Mulder's chart: what the nutrient-interaction wheel is good for

Search "nutrient antagonism cannabis" in any growing forum and a circular diagram turns up within a few replies: a dozen-odd element symbols around a rim, connected by arrows, captioned "Mulder's chart". Almost nobody posting it has read the source, because almost nobody can. The original is a 1953 conference paper, written in French, about fruit orchards, that has never been properly digitised.

Where the wheel actually comes from

The citation that circulates, when anyone bothers to give one, is "D. Mulder (1953)". Tracking the actual paper down is harder than it should be. The clearest trail runs through a translation record held by Brigham Young University: Mulder presented "Les éléments mineurs en culture fruitière" ("Minor elements in fruit growing") to the Convegno Nazionale di Frutticoltura, Italy's national fruit-growing convention, working out of the Institute for Phytopathological Research (I.P.O.) in Wageningen, the Netherlands ^[1]. That detail matters more than it looks. Grower folklore usually says the chart came from potato or arable-crop research; what the record actually shows is an orchard scientist writing about fruit trees, at a fruit-growers' convention, in French. The translation itself has since been withdrawn from the repository that held it, which is a fair summary of how thin the trail gets the moment you try to walk it: everyone quotes the chart, almost nobody has stood in front of the original.

What has survived is not the diagram itself but redrawings of redrawings. Line up five current versions and you'll find eleven elements in one, fifteen in another, and arrows drawn between different pairs in each. That's not a sign the science has moved on since 1953; it's a sign that most of what's labelled "Mulder's chart" today is a copy of somebody else's copy, not a citation of the paper.

What the wheel actually claims

Strip away the branding and the idea underneath is simple, and it predates Mulder by decades: push one nutrient up and you can pull another one down, or occasionally up, before either reaches a deficiency or toxicity threshold on its own. The chart labels each pairing antagonistic (heavy supply of one element reduces uptake of the other) or synergistic (one supports the other), covering the same nitrogen, phosphorus, potassium, calcium, magnesium, sulfur and micronutrient set laid out in the elements and mobility explainer. It is, in short, a map of competition and cooperation at the point where a root pulls ions out of the solution around it.

Pairs discussed in the article

1. Potassium – Magnesium: Holds outside soil (solid line)
2. Ammonium – Potassium: Holds outside soil (solid line)Ammonium (NH₄⁺) is one form of nitrogen; the line runs from the N node.
3. Potassium – Calcium: Soil / orchard evidence mainly (dashed line)
4. Phosphorus – Zinc: Soil-chemistry effect (dashed line)

Line style

• Solid line: Modern evidence supports this outside soil too
• Dashed line: Evidence is mainly from field soil

Other lines, not assessed here

• Thin grey line: Antagonism, as commonly reproduced
• Dotted grey line: Synergism, as commonly reproduced

Four pairs discussed in the article are highlighted; all other lines are as commonly reproduced online and are not individually assessed here.

Highlighted pairs and support levels per the article's sources [2] and [3]. Grey lines follow the pairs listed on widely circulated versions (e.g. Mosaic Crop Nutrition, NutriAg; accessed 2026-09-27). Diagram is an original redraw, not a reproduction of any single published chart.

Four relationships that still hold up, and why

Two of the four pairs cannabis growers ask about most hold up outside soil because the mechanism sits in the root membrane, not the medium; two weaken outside soil because part of what drives them depends on soil chemistry a coco slab or hydroponic reservoir doesn't have ^[2][3].

Potassium against calcium is the one with the most direct line back to Mulder's own subject: fruit-tree calcium disorders driven by a high potassium-to-calcium ratio are a textbook case in pomology, which is presumably why an orchard scientist noticed it in the first place. If the symptoms have already shown on your plants rather than in a lab report, magnesium vs potassium deficiency covers telling the two apart on the leaf.

Why nobody has tested the chart on cannabis

Cannabis-specific nutrient research is thin, but not empty, and what exists doesn't map neatly onto the wheel. Bernstein and colleagues ran a controlled trial supplementing phosphorus, and nitrogen-phosphorus-potassium together, against a standard commercial feed on a high-THC cultivar, then measured tissue mineral content by plant organ ^[4]. Phosphorus supplementation didn't leave calcium alone or push it down the way a simple antagonism story would predict: flower-tissue calcium more than doubled, from about 13 to 29 mg per gram dry weight ^[4] (the phosphorus dose itself is covered on its own terms in phosphorus and bloom boosters). That result doesn't overturn anything on the chart, phosphorus against calcium isn't one of the four pairs above, but it's a reminder that one line on a seventy-year-old orchard diagram can't anticipate what a specific cultivar does at a specific stage under a specific feed. Nobody has run the equivalent trial for potassium against magnesium in cannabis. Until somebody does, treat every number in this section as horticultural research borrowed from other crops, not as a cannabis finding.

Reading a tissue report against the wheel, instead of following it

The useful version of Mulder's chart for a commercial grower isn't a diagram to correct against on faith; it's a short list of ratios worth watching alongside the individual nutrient numbers a lab report already gives you: potassium to magnesium, potassium to calcium, and ammonium to potassium in the feed itself. None of the four pairs above comes with a validated target ratio for cannabis, so the useful move is tracking the trend across sampling rounds, not hitting a number, using the method in tissue and sap analysis.

Take an illustrative case, not a real facility's log. A 100 m² coco flower room runs potassium high through late flower, which is ordinary practice (see N:K ratios through the crop cycle). Quarterly tissue sampling on the most recently matured fan leaf comes back at potassium 2.6% dry weight and magnesium 0.28% dry weight. The nearest published sufficiency ranges, drawn from floral hemp rather than drug-type cultivars, put potassium at 1.8 to 2.7% and magnesium at 0.30 to 0.65% ^[5].

1. Check the sampling method before the numbers

   Confirm the same leaf position and the same days-since-feeding as last round. A different leaf or a different day invalidates the comparison before you've looked at a single figure.

2. Read magnesium on its own first

   0.28% sits just under the bottom of the 0.30 to 0.65% range: a miss, but a small one, not a collapse.

3. Read potassium alongside it

   2.6% sits at the top of its range, not over it. That rules out a blown-out feed and points instead at the ratio itself running potassium-heavy relative to magnesium, the pattern Mulder's chart flags as antagonistic.

4. Fix the ratio, not just the low number

   The commercial move is usually to raise magnesium supply (see cal-mag supplements) rather than cut potassium the crop needs at this stage, then re-sample next cycle to see whether the ratio, not just the single number, has moved.

   Warning Cutting potassium in late flower to chase one magnesium reading can cost more than the deficiency would.

What the chart cannot tell you

A line on a 1950s diagram carries no dose-response curve and no timing. It cannot tell you whether a magnesium reading 0.02 percentage points under range in week 3 of flower needs correcting this week or will resolve on its own by week 6, and it has nothing to say about how a growth-stage swing in demand, the same crop pulling far more potassium in late flower than in veg, changes what "antagonistic" even means at that point in the cycle. Closing that gap is what tissue and sap analysis and nutrient interactions and antagonisms are for: trend data gathered across a season, not a wiring diagram from an orchard convention seventy years ago.

The practical output of all this is a short watchlist, not a poster on the wall: track potassium against magnesium, potassium against calcium and ammonium against potassium in your feed and your tissue results the way you already track EC and pH, and let a name like Mulder's chart earn a ratio a second look, never stand in for a citation. None of this changes what you're allowed to grow or where, and fertiliser and lab-reporting rules differ by market: check the law section for what your jurisdiction expects from a licensed feeding programme before you change one.