Boron: why the deficiency persists even though you fertilise

Boron is not mobile in the plant. Once it has been used, it is not resupplied from older leaves – the plant needs a steady supply, otherwise the deficiency comes back, no matter how well you fertilised in autumn.

Why boron is missing even though you fertilised

Boron is water-soluble. That is exactly what makes it mobile in the soil and unreliable as a reserve: winter rain washes part of it out of the root zone before the plant becomes active again in spring.

The second reason lies in the root itself. In dry conditions, uptake through the root drops noticeably, because boron reaches the plant mainly with the flow of water. Dry spells in spring therefore hit exactly the phase in which cereals and oilseed rape need the most boron.

Water-solubleBoron is leached from the topsoil over the winter. That is why a field can be well supplied in autumn and still run short in spring – the reserve has long since moved on.

What boron does in the plant

Boron helps build cell walls and keeps them stable. If it is missing, the plant becomes stunted at the growing tips, because that is exactly where new cell walls have to form all the time.

Boron is indispensable for flowering and fruit set: it supports the growth of pollen tubes and fertilisation. Without enough boron, flowering stays uneven and fruit set weak – a pattern I see again and again in oilseed rape and pome fruit.

Then there is sugar transport. Boron regulates how sugar is moved from the leaves to the rest of the plant. When boron is short, the sugar stays in the leaf instead of reaching the places where it is needed for growth or grain filling. And because boron is also involved in nitrogen metabolism, a deficiency reaches all the way into protein synthesis.

The link to silicon

Without enough boron, silicon is poorly activated in the plant. The two are linked – and since silicon is abundant in almost every soil but poorly available to plants, a boron deficiency makes exactly this problem worse. I described what a low silicon value in leaf sap means in my last post on leaf sap analysis.

The analysis sheet from Ruswil

On one of our own fields in Ruswil, boron in the leaf sap was below the detection limit of 0.20 ppm – the optimum is 0.40 to 1.20.

Striking: on the same sheet, silicon is also below the optimum, 30.4 instead of 36.0 to 63.9 ppm. Exactly the pattern from the last section – where boron is missing, available silicon is usually missing too. The plant was by no means in poor shape overall: sugar, potassium and nitrogen were in the green zone. The deficiency hit precisely the two elements that need each other.

Without the analysis, nobody would have noticed. A plant does not show an incipient boron deficiency in its appearance first, but in its sap.

When and how much

The application rate depends on crop and timing; the basic pattern is the same everywhere: act early, before flowering begins.

In cereals, the application during stem elongation pays off especially when spring starts dry – exactly when the root supplies less anyway because of the drought. In oilseed rape, the autumn application matters most, because flower and pod formation are decided in autumn.

Borax or boron ethanolamine

Most boron fertilisers on the market are based on borax. Boron ethanolamine is more water-soluble and is taken up faster – an advantage precisely when the soil supplies little boron or conditions are difficult.

That is why, in Phytogreen-Bor, we use boron ethanolamine – water-soluble with 11% boron, and listed as an input by FiBL.

The next step

Do you want to know whether your own field is short of boron, or are you just guessing? A leaf sap analysis shows you in black and white, like the sheet from Ruswil above.

If boron is low, correct it in a targeted way with Phytogreen-Bor – and while you are at it, check silicon too. Questions about the rate for your crop: get in touch for advice.