Magnesium: why the deficiency comes in spring even though the soil is full

In cold soil, magnesium does not reach the plant – not even when there is plenty of it in the soil. This is not a supply problem but a transport problem, and it is not solved by putting more into the soil.

Why the soil does not deliver in spring

Magnesium reaches the root mainly with the soil water. The plant pulls water through, and the magnesium comes along with it. In February and March this flow is weak: evaporation is low, the root hardly grows, and the soil is cold.

The result is a deficiency in the midst of plenty. The soil reserve is fine, the soil analysis looks good, and the plant is still short. A soil analysis does not show you this gap – it measures the reserve. The leaf sap analysis measures what has actually arrived up top.

What a magnesium deficiency costs

Magnesium sits at the centre of the chlorophyll molecule. No magnesium, no photosynthesis – and therefore no energy for everything that follows.

You see it first in the colour: the crop stays pale instead of deep green, growth is more subdued, and at the end there is less yield. But these symptoms come late. By the time you see the paling, the plant has been running at part load for weeks.

The more expensive part lies underneath. A plant with full photosynthetic capacity has energy to spare and sends part of it into the soil as root exudates, where the soil life feeds on it. If the energy is missing, this share is the first to go. High protein levels in bread wheat depend on it too: nitrogen only becomes protein if magnesium, iron, manganese and phosphorus keep photosynthesis going.

The analysis sheet from February

On our own plot in Ruswil, we sampled winter wheat on 14 February, young leaf. Magnesium was at 237 ppm, the optimum is 250 to 430.

That is a slight deficiency, not a crisis – and I am saying so deliberately. The crop was not in bad shape: sugar, potassium, calcium, sulphur, phosphorus, manganese and zinc were all in the green zone.

Five values were below the optimum: magnesium, boron at 0.23 instead of 0.40 to 1.20 ppm, ammonium at 233 instead of 280 to 655, plus chloride and silicon. You can tick off chloride – a low value causes no problems in arable farming; it is the opposite case that causes trouble. Silicon depends on microbial activity in the soil and is a topic of its own.

That leaves magnesium and boron. I see exactly this pattern on our farm almost every year at the same time: not a crop on its knees, but two elements that are short while the rest is fine. The causes are different, though. Boron is leached over the winter – the reserve itself is gone. With magnesium, the reserve is still there; it just does not arrive.

Without an analysis I would not have noticed either of them in February. In February, winter wheat looks unremarkable anyway.

Why kieserite does little in spring

The obvious answer to a magnesium deficiency is kieserite on the soil. In my opinion, that is the wrong approach in early spring, for the reason given above: the problem is not the reserve but transport. More reserve does not solve a transport problem.

Via the leaf, you bypass the route through the cold soil completely. The nutrient goes where it is needed, without depending on root activity and soil temperature.

When and how much

The first application goes on as soon as the winter crops resume vegetative growth and have enough leaf mass. Before that, the spray hits too little leaf – and the gap between reserve and uptake is greatest in exactly this phase.

The sulphur content is not a side issue. Sulphur is leached in wet winters, and the plant needs it in spring for protein synthesis – with one pass you close two gaps that occur at the same time.

One pass is not enough, though. In our own April samples, magnesium was still below the optimum, in one case clearly lower than in February. The soil takes over later than you would think – plan the second application for stem elongation.

What goes into the first spray mix

Magnesium never goes on alone on our farm. Boron goes in too, because it is missing after the winter anyway – for winter cereals, the product information gives stem elongation, BBCH 30 to 37, as the timing. So it belongs in the mix at the second pass at the latest. Why boron is missing despite autumn fertilisation is explained in my last post.

Plus microbiology and a carbon source. For carbon, I put molasses into the spray mix – and only there. In compost tea it would be a mistake: the sugar is immediately available, and the bacteria multiply so fast that oxygen collapses between hour 8 and 12, especially above 20 °C. I took the difference apart in the post on biostimulants from your own farm.

With compost tea, it pays to check the thermometer. It belongs on the leaf as soon as the soil is warm and the plant is active. If you want to work on the microbiology earlier, do it via the soil – that is what compost extract is made for, not the foliar spray.

One more connection that matters in spring: a crop with surplus nitrate gets into drought stress much earlier than one that has enough magnesium, sulphur, manganese, iron and molybdenum. If you get the micronutrients right in February, you buy yourself a reserve for a dry May. A nitrate surplus also attracts the cereal leaf beetle.

The next step

The February deficiency is the easiest one to fix all year – if you know it is there. And you only know that once you measure.

Take a leaf sap analysis as soon as your winter crops start growing. If magnesium is low, correct it with Carbo-Eco-Mg and check boron while you are at it. Questions about the amount for your crop: get in touch for advice.