Borgmann Aquaponics Hydroponics
Nutrient Solution: Chelates, Ion Ratios and Practice
Why chelates are necessary, which ones work at which pH, how critical ion ratios are derived – and what the Long Ashton / Hewitt (1966) solution demonstrates as an example.
Article 1 shows that Fe, Mn, Zn and Cu have their availability optimum in the acidic range (pH 5.0–6.5), while Ca²⁺, Mg²⁺ and MoO₄²⁻ are more available near neutral. The compromise targeted in hydroponics at pH 5.8–6.2 means for iron: at this pH it is predominantly present as sparingly soluble Fe³⁺ and would be practically unavailable to plants without complexation. Chelates solve this problem – but not every chelate at every pH.
Chelates – Function, Types and pH Stability
A chelate is an organic ligand that binds a metal ion (here: Fe³⁺ or Fe²⁺) simultaneously via multiple coordination sites, keeping it in solution that would otherwise lead to precipitation. The strength of the bond is described by the conditional stability constant K', which is pH-dependent: as pH decreases, H⁺ increasingly competes with Fe³⁺ for the binding sites of the ligand.
The thermodynamic stability constants (log K, at 25°C, I = 0.1 mol/L) are tabulated in the NIST reference database (Martell & Smith 1974–1989) [2]:
log K (Fe³⁺-EDTA) = 25.1
Effective pH range: 4.0 – 6.5
Most economical chelate form. Above pH 6.5, Fe³⁺ begins to be released from the complex via hydrolysis (Fe(OH)₃) before it can be taken up. The standard choice for systems at pH 5.8–6.2.
log K (Fe³⁺-DTPA) = 28.0
Effective pH range: 4.0 – 7.0
Higher stability constant than EDTA → Fe³⁺ remains complexed up to pH 7.0. Relevant for aquaponic systems (pH 6.8–7.2) or calcareous irrigation water.
log K (Fe³⁺-EDDHA) ≈ 35
Effective pH range: 4.0 – 9.0
Strongest available Fe chelate form. The ortho-ortho isomer is more effective than ortho-para. More expensive, but necessary for alkaline systems or where pH control is difficult. Identifiable by its reddish-brown solution.
Chelates (EDTA forms) are also available for Mn, Zn and Cu, but are generally less critical: these ions are still sufficiently soluble as free ions at pH 5.0–6.5. At higher pH or in cases of confirmed deficiency despite adequate solution concentrations, Mn-EDTA and Zn-EDTA are advisable.
Critical Ion Ratios
The antagonism mechanisms from Article 1 can be translated into concrete target ratios. The following table lists the most important ones – each with its mechanistic justification and the guideline value for hydroponic systems (molar concentrations, since transporter kinetics are molar). These values are not absolute limits, but flagging thresholds above which antagonism effects become measurably apparent.
| Ratio | Guideline (molar) | Warning threshold | Mechanism (→ Article 1) |
|---|---|---|---|
| Ca²⁺ : Mg²⁺ | 2.0 – 4.0 : 1 | < 1.5 or > 5.0 | NSCC competition; Ca²⁺ excess inhibits Mg²⁺ at the binding site, Mg²⁺ excess reciprocally. |
| K⁺ : Ca²⁺ | < 1.5 : 1 | > 2.0 | K⁺ excess inhibits Ca²⁺ at NSCC; the Viets effect (Ca²⁺ cell wall stabilisation) is weakened. |
| NO₃⁻ : NH₄⁺ | ≥ 3 : 1 | NH₄⁺ > 25% of total N | NH₄⁺ excess → H⁺ extrusion → rhizosphere pH drops → Ca²⁺/Mg²⁺ antagonism as a secondary effect. [4] |
| Fe²⁺ : Mn²⁺ | 2.0 – 5.0 : 1 | < 1.5 | IRT1 competition; at a low Fe:Mn ratio, Mn²⁺ accumulates to toxic levels. [3] |
| P (H₂PO₄⁻) : Fe | < 100 : 1 | > 200 : 1 | Precipitation of FePO₄ (Ksp ≈ 10⁻²²) in solution; significant above this ratio, pH-dependent. |
| P (H₂PO₄⁻) : Zn²⁺ | < 200 : 1 | > 400 : 1 | Precipitation of Zn₃(PO₄)₂; simultaneously IRT1 suppression at P excess. |
| SO₄²⁻ : MoO₄²⁻ | < 1000 : 1 | > 2000 : 1 | SULTR1;2 competition: SO₄²⁻ almost completely displaces MoO₄²⁻. |
Example: Long Ashton / Hewitt (1966) – Ratio Analysis
The following table shows the Hewitt solution in g/L and mmol/L and checks the critical ratios from the previous section. The source data are in g element/L (per litre of water).
| Element | g/L | Molar mass (g/mol) | mmol/L | Note |
|---|---|---|---|---|
| Ca | 0.160 | 40.08 | 3.99 | |
| Mg | 0.036 | 24.31 | 1.48 | |
| K | 0.156 | 39.10 | 3.99 | |
| N | 0.168 | 14.01 | 12.0 | Total N; NO₃⁻/NH₄⁺ split not specified |
| P | 0.041 | 30.97 | 1.32 | as H₂PO₄⁻ |
| S | 0.048 | 32.06 | 1.50 | as SO₄²⁻ |
| Fe | 0.0028 | 55.85 | 0.050 | at upper limit; chelate form is decisive |
| B | 0.00054 | 10.81 | 0.050 | |
| Mn | 0.000064 | 54.94 | 0.0012 | |
| Cu | 0.000065 | 63.55 | 0.0010 | |
| Mo | 0.000048 | 95.96 | 0.0005 | |
| Cl | 0.00015 | 35.45 | 0.0042 | Trace amount |
Ratio check for Hewitt:
| Ratio | Calculated (molar) | Guideline | Assessment |
|---|---|---|---|
| Ca : Mg | 3.99 : 1.48 = 2.70 : 1 | 2.0 – 4.0 | ✓ within range |
| K : Ca | 3.99 : 3.99 = 1.00 : 1 | < 1.5 | ✓ within range |
| Fe : Mn | 0.050 : 0.0012 = 41.7 : 1 | 2.0 – 5.0 | ⚠ Fe significantly in excess |
| P : Fe | 1.32 : 0.050 = 26.4 : 1 | < 100 | ✓ no precipitation risk |
| P : Zn | Zn not specified | < 200 | – no data |
| SO₄²⁻ : MoO₄²⁻ | 1.50 : 0.0005 = 3000 : 1 | < 1000 | ⚠ SULTR competition critical |
Interactive Tools
Tool A – Ion Ratio Checker
Enter ion concentrations in mmol/L or g/L. Automatic calculation of all critical ratios and flagging according to the thresholds in this table. This tool is intended for the Fertiliser / Nutrient Calculator but can also be used independently.
Go to Tool A...
Tool B – Chelate Stability Window
Enter the pH value and instantly see which chelate form (EDTA / DTPA / EDDHA) remains effective at that pH, including the conditional stability constant. This version covers iron (Fe) only. Other chelates available on request.
Go to Tool B...
Primary Sources
- Hoagland, D.R. & Arnon, D.I. (1950). The water-culture method for growing plants without soil. California Agricultural Experiment Station Circular 347, 2nd ed. University of California, Berkeley. (Public domain) Local copy
- Martell, A.E. & Smith, R.M. (1974–1989). Critical Stability Constants, Vols. 1–6. Plenum Press, New York. [NIST reference database for log-K values; accessible online via nist.gov/srd/nist46]
- Vert, G. et al. (2002). IRT1, an Arabidopsis transporter essential for iron uptake. The Plant Journal 31(4):529–537. DOI: 10.1046/j.1365-313X.2002.01381.x
- Mulder, E.G. (1953). Inorganic nitrogen compounds and soil fertility. Plant and Soil 4(4):368–415. DOI: 10.1007/BF01373584
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