Short answer. Mulder's chart is a circular diagram, attributed to the Dutch soil scientist D. Mulder in the early 1950s, showing which nutrients suppress (antagonise) or help (synergise) each other [7]. It is a good reminder that excess of one nutrient can cause shortage of another. But it is a teaching sketch, not a calculator. A review of 117 published interactions found synergy more often than antagonism; the antagonisms that reliably matter are among the cations (K, Ca, Mg, NH₄, Na), phosphorus with zinc and iron, and iron with manganese [1][6].
What's happening
"Antagonism" covers several different mechanisms that the chart lumps together:
- Competition at root uptake. Ions with similar charge compete for transporters and for the root's charge balance. K⁺, NH₄⁺ and Na⁺ suppress Ca²⁺ and Mg²⁺ uptake; Ca²⁺ suppresses Mg²⁺ [6]. In cannabis, higher K supply was linked to lower Ca and Mg uptake [3], and high ammonium reduced cation uptake and yield [4].
- Precipitation in the root zone or tank. Ca with phosphate or sulfate; Fe with phosphate at high pH. That is chemistry, not physiology, and pH controls most of it (see pH and availability).
- Interference inside the plant. High P is associated with reduced Zn effectiveness and Fe translocation in many crops [6]; Fe and Mn compete; Cu and Zn compete.
- Dilution. Adding a growth-limiting nutrient makes the plant bigger, which lowers the concentration of others in tissue without any real antagonism. Shiponi and Bernstein saw P supply reshape the whole cannabis ionome, partly this way [5].
Rietra et al. reviewed 94 papers describing 117 interactions across all macro- and micronutrients: 43 were synergistic, 17 antagonistic, 35 zero-interaction, the rest non-significant or negative. Antagonistic effects on yield were found most often for divalent cations [1]. The authors also note that the number of studied interactions is low, which makes firm conclusions difficult [1].
The interactions worth knowing
| Excess of | Can suppress | Mechanism | Strength of evidence |
|---|---|---|---|
| K | Mg, Ca | Cation competition | Established [6]; observed in cannabis [3] |
| NH₄⁺ | K, Ca, Mg | Cation competition, root damage | Established [6]; cannabis [4] |
| Ca | Mg, (K) | Cation competition | Established [6] |
| Na | K, Ca | Cation competition, membrane displacement | Established [6] |
| P | Zn, Fe, (Cu) | Precipitation and in-plant interference | Supported in crops [6]; untested in cannabis |
| Fe | Mn (and the reverse) | Uptake competition | Supported [6] |
| Cu, Zn | Fe | Competition | Supported [6] |
| N (adequate) | Most others improve | Synergy via growth | Established [1] |
| High bicarbonate/pH | Fe, Mn, Zn, Cu, B, P | Solubility, not antagonism | Established; see pH and availability |
How to use this
- Keep ratios sensible. A standard Hoagland-type solution runs roughly K 235, Ca 200, Mg 49 mg/L (K:Ca:Mg ≈ 1.2:1:0.25). Big departures from that, especially K far above Ca, are where antagonism bites.
- Fix pH before any "lockout" theory. Most so-called lockout is pH or root-zone failure.
- Don't stack boosters. A PK booster plus a bloom base plus Cal-Mag can push K, P and Ca far beyond need.
- Confirm with numbers. Runoff EC, a lab solution test, or a leaf tissue test beat reading a wheel.
Common mistakes
- Treating Mulder's chart as quantitative. It has no doses.
- Calling every deficiency "lockout" and flushing with plain water, which removes everything. A root-zone flush is for real salt build-up (runoff EC far above input), done with a light solution in soilless media (see the flush debate).
- Ignoring dilution when reading tissue tests.
Evidence
- Cation antagonism (K/NH₄/Na vs Ca/Mg): Established in plant nutrition [6], Supported in cannabis [3][4].
- Synergism being more common than antagonism in yield studies: Supported (one large review [1]).
- Mulder's chart as a diagnostic tool: Grower practice; origin detailsbeing reviewed
- Fageria 2001 citation detailsbeing reviewed
