Protein in bread wheat: why nitrogen alone is not enough

More nitrogen does not automatically mean more protein. In the leaf, the nitrogen has to become amino acids, and for that the plant needs partners: magnesium, boron, sulphur, molybdenum. If one is missing, the nitrogen stays unused. Here is where that happened in our own wheat.

Two stages from light to protein

The plant builds protein in two stages. First photosynthesis: it delivers sugar and with it the energy. For this it needs magnesium, iron, manganese, nitrogen and phosphorus. What the plant does not use itself, it sends into the soil as root exudates and so feeds the microbiology.

Then protein synthesis: sugar and nitrogen become amino acids, and the amino acids become protein. This is where boron, sulphur and molybdenum come in.

The mechanism is clearest for molybdenum. It is a cofactor of nitrate reductase, the enzyme that converts nitrate into ammonium, the first step towards an amino acid. Sulphur is part of the amino acids cysteine and methionine. Boron is involved in nitrogen metabolism and sugar transport.

What our leaf showed on 7 April

On 7 April 2025 we sampled young leaves of our winter wheat in Ruswil. I explain how the leaf is built up in the post on leaf sap analysis. Here we look at the values of the protein chain.

Sugar was at 5.3 %, with an optimum of 0.5 to 2.8 %. Nitrate was low at 44 ppm (optimum below 150), and total nitrogen was in range at 2796 ppm. So photosynthesis was running, and the nitrogen was being processed.

Below the optimum were three partners of the protein chain: magnesium at 128 ppm (optimum 250 to 430), sulphur at 164 ppm (360 to 590) and boron at 0.37 ppm (0.40 to 1.20). Molybdenum, at 0.07 ppm, was within the optimum but at the lower end (0.05 to 0.20).

In my opinion this is exactly the situation in which a crop looks good and still has room to improve on protein. We did not measure what this means in the grain. The leaf shows where the bottleneck probably lies, not the protein value at harvest.

What you can do in spring

Make up the missing nutrients through the leaf. This is faster than through the soil, which is often cold or dry in spring. Our guideline values for cereals:

• Magnesium: 5 l Carbo Eco Mg per hectare. On cold soils magnesium is hardly mobile; the leaf takes it up better than Epsom salt.
• Boron: 0.5 l Phytogreen Boron per hectare covers half of the seasonal requirement in cereals.
• Molybdenum: 0.25 l per hectare, once a year. More on the foliar fertiliser page.

We have no standard rate for sulphur. We clarify it individually after the analysis.

If you also need nitrogen, there is the N foliar fertiliser based on amino acids and peptides. We recommend it for weak tillering and at stem elongation. In field and lab tests it gave winter wheat around 11 % more yield, a higher protein content and better baking quality. Mix the spray following the six rules of foliar feeding, otherwise little of the fertiliser arrives in the leaf.

Where the limits are

Protein content also depends on variety, nitrogen rate and weather. Foliar spraying replaces none of that. It closes gaps you would otherwise not see.

In dry weather, postpone planned nitrogen applications by one to two weeks until rain returns. Otherwise you increase the drought stress. The leaf sap analyses in April 2025 showed us this.

The next step

Take leaf samples as soon as the crop has enough leaf mass, at tillering or the start of stem elongation. The leaf sap analysis then tells you which of the seven nutrients are missing on your farm. How this looks for magnesium is covered in the post on magnesium deficiency, for boron in the post on boron deficiency. Questions about your crop: come to the advisory service.

Frequently asked questions about protein in bread wheat

Why does more nitrogen not automatically bring more protein?

In the leaf, the nitrogen has to become amino acids, and for that the plant needs partners: magnesium, boron, sulphur and molybdenum. If one is missing, the nitrogen stays unused.

Which nutrients does the plant need for protein?

The plant builds protein in two stages. Photosynthesis needs magnesium, iron, manganese, nitrogen and phosphorus. Protein synthesis needs boron, sulphur and molybdenum. Molybdenum is a cofactor of nitrate reductase, and sulphur is part of the amino acids cysteine and methionine.

What did the leaf of our winter wheat show on 7 April 2025?

Sugar 5.3 %, nitrate 44 ppm and total nitrogen 2796 ppm were fine. Below the optimum were magnesium at 128 ppm (optimum 250 to 430), sulphur at 164 ppm (360 to 590) and boron at 0.37 ppm (0.40 to 1.20). We did not measure what this means in the grain.

Which foliar fertiliser rates do I use in cereals in spring?

Magnesium: 5 l Carbo Eco Mg per hectare. Boron: 0.5 l Phytogreen Boron per hectare, which covers half of the seasonal requirement in cereals. Molybdenum: 0.25 l per hectare, once a year. There is no standard rate for sulphur; we clarify it after the analysis.

What do I do with planned nitrogen applications in dry weather?

Postpone them by one to two weeks until rain returns. Otherwise you increase the drought stress. The leaf sap analyses in April 2025 showed us this.

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