NPK Ratio in Cannabis: Basics, Myths, and Evidence-Based Recommendations

Introduction

The ratio of nitrogen (N), phosphorus (P), and potassium (K) is one of the central control variables in cannabis cultivation. The number of commercially available fertiliser systems is enormous, yet their formulations differ considerably – and they do not always reflect what the plant actually requires. This article summarises the current state of science and the insights of experienced practitioners, providing a differentiated assessment based on plant tissue analyses, controlled growing trials, and foundational agronomic research.

1. What Plant Tissue Analyses Reveal

A methodologically sound starting point for nutrient recommendations is the analysis of dried plant material. For Cannabis sativa (hemp), standardised reference values from scientific studies are available:

Source: Szarvas, 1999, 2003 – Plant nutrient element status for hemp

From this table the actual N:P:K ratio in plant tissue can be derived: at midpoint values of N ≈ 5.5 %, P ≈ 0.55 %, and K ≈ 2.85 %, the ratio works out to roughly 10:1:5 (5.5÷0.55 : 1 : 2.85÷0.55) on a tissue mass basis. In nutrient solutions for hydroponic cultivation, where nutrients are directly and fully available, this ratio is typically scaled down to 3:1:2 – since the determining factor is not the absolute concentration in the tissue but the relative availability and uptake efficiency.

For a general plant comparison: typical dry-matter contents in crop plants are N ≈ 4 %, P ≈ 0.5 %, K ≈ 4 %, Ca ≈ 1 %, Mg ≈ 0.25 % – phosphorus is therefore, as in cannabis, eight times lower in concentration than nitrogen or potassium (cf. Essential Fertilizer Nutrients, Floriculture Research).

2. The Classic 3:1:2 Ratio – Origin and Validity

The 3:1:2 ratio (N:P:K) has served as a baseline orientation in hydroponic practice for decades. According to the practitioner fatman7574 (rollitup.org), hydroponic trial series on greenhouse foliage plants in the 1950s and 1960s had already shown similar optimal points – a claim that is not backed by primary literature but is consistent with modern tissue analysis data. The nutrient manufacturer Advanced Nutrients (AN) has also moved closer to this ratio with its newer “Sensi” formulations, after earlier products had deviated significantly.

Independent tissue tests with various cannabis genotypes, conducted by practitioners such as the forum member known as fatman7574 on rollitup.org, confirm: the 3:1:2 ratio represents one of the most reliable formulations for typical growing conditions (normal temperatures, moderate humidity). For high-performance cultivation with elevated CO₂, high transpiration, and lower humidity (35–40 % relative humidity in veg, 25–30 % in flower), the potassium ratio can be raised to a 3:1:4 ratio.

Scientific Confirmation (RSM Study, Frontiers in Plant Science 2021)

An optimisation study using response surface analysis (RSA) in a deep-water-culture system (DWC) systematically investigated the NPK requirements of Cannabis sativa during flowering. The results showed that flower yield responded quadratically to nitrogen and phosphorus, with optimal concentrations of N = 194 mg/L and P = 59 mg/L in the nutrient solution. This corresponds to an N:P ratio of approximately 3.3:1 – consistent with the classic 3:1:2 approach (Bevan et al., 2021, Frontiers in Plant Science, DOI: 10.3389/fpls.2021.764103).

3. The Phosphorus Myth: Why High P Rates During Flowering Are Not Necessary

One of the most widespread misconceptions in cannabis cultivation is the belief that phosphorus must be massively increased during flowering to maximise “bud size” and resin production. This assumption has its origin in older soil-based trials, where phosphate is immobilised by lime and other soil constituents and is therefore poorly available to the plant – overdosing was a compensation strategy.

What the research shows

Cockson et al. (2020) tested cannabis at six different P concentrations (3.75 to 30 ppm) in a soilless system. The result: fresh weight of the flowers did not increase significantly beyond 11.25–30 ppm P. What was observed was classic luxury uptake – the plant takes up more P when it is offered without that translating into additional yield. Higher P rates merely increased the P concentration in the leaf tissue without any agronomic benefit (Impact of Phosphorus on Cannabis sativa Reproduction, Cannabinoids, and Terpenes, MDPI Applied Sciences, 10(21), 2020).

A further study (PMC 2022) found that elevated P in the root zone significantly increased the P concentration in the leachate – primarily generating environmental pollution, not plant growth.

Important: Cannabis preferentially accumulates P during the first half of the growing season and translocates it internally as needed (Cannabis Business Times, 2020). A concentrated P application in late flowering is therefore largely unnecessary in well-buffered systems.

Key takeaway: In closed soilless systems and in hydroponics, high phosphorus rates are not scientifically justifiable. A moderate, constant P level (approximately 11–30 ppm in the nutrient solution) over the entire cycle is more efficient than a boost strategy.

4. The Role of Calcium and Magnesium

The Dutch Formula (widely used in Dutch commercial cultivation) largely does not address Ca and Mg directly – yet these elements are critical for cannabis.

Ca:N ratio

The Ca:N ratio in the nutrient solution recommended by experienced practitioners (fatman7574, rollitup.org) is 0.8–1.0. For comparison: in the plant tissue itself, Ca:N according to Szarvas (1999) is approximately 0.4–0.6:1 – the nutrient solution recommendation is therefore higher, since not all offered calcium is taken up. This target has no direct peer-reviewed basis for cannabis but is consistent with the general logic of high Ca availability in solution.

Ca:Mg ratio

For cannabis – as a starting point until cannabis-specific research data become available – a K:Ca:Mg ratio of 4:2:1 is recommended in order to avoid antagonistic interactions. This recommendation is extrapolated from poinsettia research (Cannabis Business Times, 2020) and represents a pragmatic reference value pending the availability of cannabis-specific data. In efficient systems with low TDS, calcium additionally acts as a regulator of overall uptake: a higher Ca share dampens the uptake of other ions, a lower Ca share increases it – a useful control mechanism that requires expertise in practice.

Signs of Ca/Mg problems from low-N formulations

That growers increasingly report being able to work with tap water without supplementation is not a quality marker – it primarily shows that many commercial fertilisers contain so little nitrogen and calcium that the tap water’s Ca actually improves the formula.

5. Target Ratios: All Primary Nutrients at a Glance

Beyond the pure N:P:K ratio, the relationships between all primary nutrients are critical – particularly to avoid nutrient antagonisms. The following table summarises the target values as derived from tissue analyses and agronomic practice for cannabis:

Source: practitioner analysis based on GH Flora formulations and tissue standards (fatman7574, rollitup.org). Note: the original notation for N:P in the source table was “1:3-8” – this entry was inverted/ambiguously worded and has been correctly rendered here as “N:P = 3–8:1” (N dominates), since all other data (Szarvas, Bevan et al.) confirm an N/P ratio of 3–10:1.

Analysis of the Lucas Formula vs. 1:1 Blend (Micro:Bloom)

A detailed comparison of both GH approaches with the target ratios shows where the formulas sit and where they deviate:

Conclusion: Both blends produce usable but non-optimal ratios. The Lucas formula has an excessively high K value and a poor P:S ratio (too little sulphur relative to phosphorus). The 1:1 blend improves the K ratio but has a magnesium shortfall. Neither approach delivers what a plant-oriented formula would provide.

Note on source interpretation: In the original source (fatman7574) the introductory comment reads “Phosphorus is the grossly high ppm” – meaning that P in absolute terms (ppm) is far above the requirement. This refers in particular to the Lucas formula and is confirmed by the P:S ratio (5:1 vs. target 0.6:1) and by the results of Cockson et al. (2020).

6. Mixing Your Own Nutrients – the Commercial Model

A pragmatic conclusion from commercial greenhouse cultivation that neatly captures the weaknesses of the hobby fertiliser industry:

“The simplest approach is to mix your own nutrients or have someone mix them for you, rather than trying to fix ready-made fertilisers. Formulas should be developed on the basis of your own tap water – unless that water is truly bad. With bad tap water, you remove everything and add some soluble calcium. Then you develop a formula for your own needs that is based on water that already contains some soluble calcium. That gives the flexibility needed for correct ratios. This is how it is done throughout commercial greenhouse cultivation. The way it is done in the cannabis hobby serves the convenience of the manufacturers – simpler and more profitable for them.”

– fatman7574 (rollitup.org forum)

Why tap water as the base matters

Tap water in most regions already contains dissolved calcium (often 30–120 ppm Ca²⁺, depending on hardness). Anyone who develops a nutrient formula without knowing the water analysis risks either calcium excess (with very hard water + Ca-rich fertilisers) or Ca deficiency (with reverse osmosis water without supplementation). The correct approach – as standard practice in commercial horticulture:

1. Water analysis – have one done (Ca²⁺, Mg²⁺, HCO₃⁻, EC)
2. Base water as the starting point, not to be ignored
3. Supplement the formula with what is missing – without overdosing what is already present
4. Check ratios against the target table above

Ready-made manufacturers cannot offer this individualised approach – hence they deliver one-size-fits-all formulas that work at the lowest common denominator but rarely optimise.

7. Phase-Specific Recommendations

Vegetative phase

The Dutch Formula recommends a 3-1-2 approach for veg and a 2-1-2 approach for early bloom.

Flowering phase

Source: Dutch Formula protocol (screenshot source, post)

On potassium in flowering: Potassium activates enzymes of carbohydrate metabolism, regulates ion transport across membranes, and controls the stomata (transpiration, turgor). Elevated K levels in the reproductive phase are therefore physiologically sensible – particularly at lower light levels or elevated CO₂.

Late flower / flush

In the final phase, nutrients are reduced or flushed out to avoid salt accumulation in the tissue and to optimise the flavour of the end product. More modern approaches experiment with gradual nutrient reduction rather than an abrupt flush.

8. Popular Fertiliser Systems Compared

Jack’s Nutrients A&B (321 Formula)

Jack’s Nutrients is a water-soluble three-component system. Part A (5-12-26, stated as N-P₂O₅-K₂O) is the primary macronutrient carrier (nitrogen, phosphorus, potassium) plus trace elements. Part B (calcium nitrate 15-0-0) supplies additional nitrogen and, above all, calcium. The third component is magnesium sulphate (Epsom salt).

The designation “321” gives the mixing ratios in grams (3 g Part A + 2 g Part B + 1 g Epsom) – not the NPK ratio. The actual resulting elemental N:P:K ratio of the solution works out to approximately 3:1:4 (N ≈ 45, P elemental ≈ 15.7, K elemental ≈ 64.7 relative units). The system is suitable for hydroponics, coco, and other soilless systems, and can often be used without modification throughout the entire cycle.

General Hydroponics Flora Series (Lucas Formula)

The so-called Lucas formula (2 parts FloraBloom : 1 part FloraMicro) yields a calculated N:P₂O₅:K₂O ratio of approximately 5:10:9 (normalised to K₂O=1: approx. 0.56:1.11:1). The figure of 0.42:0.83:1.0 cited in the fatman7574 post is a differing approximation that cannot be exactly reproduced from the standard GH product formulations and should be classified as a forum figure. The fundamental finding remains the same: the formula delivers relatively high phosphorus and potassium at relatively low nitrogen – far from a plant-requirement-oriented balance. It gained popularity because it is easy to use, not because it is optimal.

GH FloraNova Grow

FloraNova Grow has a ratio of approximately 1.75:1:2.5 and thus approaches the 3:1:2 ratio more closely than the classic Micro/Grow combination (3.5:0.5:3.5). The addition of humic and fulvic acids from leonardite coal (lignite, dissolved with potassium hydroxide and phosphoric acid) is marketed by the manufacturer as “organic” – although it is an industrial extraction process.

9. Hydroponics vs. Soil

In soil, phosphate molecules are strongly immobilised through:

• Adsorption onto soil particles (dominant at low P concentrations)
• Chemical binding to calcium (at high pH) or iron/aluminium (at low pH)

The historical solution was overdosing. In closed soilless systems (hydroponics, coco, perlite) this immobilisation is largely eliminated. The plant receives what is offered. Therefore: in hydroponics the optimal P concentration in the nutrient solution is considerably lower than the figures on soil-based fertiliser recommendations suggest.

The optimal P concentration in the nutrient solution for cannabis, according to the current state of research, lies at 11–60 mg/L depending on growth phase and system – not in the three-digit ranges implied by some “bloom boosters.”

10. Cannabinoids, Potency, and Nutrient Supply

An important finding from the research (Saloner & Bernstein, 2021; Frontiers 2021): there is an inverse relationship between flower yield and cannabinoid content. Plants supplied with 160 mg/L N showed approximately 20–30 % lower THCA and CBDA concentrations than plants at 30 mg/L N – but achieved roughly double the dry flower yield. The optimal strategy therefore depends on the growing objective:

• Maximum total yield → higher N supply
• Maximum cannabinoid concentration → more moderate N supply with controlled stress moments

Cannabinoid levels themselves, according to the RSA study (Bevan et al., 2021), did not respond significantly to NPK concentrations within the tested range – pointing to other limiting factors (light, genetics, temperature).

11. High VPD, Low EC: a Strategy for Maximum Nutrient Efficiency

The mechanism: turgor and transpiration

Plant cells are filled with water and are under constant positive internal pressure – turgor (turgor pressure). This arises through osmosis: the cell fluid contains more dissolved substances than the surrounding solution. The surroundings are thus hypotonic relative to the cell interior – water flows through semipermeable membranes into the cell until the cell interior presses against the cell wall under pressure. This state is called turgescence; the plant is upright and rigid. If the plant loses its turgor pressure, it wilts.

Terms defined clearly: Hypotonic describes the external solution (lower solute concentration than the cell interior → water flows in). The state of the turgid cell itself is called turgescent, not “hypotonic.” Turgor (turgor pressure, hydrostatic pressure) and osmotic pressure are related but distinct quantities: osmotic pressure describes the driving force for water uptake, turgor pressure the actual counter-pressure built up by the cell wall.

The bulk of the energy for maintaining turgescence comes from transpiration: the evaporation of water through the stomata, combined with the cohesive and capillary forces of water in the vascular system (xylem). Additionally, root cells actively pump water into the plant – an energy-dependent process (ATP). The water inside the plant already carries nutrients. The transpired water moves this nutrient-laden water through the entire plant. For the actual nutrient transport only minimal additional water volumes are therefore needed – the transpired water is the pump.

Core statement (fatman7574): “Water is mainly needed to maintain plant cooling through transpiration and to transport nutrients. The water that is transpired moves the nutrient water and its nutrients through the entire plant.”

Humidity and temperature: what actually happens

Low humidity with an adequate water supply and a healthy root system does not lead to stress reactions – it simply increases transpiration. The stomata close only when the water supply at the roots is no longer sufficient to maintain turgor. In a hydroponic system with a well-developed root system and continuous water availability, this is not a problem.

The importance of root hairs: Root hairs are tubular extensions of the root epidermis, typically 10–17 µm in diameter and 0.08–1.5 mm in length (Wikipedia: Root hair; ScienceDirect). They increase the effective absorption surface considerably and can account for up to 80 % of phosphorus uptake (Soil Ecology Wiki, 2021). Even though the absolute area per millimetre of root lies in the square-millimetre range, the totality of a well-developed root system with dense root hairs is highly efficient – and in hydroponics with continuous water availability rarely a limiting factor for water supply.

Note: The figure cited in the original source (fatman7574) of “over 0.33 m² surface area per 5 mm of root” cannot be reproduced from published data on root hair density and length and probably represents a considerably exaggerated estimate. The core statement – that root hairs provide a substantial surface area and that a well-maintained root system supports high transpiration and uptake rates – remains biologically correct.

Forum misconception: The oft-cited warning against low humidity originates from soil cultivation, where water availability is limited by the substrate’s water-holding capacity. In hydroponics this limitation is absent, as long as irrigation is frequent enough. A good grow has no problems with low humidity – a poor grow does, because the root system is inadequate, not because of the humidity itself.

High VPD + low EC: a confirmed strategy

Vapor Pressure Deficit (VPD) describes the difference between the water vapour pressure inside the leaf (approximately 100 % RH) and that of the ambient air. The higher the VPD, the stronger the transpiration. This means that at high VPD the absolute amount of nutrient transported through the plant also increases – even at a lower EC concentration in the solution.

FulCrop Sciences (2024) confirms: at high VPD, plants can take up nutrients more efficiently because the faster water flow increases mass flow of nutrients to the root. This permits running lower EC values without risking deficiency symptoms.

This principle explains the approach of fatman7574: operating at 88–92 °F, 35–40 % RH (veg) or 25–30 % RH (flower) with CO₂ supplementation and low ppm values – because high transpiration + an efficient root system deliver the required absolute nutrient quantity despite a lower concentration. Realistic increase in water uptake: 50–100 % over standard conditions – provided temperatures lie in the 27–33 °C range and RH is below 30–40 %.

Scientific verification (Frontiers in Plant Science, 2025)

Bugbee et al. (2025, Colorado State University) explicitly investigated the transpiration-based mass-flow model for cannabis nutrient uptake during the vegetative phase. Result: nutrient uptake can be directly derived from water uptake (transpiration) – a biophysically grounded fertigation model that can replace static EC recipes (Frontiers in Plant Science, DOI: 10.3389/fpls.2025.1753553).

Limits of the strategy

Excessively high VPD (> 1.6–2.0 kPa) triggers stress responses: the stomata close actively to limit water loss – transpiration collapses, CO₂ uptake and nutrient transport stop. The optimal window for cannabis is:

• Veg: 0.8–1.2 kPa
• Flower: 0.8–1.6 kPa (depending on light intensity and temperature)
• Lower limit: < 0.4 kPa → mould risk, nutrient transport stagnates
• Upper limit: > 2.0 kPa → stomatal closure, stress cascade

12. Carbonate Chemistry, pH Drift, and Nutrient Lockout

How pH drift occurs: ion balance at the root

Plant roots take up ions and release counter-ions to maintain ionic balance. The direction of the drift depends on which ions dominate:

Case 1 – Nitrate-dominated uptake → pH rises: When the plant takes up more negatively charged ions (primarily NO₃⁻) than positively charged ions, it releases carbonate ions (CO₃²⁻) to compensate. These rapidly convert to bicarbonate (HCO₃⁻) in water. The bicarbonate then binds positively charged cations (Ca²⁺, Mg²⁺, K⁺) – the result is fewer free H⁺ protons relative to OH⁻ ions: the pH rises. At the same time, the bound calcium and magnesium ions are no longer available to the plant.

Case 2 – Ammonium-dominated uptake → pH drops: When the plant takes up many positively charged ions (NH₄⁺), it releases H⁺ protons. These react with the negatively charged phosphate ions in the solution, forming phosphoric acid (H₃PO₄). The excess of H⁺ protons shifts the equilibrium: the pH drops.

(fatman7574, rollitup.org)

This explains why well-formulated solutions with a mixed nitrogen source (calcium nitrate + some ammonium nitrate, as in Jack’s 321) maintain a more stable pH: nitrate pushes the pH upward, ammonium pushes it downward – the two partially cancel out. Pure nitrate solutions require daily pH reduction, pure ammonium solutions require daily pH increase.

Bicarbonate as buffer and lockout mechanism

At pH values below 8.3, carbonates are present almost entirely as bicarbonate (HCO₃⁻), carrying a charge of 1−. Bicarbonate can form soluble complexes with divalent cations (Ca²⁺, Mg²⁺) – e.g. Ca(HCO₃)₂ (dissolved, neutral) – and at rising pH can also precipitate sparingly soluble carbonates (CaCO₃ at pH > 8.3). The preferential binding of Ca²⁺ over Mg²⁺ in water has several causes:

• Higher concentration: Ca²⁺ is more abundant than Mg²⁺ in typical nutrient solutions → statistically more frequent contact with HCO₃⁻
• Smaller hydration shell: Mg²⁺, despite having a smaller ionic radius, has a considerably larger hydration shell than Ca²⁺ (stronger hydration), which kinetically slows its reaction with anions
• K⁺ carries only a single positive charge (1+) and does not form stable complexes with HCO₃⁻ – its contribution to complex formation is small

Correction note: In the original source (fatman7574) the formulation states that bicarbonate “forms pairs with a charge of 2−.” This is chemically incorrect: HCO₃⁻ carries the charge 1−. Two bicarbonate ions together have 2−, but they do not occur as stable ion pairs. Similarly, the statement “larger ionic radius = stronger charge density” is physically reversed (charge density = charge/volume, therefore: larger radius → lower charge density). The fundamental point – that Ca²⁺ is preferentially bound by bicarbonate and thereby becomes less available to the plant – is nevertheless factually correct and well documented.

This means: high bicarbonate concentration (= high carbonate hardness of the tap water or pH drift upward) blocks primarily calcium and magnesium, not potassium. Some manufacturers respond by adding a lot of potassium to their formulas – so that the less vulnerable K⁺ substitutes for calcium and magnesium when these are bound. This is, however, not an optimal approach.

“Every time more positive ions are taken up by the roots (e.g. nitrate), a carbonate ion is released that attaches itself to positively charged ion(s) – i.e. bound + ions and a higher pH, since the equilibrium shifts to fewer free H⁺ protons.” – fatman7574

pH lockout: mechanism at a glance

The optimal pH window for cannabis (5.8–6.3) lies precisely where bicarbonate is present but has not yet accumulated to lockout-relevant levels – and where at the same time phosphate is well soluble and micronutrients (Fe, Mn, Zn) are still sufficiently available (Emerald Harvest, 2025).

Consequences for practice

• Keep TDS/EC low → lower buffering capacity of the solution → carbonate chemistry is easier to control
• Water analysis before formula development → carbonate hardness (KH, HCO₃⁻) determines how much pH reduction is structurally needed
• pH drift is an indicator: if the pH rises by 0.3–0.5 units daily, the plants are actively taking up nitrate → system is working. If the pH stagnates, uptake may be faltering.
• Soft water (RO) has minimal buffering → pH instability, but lower lockout risk; adding some calcium nitrate stabilises

13. pH-Up and pH-Down: the Nutrient Contribution of Corrective Agents

pH corrections are not neutral interventions

A frequently underestimated variable: every pH correction adds nutrients to the reservoir. Depending on the agent used, significant amounts of additional ions can enter the solution daily – shifting the carefully calculated element ratios.

pH-Up: potassium sources dominate

The common pH-Up agents at a glance:

Source: Science in Hydroponics, 2022

Practical consequence: Anyone dosing pH-Up daily inevitably raises the K content of the solution. fatman7574 points out: since many commercial formulas require daily potassium hydroxide addition anyway (pH drift upward due to plant uptake of anions), the potassium can be factored into the formula from the start – rather than adding it unplanned via pH-Up.

Note: Potassium carbonate is preferable to KOH when the pH in recirculating systems tends to rise: CO₃²⁻ forms a buffer system and stabilises the pH over a longer period (CANNA Gardening).

pH-Down: phosphoric acid as the main problem source

By far the most common pH-Down option is phosphoric acid (H₃PO₄). It is inexpensive, food-grade, stable – but it is a nutrient that raises the P content of the solution:

Source: Science in Hydroponics, 2020; CANNA Gardening

Critical consequence for the nutrient balance: Anyone dosing phosphoric acid daily is systematically adding P – without accounting for it in the fertilisation plan. In formulas that are already P-heavy (like Lucas), this can lead to a chronically excessive P:N ratio. CANNA therefore recommends: nitric acid in the vegetative phase (adds N, does not consume K), phosphoric acid in flowering (adds P, which is more welcome in flowering) – a phase-specific choice that actively manages the nutrient balance.

Nitrogen source influences pH drift

An additional mechanism: the type of nitrogen source determines the pH tendency in the solution.

• Nitrate-N (NO₃⁻): plant uptake causes the pH to rise (anions are taken up, hydroxide remains behind) → typical cause of upward pH drift in hydroponics → pH-Down needed
• Ammonium-N (NH₄⁺): plant uptake causes the pH to drop → downward pH drift → pH-Up needed

This explains why well-formulated blends (such as Jack’s 321, which combines calcium nitrate + some ammonium nitrate) maintain a more stable pH: the mixed nitrogen source balances the drift.

14. Case Study: Air-Atomised Aeroponics vs. Medium-Pressure System

Experimental setup

The following comparison is based on a controlled grow by fatman7574 (rollitup.org), in which 144 plants in SOG (Sea of Green) were cultivated with identical clones, the same environmental parameters (same thermostat, humidistat, and CO₂ ppm settings), and the same genetics (Indica/Afghani) in two different systems:

The thermostats, humidistats, and CO₂ controllers were left at identical setpoints. Only the nutrient delivery system was changed. CO₂ consumption was determined gravimetrically using a scale beneath the tank.

Projection to a 6-week flowering phase (darker resin)

By extending to 6 weeks of flowering (darker, more potent resin) and adjusting for continuous rotation (swapping half the plants every 3 weeks):

Interpretation in the context of this article

These data illustrate several principles discussed in the article in practice:

Efficient nutrient delivery reduces total consumption: The air-atomised system delivers nutrients directly and uniformly to the root surface – with 50 % less water and nutrient solution the same yield is achieved. This is the practical proof of the “low EC + high transpiration system” principle.

Shorter flowering time at the same yield: 5 instead of 8 weeks of flowering at the same dry weight – enabled by optimised nutrient availability (no lockout, correct ratios) and more efficient gas exchange.

Less light, more efficiency: 48.61 vs. 76.38 W/ft² – plant height in the SOG is lower (shorter veg due to optimal root supply), hence less light demand per area. The g/W ratio improves radically.

CO₂ savings through efficiency: At the same CO₂ concentration, 30 % less CO₂ is consumed – because the system has less air volume and less leakage, and the plants use CO₂ more efficiently through optimised gas exchange.

Methodological strength of these data: The comparison was conducted with identical clones, the same genetics, the same room, and the same climate settings. The only changed parameter was the nutrient delivery system. This makes the effects directly attributable to the system, even though it is not a controlled trial in the academic sense.

15. Water Alkalinity, Water Hardness, and EC: Interactions and Interdependencies

One of the most frequently underestimated factors in hydroponics is the source water. Anyone who does not know what is already in the water is optimising blind – regardless of how precisely the nutrient formula has been calculated.

Alkalinity, water hardness, and EC – three different quantities

Alkalinity refers to the concentration of bicarbonate (at pH > 8.2 also carbonate) in natural water. Bicarbonate is alkaline and raises the pH. High alkalinity (> 75 ppm) causes the pH in nutrient solutions to rise. Water hardness captures exclusively calcium, magnesium, and iron – those elements that carry a 2+ charge and contribute to alkalinity.

Phantom EC: when the EC reading is misleading

A critical practical problem: alkalinity and hardness raise the EC of the source water without necessarily contributing useful plant nutrients. The EC value alone says nothing about the quality of the water – sometimes hard water with a high EC is still usable, while other water at the same EC is harmful because it contains too many undesirable elements.

The source water contributes its own EC share to the nutrient solution. The EC contribution of the irrigation water must be added to the fertiliser EC to determine the correct target EC.

Practical example:

• Tap water EC: 0.40 mS/cm (Ca bicarbonate, Mg bicarbonate, Na, Cl)
• Target EC of the nutrient solution: 1.60 mS/cm
• Actually required fertiliser EC: only 1.20 mS/cm

Anyone who doses the fertiliser to 1.60 mS/cm without accounting for the source water is systematically overfeeding.

Is Ca and Mg from hard water available?

A widespread myth: calcium and magnesium from hard tap water are not available to plants. This is incorrect. Calcium and magnesium from hard water are fully available to plants once the pH is lowered to the hydroponic target value. It is a common error to consider these ions unavailable.

This has direct consequences for formula development with hard water: the Ca and Mg share in the fertiliser must be reduced accordingly. With very hard water (> 180 ppm), Ca can be in excess and antagonistically inhibit other cations.

Alkalinity and pH buffering

Alkalinity measures the ability of the water to resist changes in pH. The higher the alkalinity, the harder it is to raise or lower the pH.

• Low KH (< 30 ppm): pH unstable, jumps quickly → frequent adjustment needed, but easily controllable
• Medium KH (30–100 ppm): good buffering → stable pH; ideal conditions for most hydro systems
• High KH (> 150 ppm): persistent upward pH drift; large amounts of acid needed; high alkalinity leads to rapid pH swings and can cause nutrient lockout and corresponding deficiency symptoms

Acid treatment of hard water

Acids for pH reduction chemically neutralise the bicarbonates in the water:

Nitric acid is the preferred choice in commercial greenhouse operations for hard water – the reaction converts non-plant-available Ca bicarbonate into fully available calcium nitrate, one of the most valuable nutrients of all. (Science in Hydroponics, 2022)

Water analysis checklist

The first step for successful hydroponics is a water analysis: pH, EC, and alkalinity should be tested. Poor source water can cause nutrient toxicity or deficiency symptoms from the start or over the course of cultivation.

The four EC sources in every hydro solution

1. Source water EC (lime, Na, Cl, Mg, Ca from the mains) – often ignored
2. Fertiliser EC (nutrient salts – the only fully controllable component)
3. pH corrective EC (KOH, H₃PO₄ etc. add their own ions, → section 13)
4. Accumulation EC (salt build-up through transpiration in closed systems)

Only those who know and account for all four sources can formulate a nutrient solution precisely.

Sources: OSU Extension (2017); CANNA Gardening (2024); Emerald Harvest (2024); Science in Hydroponics (2022)

16. Conclusion: Practical Recommendations

Key takeaways:

1. P is overrated. Cannabis in soilless systems does not need high phosphorus rates. 11–30 ppm is sufficient for optimal flower set.
2. 3:1:2 is a proven starting point in the nutrient solution. For high-performance cultivation (high CO₂, low humidity) experienced growers tend toward 3:1:4. Jack’s 321 delivers an elemental ratio of approximately 3:1:4 as well – the “321” refers to the mixing ratios in grams, not the NPK ratio.
3. Ca and Mg are critical. The target Ca:N ratio in the nutrient solution is 0.8–1.0 (practical value); the Ca:Mg ratio 2:1; K:Ca:Mg ideally 4:2:1 (extrapolated from poinsettia research).
4. Keep all element ratios in view. The target table (Ca:Mg, K:Mg, N:P, P:S, Mg:N) matters more than individual ppm values.
5. Include tap water. Anyone who develops a formula without a water analysis is optimising blind.
6. Mixing your own is superior. Commercial one-size-fits-all formulas are optimised for manufacturer convenience, not for the plant’s optimum.
7. The Dutch Formula is a pragmatic orientation but does not address Ca/Mg directly – supplementation is therefore usually required.
8. Lucas has a poor P:S ratio (too little sulphur, too much phosphorus) and excessive potassium; the 1:1 blend neglects magnesium.

Sources

• Szarvas, I. (1999, 2003) – Plant nutrient element status for hemp, Journal of the International Hemp Association
• Bevan, L. et al. (2021) – Optimisation of N, P, and K for Cannabis sativa in the Flowering Stage, Frontiers in Plant Science
• Bevan, L. et al. (2024) – Mineral nutrition for Cannabis sativa in the vegetative stage, Frontiers in Plant Science
• Cockson, P. et al. (2020) – Impact of Phosphorus on Cannabis sativa Reproduction, Cannabinoids, and Terpenes, Applied Sciences 10(21)
• Saloner, A. & Bernstein, N. (2021) – Response of Medical Cannabis to Nitrogen Supply, Frontiers in Plant Science
• Whipker, B.E. et al. (2021) – How to Optimize Phosphorus Fertility for Cannabis, Cannabis Business Times
• Cannabis Business Times (2020) – Balancing the Nutrient Equation
• Sustainable Cannabis Nutrition (PMC, 2022) – Elevated root-zone phosphorus increases leachate P without yield improvement
• fatman7574 (rollitup.org) – Lucas Formula Recipe from Scratch “Really”
• fatman7574 (rollitup.org) – Methodical, scientific approach to nutrients and nutrient formulations discussion
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• Emerald Harvest (2024) – Irrigation Water Quality in Hydroponics
• Whipker, B.E. et al. (NCSU, 2019) – Optimizing Electrical Conductivity (EC) in Cannabis Cultivation, Cannabis Business Times
• GardenMyths (2024) – Using Conductivity (EC) and pH Measurements to Control Hydroponic Solutions
• Wikipedia – Root hair
• Wikipedia – Turgor pressure
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• Science in Hydroponics (2022) – A guide to different pH up options in hydroponics
• Science in Hydroponics (2020) – A guide to different pH down options in hydroponics
• CANNA Gardening – The Ideal pH level
• Oklahoma State University Extension (2017) – Electrical Conductivity and pH Guide for Hydroponics
• Emerald Harvest (2025) – Optimizing pH in Hydroponics