm. That is more than twice as much. Median means: half of the samples lie below, half above. The difference is statistically clear; chance is practically ruled out.

One limitation belongs here: this is a comparison of many samples, not a fertiliser trial. It shows an association, and other things such as nitrogen fertilisation play a part. But nitrate reductase explains why the association is so clear.

Molybdenum has two special features. It is the only trace element that becomes more available at a higher soil pH; below pH 6.5 it becomes scarce. And legumes need it twice over: their nodule bacteria fix atmospheric nitrogen with nitrogenase, an enzyme that also contains molybdenum.

Where the limits are

A catalyst speeds up what is possible anyway. It does not replace a building material. If nitrogen is missing, even the best molybdenum does not help.

A little also does not mean: the more, the better. With trace elements there is often only a small step between deficiency and too much.

And soil analysis helps only to a limited extent with trace elements. Manganese, for example, oxidises in the sample until the analysis and then no longer shows what the plant can take up. What really arrives is shown by the leaf sap.

The next step

Measure before you fertilise. The leaf sap analysis shows you which trace element is missing. Then you give exactly that one through the leaf, in a small amount and chelated. You find all trace elements among the foliar fertilisers. How to mix the spray correctly is explained in the six rules of foliar feeding. Questions about your crop: come to the advisory service.

Frequently asked questions about trace elements

Why do trace elements work even though the plant only needs grams of them?

Trace elements work as catalysts in enzymes and are not used up in the process. A tonne of plant dry matter contains around 15 kg of nitrogen but only 0.1 g of molybdenum. Yet this tenth of a gram decides whether the nitrogen becomes protein.

What does molybdenum do in the plant?

Molybdenum is the heart of nitrate reductase, which performs the first step from nitrate to protein. If it is missing, the nitrogen piles up as nitrate in the leaf. In our 1090 leaf sap analyses from 2019 to 2026, nitrate had a median of 68 ppm with molybdenum deficiency and 32 ppm with enough molybdenum. This is a comparison of many samples, not a fertiliser trial.

What are the consequences of a nitrate backlog in the leaf?

The plant becomes more susceptible to disease, because free nitrogen in the sap is food for fungi and bacteria. It becomes more susceptible to drought, because soft, watery tissue loses water faster in heat. And it attracts sucking insects such as aphids, thrips and spider mites.

How much molybdenum foliar fertiliser do I need?

0.25 to 0.5 l molybdenum foliar fertiliser per hectare, once a year. Molybdenum is the only trace element that becomes more available at a higher soil pH. Below pH 6.5 it becomes scarce. Legumes need it twice over.

Where are the limits of trace element fertilisation?

A catalyst does not replace a building material: if nitrogen is missing, even the best molybdenum does not help. More is not better, there is often only a small step between deficiency and too much. And soil analysis helps only to a limited extent with trace elements.

Last updated: November 2026. Author: EDAPRO GmbH, Ruswil, Switzerland. Sources: EDAPRO trials, leaf sap analyses and field experience.