Leaf sap analysis: when to do it and what it shows you

A leaf sap analysis shows you what the plant has actually taken up – not what is in the soil. That is the whole difference. In the plant sap you see a deficiency about two weeks earlier than in dry matter, and you can still react in the same crop.

Why plant sap and not dry matter

The classic leaf analysis dries the material and measures everything that is in it – including what has long been built in and is immobile. Leaf sap measures the dissolved fraction. That is the part the plant is working with right now.

That is where the head start comes from. What still looks unremarkable in dry matter is already short in the sap.

The second difference is nitrogen. In leaf sap it is split into nitrate, ammonium and total nitrogen. So you see not only how much nitrogen is there, but whether the plant is incorporating it. When protein synthesis is running, nitrate and ammonium stay low in the leaf.

What a soil analysis does not show

A soil analysis shows the reserve. Whether that reserve actually gets into the plant is another matter – and that is exactly where the two methods part ways.

With manganese it is particularly clear: by the time the soil sample reaches the lab, the manganese has oxidised. The value then no longer reflects what was available to the plant. That is why I give no weight to manganese from a soil analysis.

Same pattern with calcium. The cation exchange capacity often shows plenty of calcium, yet the plant is still short of it. If you lime then, you are fertilising past the cause.

Together, the two analyses give a picture. The soil analysis shows you what is in the soil, the leaf sap analysis what of it is really taken up. Only with both can you design fertilisation efficiently – while promoting plant health and humus build-up at the same time. We can help you with that.

When to measure

As soon as the winter crops start their vegetative growth, you can take the first sample. On our farm, that is the end of February, depending on the year. In oilseed rape, October is the second good time, because you can still influence autumn development then.

I take the second sample two to three weeks after the slurry application. Then I can see whether nitrogen incorporation is working or whether nitrate is left standing in the leaf.

And when it turns dry, the analysis is an early warning system: crops under drought stress reduce sap flow before you can see anything in the field.

How to sample correctly

Sampling determines how meaningful the result is, and timing is critical. If you get sloppy here, you get clean numbers from the wrong crop.

Sample before nine in the morning. You need around 120 grams of leaf material, from fully developed, healthy leaves of the same age, spread across the whole field. Leave out the field edge, and with trees the inside of the crown and the shady side.

Keep at least a week between the last fungicide or foliar feed and sampling, otherwise you are measuring your own spray. Break off the stalks, pat the leaves dry, label the bag itself – no slip of paper in the bag, it goes soggy.

In spring, young leaves are enough. Later in the season, it is better to collect both and have young and old leaves analysed separately – only this comparison shows whether the plant is moving nutrients from old to young. But even a sample of the young leaves allows many conclusions.

How to read the sheet

The order is always the same, and it matters more than it looks. If you start with the trace elements, you correct symptoms instead of causes.

1. Are there surpluses? Too much nitrogen pushes sugar down and displaces magnesium.
2. Is photosynthesis running? The sugar value tells you.
3. Are pH and EC balanced? Together, the two show the plant’s basic condition: the EC how many nutrients in total are dissolved in the sap, the pH whether the plant has enough energy to keep its metabolism in balance.
4. Only then the deficiencies – and pay attention to mobility, young leaf versus old leaf.

There are two values I always read first. Nitrate should be at zero; what counts is total nitrogen. If there is nitrate in the leaf, what is usually missing is molybdenum – without molybdenum, nitrate is not built into peptides.

If you grow forage, you have a second reason to look at this value. Nitrate and ammonium in the leaf are non-protein nitrogen, and the cow cannot convert it cleanly into protein in the rumen – it ends up as urea in the milk and puts a strain on the liver. A high urea content in milk is therefore often not a ration problem but a nitrate problem from the crop. If boron and molybdenum are missing, protein synthesis in the plant slows down and non-protein nitrogen rises.

Then there is crop protection. Sucking insects such as aphids, thrips and flea beetles are after exactly this unincorporated nitrogen: dissolved nitrate and ammonium are food they can use directly, without having to break it down first. A plant that has built its nitrogen into proteins is much less interesting to them – so with targeted foliar feeding you make the crop unattractive.

And aluminium should also be at zero. If it is elevated, together with iron and manganese, the sheet is telling you something about the soil: too acidic, compacted or waterlogged.

With silicon, I read the soil more than the nutrient. Almost every soil has plenty of silicon, but not in a form plants can use – only the microbiology makes it available. A low silicon value in leaf sap is therefore the best indicator of microbial activity in the soil that the analysis sheet provides.

An example from the polytunnel

Aubergines in a polytunnel, May 2024. The analysis sheet shows sodium at 150 ppm; the optimum is 4 to 8. Chloride is at 1658 instead of 550 to 1260.

What is interesting is what happens alongside. Magnesium is at 210 instead of 540 to 900, phosphorus at 153 instead of 470 to 780. The sodium blocks magnesium uptake, the chloride inhibits phosphorus and probably nitrogen too. The crop does not have a nutrient problem – it has a salt problem.

Salts like these often come from spring water. The simple answer is rainwater, the expensive one a reverse osmosis unit. To rid the soil of the salts in the short term, I work with fulvic acid: it precipitates the salts and makes the nutrients available again.

Without the analysis, you would have fertilised here. And nothing would have arrived.

The next step

Start with two samples, not one. One field that worries you and one that is doing fine – the comparison tells you more than a single value without a reference.

If there is nitrate in the leaf and sugar is low, molybdenum and magnesium are the first candidates. And if you want the nutrients to get into the plant in the first place, work on soil biology at the same time – compost tea mainly improves the sugar and silicon values in our analyses.

If you want someone at your side for the interpretation who has seen these numbers many times: get in touch for advice. In 2021, we were the first provider of leaf sap analyses in Switzerland.