Introduction

Auxins can modulate root architecture, fruiting and stress responses. In hydroponic and substrate soilless systems, exogenous root-zone applications at very low ppm sometimes boost yield or quality. Push the dose and you flip the response. Below I review peer-reviewed work on widely grown crops, focusing on species, timing, exact dosages converted to ppm, and toxic thresholds. Where possible I prioritize reviews to frame context, but yield data come from primary trials.

Evidence & discussion

Sweet pepper. Two lines of evidence exist. First, fertigation with a commercial IBA product at 0.4 percent active (4000 ppm in the stock) applied weekly from early fruit development at 0.5 L ha⁻¹ outperformed 1.0 L ha⁻¹, increasing marketable yield while improving root mass and water and nutrient uptake in perlite culture (1). Second, a separate trial compared root fertigation vs foliar using a formulation containing 6.75 g L⁻¹ NAA and 18 g L⁻¹ NAA-amide. The fertigation rate was 0.6 mL L⁻¹ of product in the solution, equal to ~4 ppm NAA plus ~10.8 ppm NAA-amide per application; foliar used 0.4 mL L⁻¹ or ~2.7 ppm NAA plus ~7.2 ppm NAA-amide. Early and total yield were higher with fertigation, while foliar favored some quality traits like firmness and soluble solids (5). Practical read: peppers respond to root-zone auxin in the single-digit ppm range, but more is not better.

Melon. The same IBA approach that helped pepper flopped in melon. In perlite greenhouse culture, 0.4 percent IBA applied weekly at 0.5 or 1.0 L ha⁻¹ did not improve yield or water or nutrient relations. Authors concluded it is not an effective tool for commercial melon in soilless culture (2). Species matter.

Strawberry. In long recirculating systems, autotoxic phenolics depress growth and fruiting. A one-time root or crown dip in NAA before transplant at 5.4 μM NAA, which is ~1 ppm, mitigated autotoxicity and restored flower and fruit numbers compared with untreated plants. A higher 54 μM dose, about 10 ppm, was less effective (3). Timing was everything.

Toxic thresholds from hydroponic seedlings. While not a yield trial, maize in nutrient solution shows the margins. IBA at 10⁻¹¹ M is ~0.000002 ppm and stimulated root growth, but 10⁻⁷ M is ~0.02 ppm and significantly stunted primary root elongation and biomass. The same hormone switches from helpful to harmful across four orders of magnitude (4). That narrow window explains why melon trials can miss and pepper trials can hit. For broader context on root-zone biostimulation via fertigation programs, see this review (6).

Tables

Table 1. Positive responses to exogenous auxin at the root zone in soilless crops

Crop & system	Auxin and delivery	Dose in root zone (ppm)	Timing	Outcome
Sweet pepper, perlite	IBA 0.4 percent product via fertigation	Stock is 4000; applied 0.5 L ha⁻¹ weekly	From early fruit development	Higher marketable yield at 0.5 vs 1.0 L ha⁻¹; improved root mass and water and nutrient uptake (1)
Sweet pepper, soilless	NAA + NAA-amide via fertigation	~4 NAA + ~10.8 NAA-amide per application	Weekly during production	Higher early and total yield vs foliar; foliar favored firmness and °Brix (5)
Strawberry, recirculating hydroponics	NAA root or crown dip	~1 optimal; ~10 less effective	One time at transplant	Mitigated autotoxic yield loss; restored flower and fruit counts under closed reuse (3)

Table 2. Null results and toxic thresholds

Crop or context	Auxin & delivery	Threshold or tested dose (ppm)	Timing	Result
Melon, perlite greenhouse	IBA 0.4 percent via fertigation	Stock 4000; 0.5 or 1.0 L ha⁻¹ weekly	Season-long	No improvement in yield or water or nutrient relations (2)
Maize seedlings, hydroponic assay	IBA in solution	0.000002 stimulatory vs 0.02 inhibitory	Continuous exposure	Root growth stimulation at ultra-low ppm but marked stunting by 0.02 ppm (4)

Conclusion

Root-applied auxins are not a silver bullet. They can raise yield or preserve quality, but only when dose and timing line up with the crop’s physiology. Peppers respond to single-digit ppm root fertigation with higher early and total yields, while melons do not. Strawberries benefit from a ~1 ppm pre-plant dip that preempts autotoxicity, whereas ~10 ppm underperforms. Hydroponic seedling work reinforces the risk: ~0.02 ppm IBA already suppresses maize roots. The safe play is to trial low, crop-specific ppm near published values, apply at the stage that matters, and stop if marketable yield does not move. If you treat auxins like a nutrient and “turn them up,” they will punish you. If you treat them as a precise signal, they can pay off.

Strawberry in controlled environments is not short on opinions. Research from the past 10 years has given us a lot of information on strategies to increase yields and reduce costs. Below I synthesize recent findings, aiming to provide you with practical information that can help you improve your crop. I focus first on mineral nutrition, then biostimulants, exogenous hormone applications, and pruning or cultural practices. When concentration units were not reported in ppm, I converted them. Where authors only gave mL L⁻¹ of a commercial product, I report ppm v/v and, when possible, ppm of active ingredients.

What the evidence says

Mineral nutrition that consistently improves output

1. Stage-specific K:N balance matters more than one static recipe. A greenhouse pot trial in soilless bags across three cultivars found that running a higher K:N balance in vegetative growth, then lowering it in production, delivered the best overall performance. Their S2 program (growth K:N 2.6, production K:N 1.0) raised yield by 30 percent and improved firmness and shelf-life metrics compared to other balances, with equal seasonal totals of N, P, K, Ca, Mg across treatments. This is one of the clearest, practical levers reported for soilless production in the last decade (1).
2. Absolute NO3⁻ and K setpoints still matter, but the optimum is not “more is better”. A hydroponic study that orthogonally varied nitrate and potassium in soilless strawberries showed that 15 mM NO3⁻ increased yield while higher K favored nutraceutical quality. Converting their molarities to ppm: 9, 12, 15 mM NO3⁻ equal 126, 168, 210 ppm N as nitrate and 558, 744, 930 ppm NO3⁻, while 5, 7, 9, 11 mM K⁺ equal 196, 274, 352, 430 ppm K. The highest yields occurred at the upper end of their NO3⁻ range, with quality improving as K approached 430 ppm K. Takeaway: push N during heavy fruiting if you can keep flavor in check, and use K to tune quality targets (2).
3. Simply cranking K in water-culture will backfire. A 2025 deep-water culture trial that stepped K from 117 to 348 ppm at constant 77 ppm N found no yield benefit and, in some cases, reduced fruit size and total yield as K rose. Translation: chasing high EC by piling on K is noise, not signal, in DWC strawberries (3).
4. The nitrate fraction can be used as a steering tool without changing total N. A 2025 soilless study that varied the percentage of total N supplied as nitrate from 0 to 100 percent across three cultivars showed meaningful shifts in plant N status and leachate pH, offering a route to manage uptake and alkalinity without changing ppm N. This is more about stability and diagnosis than raw yield, but it is actionable in recirculating systems (4).
5. System choice is not neutral. A 129-day greenhouse comparison found a coir-based substrate system substantially outperformed three water-culture systems (NFT, vertical stacked flow, aeroponics) for total yield and resource-use efficiency in ‘Florida Brilliance’ and ‘Florida Beauty’. If your priority is marketable kilograms per square meter, substrate is still the safe bet unless you have a very strong reason to go water-culture (5).

Biostimulants with greenhouse soilless data

Two solid greenhouse papers in soilless bags make this practical:

• A nutrient-limitation stress trial in soilless ‘Elsanta’ tested 10 foliar biostimulants. Several treatments improved marketable yield and fruit quality under low fertility. Doses were applied as labeled mL L⁻¹; I report them as ppm v/v. Effects were strongest for specific protein hydrolysates and seaweed extracts, with chitosan showing quality gains rather than yield spikes (6).

• A head-to-head in substrate culture directly compared commercial plant biostimulants and synthetic auxins. The best biostimulant program matched or exceeded auxin-based fruit set under the tested conditions, and the paper fully discloses active contents for the auxin products, which lets us convert to ppm actives for fair comparison (7).

Exogenous hormone applications

Soilless strawberry papers using PGRs are fewer than field studies, but the 2024 greenhouse comparison above provides what growers need: dose-disclosed auxin programs in substrate bags, with yield and quality outcomes. The synthetic auxin formulation Auxyger was listed at 6.7 g L⁻¹ NAA + 16.9 g L⁻¹ NAD. At 0.5 mL L⁻¹, that is 3.35 ppm NAA and 8.45 ppm NAD actives. In that trial, the best protein hydrolysate program rivaled or beat this auxin program on yield while improving certain quality attributes, which makes a case for biostimulant-first strategies where regulations or buyer specs frown on PGR residue (7).

Pruning and culture practices with measurable, repeatable gains

• Runner control increases yield in everbearing cultivars under tabletop tunnel production. Bi-weekly runner removal in ‘Favori’ increased total and marketable yield per plant and improved average berry size, while partial defoliation reduced both. This is not a subtle effect; it is sink management and it pays off (8).

• Planting density in greenhouse substrate is a yield vs. cull tradeoff, not a free lunch. A two-season soilless trial in troughs found 5 to 15 cm in-row spacing maximized commercial fruit and profitability for ‘Pircinque’, but the densest spacings increased small and discarded fruit percentage. If labor for canopy management is tight, 10 to 15 cm is the saner operating point (9).

• System selection again: when in doubt, choose substrate if your KPI is kilograms. The 2025 greenhouse head-to-head is clear that coir-based substrate outperformed water-culture for both yield and resource efficiency in their conditions (5).

Mineral nutrition highlights in soilless strawberries

Study & system	Factor	Setpoints converted to ppm	Observed effect
Preciado-Rangel 2020, soilless culture (2)	NO3⁻ and K in solution	NO3⁻ at 126, 168, 210 ppm N (558, 744, 930 ppm NO3⁻). K at 196, 274, 352, 430 ppm K	Higher NO3⁻ increased yield, higher K improved nutraceutical quality; best yields at 210 ppm N with K toward 430 ppm K.
Ries 2025, deep-water culture (3)	K at constant 77 ppm N	117, 194, 271, 348 ppm K	Increasing K above 117 ppm did not improve yield or fruit size; higher K often reduced fruit size and yield.
Yafuso 2025, soilless substrate (4)	Percent of total N as nitrate	0 to 100 percent of total N as NO3⁻ at a fixed total N (ppm not changed)	Adjusting nitrate fraction shifted foliar N and leachate pH, offering control without changing ppm N.
Nakro 2023, greenhouse soilless (1)	K:N balance over time	Growth phase K:N 2.6, production phase K:N 1.0 (ratios)	Program raised yield 30 percent and improved firmness and shelf-life vs other balances.

Biostimulants in soilless strawberries

Product or molecule	Type	Dose used in study (ppm)	Cultivar & system	Observed effect	Source	Notes
Protein hydrolysate (Trainer)	Amino acid hydrolysate	5000 ppm v/v (5 mL L⁻¹)	‘Elsanta’ in peat-based substrate	Increased marketable yield and improved quality under nutrient limitation	(6)	Labeled concentration is mass per kg; ppm v/v reported for transparency.
Seaweed extract	Ascophyllum-based	2500 ppm v/v (2.5 mL L⁻¹)	‘Elsanta’ in substrate	Yield and antioxidant gains under low fertility	(6)	Product-label dose.
Chitosan solution	Biopolymer	10000 ppm v/v (10 mL L⁻¹)	‘Elsanta’ in substrate	Quality improvements more than yield	(6)	DDA: NR, molar mass: NR in paper.
Protein hydrolysate program	Amino acid hydrolysate	5000 ppm v/v (5 mL L⁻¹)	Greenhouse substrate bags	Matched or exceeded auxin program on yield while improving specific quality traits	(7)	See auxin row for direct comparison.

Exogenous hormones tested in soilless conditions

Active(s)	Class	Dose as actives (ppm)	Product dose	Cultivar & system	Observed effect	Source
NAA + NAD	Synthetic auxin + cofactor	3.35 ppm NAA + 8.45 ppm NAD calculated from 6.7 g L⁻¹ NAA + 16.9 g L⁻¹ NAD at 0.5 mL L⁻¹	0.5 mL L⁻¹	Greenhouse substrate bags	Increased fruit set and yield vs water control, but best protein hydrolysate program was competitive on yield with added quality benefits	(7)

Pruning and cultural practices in soilless systems

Practice	Setting	Quantified outcome	Source
Bi-weekly runner removal	Everbearing ‘Favori’ in tabletop tunnel	Higher total and marketable yield and larger berries vs keeping runners; defoliation reduced yield	(8)
In-row spacing 5 to 15 cm	Greenhouse troughs, soilless substrate	Highest commercial yield and profitability with 5 to 15 cm, but denser plantings increased culls; 10 to 15 cm safer if labor is limited	(9)
System choice: substrate vs water-culture	Greenhouse, coir substrate vs NFT, vertical, aeroponics	Substrate system delivered the highest yield and best resource-use efficiency in both tested cultivars	(5)

Practical summary

• If you run substrate culture, start with a sane base recipe and adopt a two-phase K:N strategy. Push K:N in vegetative growth to build canopy and sink capacity, then lower K:N in production to support sustained fruiting. The 2.6 then 1.0 K:N program is the best documented template right now and lifted yield by 30 percent in greenhouse soilless conditions (1).

• For absolute targets during heavy fruiting, do not be shy about 200 ppm N as nitrate if fruit flavor is maintained, and keep K in the 350 to 430 ppm range to pull quality without sacrificing mass. That is where the 2020 hydroponic NK grid saw the best balance (2).

• Water-culture is unforgiving with K. Above roughly 120 to 200 ppm K in DWC at moderate N, returns were negative in 2025 work, so treat “more K” as a risk factor rather than a lever in water-culture strawberries (3).

• Biostimulants can be yield-positive under stress and can stand toe-to-toe with low-dose auxin programs in substrate. If you need a conservative starting point, weekly foliar protein hydrolysate at 5000 ppm v/v is the most replicated choice across the soilless greenhouse literature summarized here (6), (7).

• Exogenous auxins at single-digit ppm actives work, but they are not automatically superior to a strong biostimulant program in greenhouses. If you use auxins, be precise about actives. The 0.5 mL L⁻¹ Auxyger rate equals 3.35 ppm NAA + 8.45 ppm NAD. Compare like with like, not mL of product (7).

• Cultural practices still pay the bills. Remove runners on a schedule in everbearers and do not defoliate unless you enjoy losing yield (8). Pick a density you can actually manage. If labor is tight, 10 to 15 cm spacing is a rational compromise in tabletop or trough systems (9). If you are choosing systems with yield as the top KPI, substrate culture remains the safest option in 2025 greenhouse data (5).

Hydroponic tomato yields are already high, yet many operations still leak performance through nutrient scheduling, canopy design, and stress control. Below is a blunt, data-driven synthesis for controlled environments based on recent scientific studies. The pattern is consistent: stabilize nutrition and irrigation first, then layer biostimulants or hormones only where trials show a payoff.

Mineral nutrition and solution management

A 2024 greenhouse study across six cultivars found that a constant nutrient concentration program matched yield and improved size distribution compared with stage-based ramps when EC was well controlled (1). A 2023 review distills current best practice for recirculating systems, stressing stage-appropriate EC, ion ratios that avoid antagonisms, and disciplined monitoring in closed loops (2).

Closed systems are viable when sanitation and monitoring are tight. A greenhouse comparison showed closed hydroponics achieving similar yields with better water and fertilizer use efficiency than open run-to-waste setups (3). Calcium balance still matters. Whole-plant experiments showed that simply pushing calcium does not prevent blossom-end rot and that imbalances can backfire, so keep Ca adequate and balanced rather than excessive (4).

Irrigation and pruning practices that scale

Partial root-zone drying and moderate deficit irrigation remain the most defensible water-saving tactics in greenhouses. Grafted tomatoes under PRD or deficit regimes saved 30 to 40 percent water with only minor yield penalties and sometimes higher fruit mineral concentrations (5).

On canopy design, a low-truss high-density approach can raise kilograms per square meter. In a hydroponic sub-irrigated trial with the indeterminate hybrid Rebeca, the top treatment was two trusses per plant at 11.1 plants per square meter, reaching 22.61 kg per square meter in 134 days without harming fruit quality (6).

Biostimulants with signal, not hype

Seaweed extracts and chitosan have the most consistent tomato evidence in soilless systems.

A greenhouse study in inert substrates showed that foliar seaweed extract at 100 000 to 200 000 ppm improved chlorophyll, gas exchange, and fruit quality indices. Silicon at 75 ppm (as sodium silicate) increased firmness and yield per plant in a palm-peat mix. Effects were substrate and dose dependent, so you must calibrate to your product and spray volume per area (7). A 2022 review synthesizes similar benefits for seaweed extracts under salinity stress, with gains tied to photosynthesis and ion homeostasis rather than magic bullets (8).

For chitosan, a 2025 greenhouse study on Floradade and Candela F1 tested 500, 1000, and 2000 ppm foliar programs. Higher rates improved growth and physiology, with cultivar-specific responses. Product specs like degree of deacetylation and molar mass were not reported, so do not assume equivalence across suppliers (9).

Exogenous hormones: targeted, not blanket

If fruit set is the bottleneck during heat or low pollen viability, exogenous hormones can help. In protected cultivation of cv. Srijana, a conservative foliar program of GA3 at 50 ppm with NAA at 25 ppm increased fruit set and total yield. The response surface penalized higher rates, reminding you that timing and dose are critical (10). For mechanism and limits, a 2022 review explains how auxin and gibberellin signaling induce parthenocarpy in tomato and why misuse leads to malformed fruit (11).

Summary tables

Table 1. Mineral nutrition and system practices with yield impact in CEA tomatoes

Factor	Cultivar or type	Dose or setting (ppm)	Observed effect	Source
Constant vs stage-based nutrient supply	Six cultivars, greenhouse	Program choice rather than dose	Constant feed matched yield and improved size distribution	(1)
Nutrient solution management review	General CEA	Program design	Best practice for EC, ion ratios, and closed-loop monitoring	(2)
Closed vs open hydroponics	Determinate tomato, greenhouse	System choice	Closed loop improved water and fertilizer efficiency with comparable yield	(3)
Calcium balance	Modern genotypes	Balanced Ca supply	Lower BER risk depends on overall ion balance, not brute Ca	(4)
Partial root-zone drying and deficit irrigation	Grafted tomato, greenhouse	Irrigation scheduling	30 to 40 percent water savings with minor yield penalties	(5)

Table 2. Biostimulants in soilless tomatoes

Biostimulant	Cultivar or type	Application	Dose (ppm)	Observed effect	Source
Seaweed extract	Cherry tomato, greenhouse substrates	Foliar	100 000 to 200 000	Improved physiology and fruit quality indices under stress	(7)
Silicon as sodium silicate	Cherry tomato, greenhouse substrates	Foliar	75	Increased firmness and yield per plant in palm-peat mix	(7)
Chitosan (medium MW, commercial)	Floradade and Candela F1	Foliar, multiple sprays	500, 1000, 2000	Improved growth and physiological performance, cultivar dependent	(9)
Seaweed extract review	Multiple tomato types	Seed or foliar in soilless culture	Various	Stress tolerance and modest yield gains under salinity	(8)

Table 3. Exogenous hormone programs with documented yield or set effects

PGR	Cultivar or type	Application	Dose (ppm)	Observed effect	Source
GA3 + NAA	Srijana, protected cultivation	Foliar during flowering	GA3 50, NAA 25	Increased fruit set and total yield; higher rates underperformed	(10)
Auxin and GA context	Tomato, general	Mechanistic review	N/A	Explains parthenocarpy induction and risks of misuse	(11)

Practical takeaways

Do not chase clever ramps before you can hold EC steady. A constant, well-tuned feed can match yield and improve size distribution when the rest of the system is under control (1), (2). Closed loops pay only if you earn them with monitoring and sanitation (3). Low-truss high-density recipes push kg per square meter, provided irrigation and nutrition meet the faster sink demand (6). Seaweed extracts and silicon can help under stress, but responses are product and substrate specific. Chitosan works, yet cultivar and formulation matter, so trial first (7), (8), (9). Hormones are scalpels for set problems, not a replacement for climate and pollination management (10), (11).

You can make your Athena schedule much cheaper by replacing the pH up products with simple raw salts. Branded pH management and buffering products like Athena Balance and Athena Pro Balance are, at their core, just sources of potassium bases delivered in carbonate or silicate form. They are however, very over priced for what they are and can be a high percentage of the overall cost of running these nutrient regimes. By understanding their labels and safety data sheets, we can replicate these formulations with commodity salts, achieving equivalent nutritional and pH adjusting outcomes at a fraction of the cost.

Athena Pro Balance can be replaced with Potassium Carbonate
The powdered Pro Balance product is likely nothing more than high-purity potassium carbonate (K₂CO₃), usually 98.5–100% pure. Chemically, K₂CO₃ contains ~68% K₂O-equivalent by weight, which is exactly what the Athena Pro Balance label reflects. This means you don’t need to blend or dilute anything to make a replacement, simply sourcing food-grade or fertilizer-grade potassium carbonate is sufficient. You can dose it directly as you would the branded powder, bearing in mind it is strongly alkaline and should be added to water with care. Storage should be in sealed HDPE containers to avoid caking from atmospheric moisture.

Athena Blended Balance (liquid) can be replaced with an AgSil 16H solution
The liquid Balance label shows 2% K₂O. AgSil 16H, a common potassium silicate source, contains 32% K₂O and ~53% SiO₂. To reproduce the K₂O content of Athena Balance, you need to dilute AgSil at the correct ratio:

• Target is 2% K₂O.
• Required fraction = 2 / 32 = 0.0625.
• This means 6.25% (w/w) AgSil in water.

Translated to a practical recipe, this equals 236.6 g of AgSil 16H per US gallon of solution (3.785 L), topped up with RO water (must be RO or distilled water). Dissolve the AgSil slowly with vigorous mixing, as potassium silicate is highly viscous and alkaline. The result is essentially identical in potassium concentration to the branded Balance, with the added benefit of supplying soluble silica (~1.55% Si in the solution).

Improving stability with KOH
One common issue with potassium silicate solutions is their tendency to polymerize or precipitate over time, especially at lower concentrations or in the presence of divalent cations. To mitigate this, adding a small amount of potassium hydroxide (KOH) helps maintain a strongly alkaline environment that discourages silica gelation. For the recipe above, adding 1 g of KOH per gallon of solution is a simple way to improve stability during storage. This will not significantly change the K₂O content but will keep the solution more stable and easier to handle.

Cost Analysis
Beyond the chemistry, cost is the main driver for making these substitutions. Let’s look at a ballpark comparison based on typical retail prices (USD, 2025):

Product	Retail Price	Equivalent Raw Material	Raw Material Price	Cost per Gallon of Finished Equivalent
Athena Pro Balance (powder)	~$7 per lb	Potassium carbonate	~$2 per lb	Replacement is more than 3x cheaper
Athena Balance (liquid)	~$20-40 per gallon	AgSil 16H + 1 g KOH	~$6.4 per lb AgSil, ~$5 per lb KOH (~3$ AgSil + 1c of KOH per gal)	Replacement costs is around 10x cheaper

For the Balance liquid in particular, the price difference is striking: the branded gallon runs around $20-40, while the equivalent solution made from AgSil 16H plus a pinch of KOH comes out to under $3 per gallon, even at retail chemical pricing. The Pro Balance substitution is less dramatic in absolute terms but still represents substantial savings over time.

Take-home message
Replacing Athena Pro Balance is as simple as sourcing potassium carbonate, while Athena Balance can be reliably reproduced with a potassium silicate solution prepared from AgSil 16H plus a small stabilizing addition of KOH. For growers comfortable working with raw salts, this substitution strategy provides full control, predictable composition, and significant cost savings while providing a drop-in replacement for one of the most expensive parts of the Athena nutrient line.

In hydroponic and substrate systems chitosan can help, but only inside fairly narrow windows of dose, molecular traits, and crop context. Here is what the strongest hydroponic and soilless evidence shows for common greenhouse crops, with doses in ppm and forms that have actually been tested in peer-reviewed trials.

What matters before you dose

Form and solubility. Most horticultural studies use acid-solubilized chitosan, typically chitosan acetate prepared by dissolving chitosan in dilute acetic acid. Solubility improves as degree of deacetylation increases and molecular weight decreases. That changes biological activity and leaf penetration, which is why not all chitosans behave the same in crops grown without soil. Review data across crops confirms that activity depends on origin, degree of deacetylation, molecular weight and derivative used, not just “chitosan” on the label (1).

Degree of deacetylation and molecular weight. Higher deacetylation increases positive charge density and solubility in the acidified sprays most growers use. Lower to mid molecular weight generally penetrates tissues better; very high molecular weight tends to act more at surfaces. Reviews focused on crop plants note these relationships and explain why different products show inconsistent results if DD and MW are not controlled (1).

Application route. Foliar and rootzone applications are not interchangeable. Foliar sprays in hydroponics commonly use 50 to 200 ppm for stress mitigation and quality endpoints. Rootzone dosing inside recirculating solutions can work for disease suppression at similar or higher ppm, but the tolerance window is tighter and crop-dependent. A 2024 root-focused review flags that root exposure can inhibit growth if dose and MW are off, even while defense responses go up (2).

Source. Commercial material is generally crustacean-derived, with fungal-derived chitosan available at smaller scale. Origin mainly matters through DD, MW and impurities like ash and protein. Again, agronomic performance maps back to those properties rather than source alone (1).

What the hydroponic and soilless studies actually show

Leafy greens and fruiting vegetables most tested in soilless settings

• Lettuce, deep-flow hydroponics, foliar. In a controlled deep-flow system, foliar chitosan at 100 ppm mitigated salt stress, improved relative water content and chlorophyll, and reduced membrane damage markers. The trial used exogenous chitosan applied to leaves while plants grew in circulating nutrient solution, so the result is directly relevant to recirculating NFT or DFT growers (3).
• Cucumber, hydroponic rootzone, disease control. In a classic hydroponic study, adding 100 to 400 ppm chitosan to the nutrient solution suppressed Pythium aphanidermatum root rot and induced host defenses without visible phytotoxicity at those doses. This is one of the best-controlled demonstrations of rootzone efficacy in a soilless system (4).
• Tomato, soilless substrate, chitosan-based material at the rootzone. A soilless peat and perlite greenhouse system received a chitosan polyvinyl alcohol hydrogel with copper nanoparticles placed in the rootzone. The treatment improved growth, antioxidant capacity and yield relative to the untreated control. This is not a simple chitosan salt spray and the dose was delivered as a solid material rather than a ppm solution, but it shows chitosan-based materials can be integrated into substrate programs in practice (5).
• Context across crops. A comprehensive review of chitosan for plant protection and elicitation explains the defense activation seen above and why responses are dose and MW dependent. It also documents successful use patterns that generalize to greenhouse crops treated by foliar or root routes (6).

Practical dose ranges that align with the hydroponic evidence

If you want the odds on your side in hydroponics or inert substrates, stay inside these lanes and confirm on a small block first.

• Foliar, leafy greens and fruiting vegetables in hydroponics or inert substrate. 50 to 150 ppm per spray, usually every 7 to 10 days around stress periods. The deep-flow lettuce result sits at 100 ppm and delivered physiological benefits under salinity (3).
• Rootzone, recirculating hydroponics. 100 to 400 ppm in the circulating solution only when you have a clear disease target like Pythium in cucumber. For general biostimulation, root dosing is higher risk. The hydroponic cucumber study used 100 and 400 ppm to suppress Pythium effectively (4). Outside this range you are more likely to see growth penalties than benefits according to root-focused syntheses (2).
• Chemistry targets when purchasing. Prefer DD around 80 to 90 percent and low to mid MW material for foliar work. Verify supplier certificates rather than marketing bullets. The crop reviews explaining DD and MW effects are clear that these traits determine outcomes (1).

Summary tables

Table 1. Trials in hydroponic or soilless systems with chitosan

Crop	System	Application route	Chitosan form	Dose used (ppm)	Reported effect	Reference
Lettuce	Deep-flow hydroponics	Foliar spray	Acid-solubilized chitosan solution	100	Mitigated salinity stress, higher RWC and chlorophyll, lower oxidative damage	(3)
Cucumber	Hydroponics	Rootzone in nutrient solution	Chitosan solution in recirculating feed	100 to 400	Suppressed Pythium root rot, induced defense enzymes, no visible phytotoxicity at tested doses	(4)
Tomato	Soilless substrate, peat plus perlite	Rootzone material in substrate	Chitosan PVA hydrogel with Cu nanoparticles	not applicable as ppm	Improved growth, antioxidant capacity and yield versus control in substrate culture	(5)

Table 2. Chemistry traits that move the needle

Trait	Why it matters in soilless culture	Practical target
Degree of deacetylation	Higher DD increases solubility in dilute acids used for sprays and increases cationic charge for leaf interaction	80 to 90 percent DD for foliar sprays (1)
Molecular weight	Lower to mid MW improves penetration and reduces viscosity. Very high MW can sit on surfaces and act mainly as an elicitor	Low to mid MW for foliar, avoid very high MW for root dosing (1)
Source	Crustacean and fungal sources both work. Performance depends on DD, MW and impurities, not source alone	Buy on spec sheet, not species label (1)

Table 3. Foliar versus root applications in hydroponics and substrates

Dimension	Foliar application	Root application
Typical working range	50 to 150 ppm per spray	100 to 400 ppm in the solution when disease control is the objective
Primary targets	Stress mitigation, quality traits, mild growth stimulation	Pathogen suppression in roots and elicitation of defenses
Risk profile	Low when DD and MW are appropriate and pH is controlled	Higher. Dose and MW errors can reduce root growth and yield
Evidence base in soilless settings	Deep-flow lettuce shows clear physiological benefits at 100 ppm (3)	Hydroponic cucumber shows robust Pythium control at 100 to 400 ppm (4)

How to deploy without shooting yourself in the foot

1. Start with foliar at 100 ppm on a small block. If your chitosan is low to mid MW and 80 to 90 percent DD, you are in the same ballpark as the effective lettuce hydroponic protocol (3).
2. Reserve root dosing for disease pressure. If you are chasing Pythium in cucumber, 100 to 400 ppm in the solution is supported. For general “growth promotion”, root dosing is more likely to backfire than help in recirculating systems (4), (2).
3. Verify product specs. Ask for DD and MW. If the vendor will not provide them, find one who will. The variability you see in practice maps to those two numbers (1).
4. Do not stack unknowns. Mixing chitosan with copper, acids, or surfactants without a clear recipe can change activity. That can help in substrate programs where materials are embedded, as in the hydrogel example, but it is not a blank check (5).
5. Measure the outcome that pays. Run a side-by-side block with your limiting stress in view. If you cannot tie chitosan to a measurable gain in yield, quality or loss avoidance in your system, move on. Elicitation without payoff is just cost (6).

Introduction

Iodine sits in a weird spot in plant nutrition. It is essential for humans, not officially essential for higher plants, yet low, well chosen doses often push crops to perform better in controlled systems. The key is dose and form. Get either wrong and you tank growth. Get them right and you can see yield and stress-tolerance gains that are economically meaningful. Recent reviews lay out both the promise and the pitfalls, so let’s cut through the noise and focus on agronomically relevant hydroponic and soilless work only. (1)

Why iodine can behave like a biostimulant

Mechanistically, iodine at trace levels appears to influence redox balance and stress signaling and can even become covalently bound to plant proteins. Proteomic evidence has shown widespread protein iodination, and plants deprived of iodine under sterile hydroponics grow worse until micromolar-range iodine is restored. That does not make iodine “essential” in the strict sense, but it explains why tiny doses can trigger outsized responses. (2)

Form matters

Across multiple hydroponic tests, iodide is absorbed faster and is more phytotoxic than iodate. In basil floating culture, growth was unaffected by roughly 1.27 ppm iodine as KI or 12.69 ppm iodine as KIO₃, but KI above about 6.35 ppm iodine cut biomass hard, while KIO₃ needed far higher levels to do the same. That is a practical takeaway for nutrient solution design. Favor iodate when you are exploring a new crop or cultivar. (3)

Evidence from hydroponic and soilless crops

Lettuce

A classic water-culture study ran 0.013 to 0.129 ppm iodine in solution and saw no biomass penalty while leaf iodine rose predictably. Iodide enriched tissue more than iodate at equal iodine, which is useful if your target is biofortification, not just a biostimulant effect. (4)

Under salinity, iodate becomes more interesting. In hydroponic lettuce with 100 mM NaCl, about 2.54 to 5.08 ppm iodine as KIO₃ increased biomass and upregulated antioxidant metabolism, which is exactly what you want in salty recirculating systems. Push higher and the benefits fade. (5)

Strawberry

Hydroponic strawberry responded to very low iodine. Iodide at or below 0.25 ppm and iodate at or below 0.50 ppm improved growth and fruit quality, while higher levels reduced biomass and hurt fruit quality metrics. You do not have much headroom here. (6)

Basil

Greenhouse floating culture trials on sweet basil showed cultivar-specific tolerance but the same pattern every time. KI starts biting growth above single-digit ppm iodine, while KIO₃ is far gentler at comparable iodine. Antioxidant capacity trends are cultivar dependent, so do not generalize “more phenolics” as a guarantee of better growth. (7)

Tomato

Tomato is where yield effects get real. In growth-chamber work, fertigation with iodate at roughly 6.35 to 12.69 ppm iodine increased fruit yield by about 30 to 40 percent in a small-fruited cultivar. In a greenhouse trial with a commercial hybrid, much lower iodine in solution, around 0.025 to 1.27 ppm as KIO₃, still improved plant fitness and mitigated part of the salt penalty. Dose tolerance depends on the system and the genotype, so copy-pasting numbers between cultivars is a bad idea. (8)

Cabbage

Hydroponic Chinese cabbage tested 0.01 to 1.0 ppm iodine as KI or KIO₃. Uptake and partitioning behaved differently by species and form. The practical read is that both forms work for biofortification within that band, but I would still lean iodate first for safety. (9)

Working ranges seen in hydroponic or soilless trials

Crop	System	Iodine form used	Dose range tested in literature (ppm as I)	Observed direction of effect
Lettuce	Water culture	Iodide and iodate	0.013 to 0.129	Neutral on biomass, strong tissue enrichment at all doses tested
Lettuce under salinity	Hydroponic with 100 mM NaCl	Iodate	~2.54 to 5.08	Biomass increased, antioxidant system activation
Strawberry	Hydroponic	Iodide and iodate	Beneficial at or below 0.25 (I−) and 0.50 (IO3−)	Growth and fruit quality improved at low doses, declines above
Basil	Floating culture	Iodide and iodate	Safe near 1.27 as KI, 12.69 as KIO3; toxicity above ~6.35 as KI	KI far more phytotoxic than KIO3 at equal iodine
Tomato	Substrate fertigation and growth chamber	Iodate	~0.025 to 12.69 depending on setup	Yield and stress tolerance improved within study-specific bands
Cabbage	Hydroponic	Iodide and iodate	0.01 to 1.0	Both forms accumulated; response form-dependent

Practical setup that does not wreck a crop

Start with iodate. It is consistently less phytotoxic in solution culture than iodide at the same iodine level. Use iodide later only if you have a clear reason. (7)

Leafy greens
Conservative exploratory band: 0.03 to 0.10 ppm iodine in solution during vegetative growth. If you are running saline conditions, you can test up to about 2.5 to 5.1 ppm as iodate for stress mitigation, but do not do this blind outside a salinity trial. (4) (5)

Strawberry
Keep solution iodine low. Try 0.05 to 0.25 ppm as iodide or 0.10 to 0.50 ppm as iodate. Expect quality shifts alongside biofortification, and expect penalties if you push higher. (6)

Basil
If you work with KI, do not exceed about 1.3 ppm iodine without a reason and tight monitoring. With KIO3, you have more headroom, but benefits are not guaranteed at the higher end. (7)

Tomato
In substrate systems, exploratory fertigation bands that have shown positive responses run roughly 0.025 to 1.27 ppm iodine as iodate for commercial cultivars. Higher doses around 6.50 to 12.50 ppm have improved yield in small-fruited genotypes under controlled conditions, but those are not starting points for a commercial house. (8)

Cabbage and other Brassicas
0.01 to 1.0 ppm works for biofortification trials in solution culture. Track form-specific uptake. (9)

Common failure modes

1. Using iodide when you should have used iodate. Iodide is more phytotoxic in water culture. If you switch to iodide, cut the ppm accordingly and watch plants closely. (7)
2. Copying doses between crops or between stress contexts. Lettuce under salt stress tolerated and benefited from multi-ppm iodate that would be overkill in non-saline runs. (5)
3. Chasing biofortification at the expense of growth. Strawberry is very sensitive; the window for improvement is narrow and easy to overshoot. (6)
4. Assuming universality. Tomato shows real yield gains, but the best range depends on cultivar and system. Validate locally. (8)

Crop	Best form to start	Trial band to test next (ppm as I)	Notes you should not ignore
Lettuce	KIO3	0.03–0.10 for routine runs; up to 2.5–5.1 only in salinity trials	Tissue enrichment is easy at sub-ppm; benefits need stress context
Strawberry	KI or KIO3	0.05–0.25 as KI; 0.10–0.50 as KIO3	Quality improved at low levels; penalties above
Basil	KIO3	0.5–3.0	KI becomes risky above low single digits
Tomato	KIO3	0.025–1.27 in commercial substrate; leave 6.5–12.5 to controlled trials	Verify by cultivar; watch fruit quality metrics
Cabbage	KIO3	0.05–0.5	Uptake is efficient; track partitioning by organ

Final word

Iodine can behave like a biostimulant in hydroponics and soilless systems, but only if you respect its razor-thin margin between helpful and harmful. Start small, prefer iodate, and validate on your own cultivars and systems instead of trusting a one-size-fits-all recipe. If you need a broader framework for running precise biofortification trials in soilless production, recent reviews are clear about why controlled systems are the right place to do this work. (9)

People ask about dosing cobalt in recirculating systems to “stimulate” growth or flowering. For the crops that matter in hydroponics and soilless culture, peer-reviewed work does not show reliable growth or yield benefits from adding cobalt to the solution. What the literature does show is straightforward: cobalt is readily taken up at low ppm, it inhibits ethylene biosynthesis at pharmacological doses, and it becomes toxic fast when you push concentration. The burden of proof for agronomic benefit is still unmet. Below I summarize what high-quality studies in hydroponics and soilless systems actually report.

What cobalt does in plants

Cobalt is not established as essential for most higher plants. It is essential for N-fixing microbes and therefore matters in legumes, but for tomato, cucumber, lettuce and the like, its status is “potentially beneficial at very low levels, toxic at modest excess.” A recent review frames this clearly and compiles transport and toxicity data across species (Frontiers in Plant Science, 2021).

A second, practical point is mechanism. Cobalt ions inhibit ACC oxidase, the last step in ethylene biosynthesis. That is why physiologists use cobalt chloride in short, high-dose treatments to suppress ethylene responses in experimental tissues. Classic work documents this inhibition in cucumber and other plants (Plant Physiology, 1976).

Ethylene inhibition can, in principle, delay senescence or alter stress signaling. The catch is dose. The amounts that clearly block ethylene in lab tissues are usually far above what you want sloshing around a long-cycle greenhouse system, and benefits rarely translate to whole plants under production conditions.

What happens in hydroponics and soilless systems

Tomato

Nutrient solution exposure, subtoxic range
Tomato grown hydroponically with cobalt at 0.30 ppm and 1.18 ppm showed strong root retention and limited shoot transfer. This is uptake behavior, not a biostimulant response, and the authors did not report yield benefits. The forms used were cobalt(II) salts in solution culture (Environmental Science & Technology, 2010).

Toxicity under higher exposure
A hydroponic study imposed severe cobalt stress at 23.57 ppm and observed depressed biomass, disrupted water status, chlorophyll loss and oxidative damage in tomato. Cobalt was supplied as cobalt chloride in the nutrient media. Plant growth regulators mitigated symptoms but did not make cobalt itself beneficial (Chemosphere, 2021).

Lettuce

Toxicity in greenhouse hydroponics with inert media
Iceberg lettuce grown in a perlite based hydroponic system suffered growth and pigment losses at 11.79 ppm cobalt. Cobalt was added as cobalt salt to a modified Hoagland solution. The same paper showed nitric oxide donor treatments could blunt the damage, which again argues cobalt at this level is a stressor, not a stimulant (Chilean Journal of Agricultural Research, 2020)

Cucumber

Mechanistic ethylene work, not production benefit
Multiple peer-reviewed studies in cucumber use cobalt chloride as an ethylene biosynthesis inhibitor in explants or short assays. These demonstrate the mechanism but are not agronomic validations for dosing cobalt into a recirculating system for weeks (Plant Physiology, 1976; Forests, 2021).

Summary table of relevant studies in hydroponics and soilless culture

Crop	System	Cobalt form	Solution cobalt (ppm)	Exposure description	Main outcome
Tomato	Aerated nutrient solution	Co(II) in solution culture	0.30 and 1.18	Whole plants in controlled hydroponics	Strong root retention, limited shoot transport; no biostimulant effect reported. ES&T 2010 PubMed
Tomato	Hydroponic solution, stress test	Cobalt chloride	23.57	Whole plants, growth regulators tested for mitigation	Marked toxicity: biomass and chlorophyll decreased, oxidative stress increased. Chemosphere 2021
Lettuce	Perlite + recirculating solution	Cobalt salt in modified Hoagland	11.79	Greenhouse hydroponics with inert media	Significant growth and pigment losses at this dose; NO donor partially mitigated damage. Chilean J. Agric. Res. 2020
Cucumber	Short mechanistic assays	Cobalt chloride	used as ethylene inhibitor in short assays	Explants or detached tissues	Confirms ethylene inhibition by Co²⁺; not a production recommendation. Plant Physiology 1976

So is cobalt a biostimulant in hydroponic vegetables

For tomato, cucumber and lettuce grown hydroponically or in soilless culture, peer-reviewed journal data do not support cobalt as a legitimate biostimulant input. You can inhibit ethylene transiently with cobalt chloride in lab tissues, but that is not a recipe for higher yield in a recirculating system. The agronomic studies that actually dose solutions show either neutral responses at sub-ppm levels or clear toxicity when you push into low double digits. The general biology context from a recent cobalt review matches this picture and does not contradict it (Frontiers in Plant Science, 2021).

Practical guidance for hydroponic and soilless growers

Default practice
Do not add cobalt intentionally to non-legume hydroponic recipes. There is no reproducible benefit and real risk of toxicity in the low tens of ppm, with lettuce showing damage already at ~12 ppm and tomato at ~24 ppm under hydroponic conditions. (see here, or here)

If you want to experiment
Keep total cobalt in solution at sub-ppm levels and treat it as a research trial, not a production strategy. Track solution cobalt with ICP if you can. The only peer-reviewed hydroponic tomato data near this range are 0.30 to 1.18 ppm, which documented transport behavior, not stimulation.

Forms used in the literature
Cobalt chloride is the dominant form when researchers test ethylene inhibition or impose cobalt stress. Cobalt sulfate also appears in some soilless protocols. Neither form has peer-reviewed evidence of yield stimulation in hydroponic tomato, cucumber or lettuce. (see here or here)

Legumes are the exception
Cobalt matters indirectly via N-fixing symbionts. If you are growing legumes in soilless systems, cobalt management belongs in the microbial nutrition discussion, not as a general biostimulant for non-legumes (see here).

Crop	“Stimulant” claim in journals	Reported beneficial window	Toxicity begins around	Notes
Tomato	None in hydroponic journals	None demonstrated	~23.6 ppm in nutrient solution	Sub-ppm exposures documented uptake with no benefit. ES&T 2010; Chemosphere 2021
Lettuce	None in hydroponic journals	None demonstrated	~11.8 ppm in nutrient solution	Damage includes biomass and chlorophyll loss in greenhouse hydroponics. Chilean J. Agric. Res. 2020
Cucumber	Mechanistic ethylene inhibition only	Not applicable	Not defined for production, lab tissues often use high short-term doses	CoCl₂ used to block ethylene in explants; not a production recommendation. Plant Physiology 1976

Bottom line

If you grow tomato, cucumber or lettuce in hydroponics or inert media, cobalt is not a proven biostimulant. At sub-ppm levels you might see nothing. Push it into the low tens of ppm and you will see toxicity. The only unequivocal “effect” you can count on is ethylene inhibition during short, high-dose laboratory treatments with cobalt chloride, which is not a safe or sensible production tactic. Until robust, peer-reviewed hydroponic trials show yield or quality gains at practical ppm, the rational move is to leave cobalt out.

Introduction

We know that silicon can be a very beneficial element for many plant species (see some of my previous posts here and here). It mainly enhances disease resistance and increases the structural integrity of plant tissue. Because of these advantages, you will want to add silicon to your nutrient solution. However, there are a lot of misconceptions and questions about the use of Si in plants and the exact form of Si that you should use. In this post I am going to address some of the most common questions about silicon sources and how to use them properly.

What sources are available?

To use silicon in nutrient solutions, we will generally have 3 types of sources available.

First, we have basic potassium silicates, which are solids or solutions derived from the reactions of silica with potassium hydroxide. In this category you have popular products like AgSil 16H and liquid concentrates like Growtek Pro-Silicate. These products have a very basic pH.

Second, we have acid stabilized silicon products. These are products like PowerSi Classic and OSA28. These products are always liquids and contain monosilicic acid in an acidic environment, with stabilizing agents added to prevent the polymerization of the monosilicic acid.

Third, we have non-aqueous products with organosilicon reagents, like Grow-Genius. These products do not contain water and are derived from reagents like TEOS (tetraethyl ortho-silicate) and other Si containing compounds, mainly Si containing surfactants. They are not in forms that are plant available but will generate these forms when in contact with water.

Do potassium silicates contain “less available” silicon?

When you dissolve a potassium silicate at high concentration, it forms silicate oligomers. These are large silicon chains that get stabilized in basic solutions because of their high negative charge. This is why you can create highly concentrated potassium silicate solutions in basic pH. As a matter of fact, making the solutions more basic with added potassium hydroxide often enhances the solubility of potassium silicate solids like AgSil16H (see here for a procedure on how to do this). However, when the molar concentration decreases the silicate hydrolyzes into monomeric silicate anions.

When potassium silicate is diluted in nutrient solutions, this is exactly what happens. The reduction in concentration hydrolyzes the Silicates into monomers. If the solution pH is then lowered, the final form present will be monosilicic acid. If you properly prepare a nutrient solution with potassium silicate, the end form will be monosilicic acid, the form that is mostly available to plants.

It is a misconception that potassium silicates are somehow less “plant available”. They end up producing monosilicic acid and being perfectly available, when used properly.

How do I properly use a potassium silicate?

First, if using a solid, you need to prepare a stock solution no more concentrated than 45g/L. The recommendation with AgSil 16H would be to prepare a stock solution at 15g/gal and then using this solution at a rate of 38mL/gal of final solution (injection rate of 1%). To increase the stability of your AgSil 16H concentrate you can add 1g/gal of KOH. The end addition to your solution will be +9.8ppm of Si as elemental Si and +11.55ppm of K. The KOH addition and low 15g/gal concentration ensures that silicate will already be largely present as monomeric silicate anions.

Second, make sure to add this solution to your water first. If you add this solution after nutrients, the Si will come into contact with Ca and Mg in its concentrated form, which will cause problems with its stability in solution. Add it first, then add your lowest pH fertilizer concentrate, then your Ca containing concentrate, then finally decrease the pH with an acid to the desired level if needed.

This procedure ensures you get a final solution containing monosilicic acid that will be stable. If you increase the Si in the stock solution, change the injection order, or increase the Si in the end solution beyond 20ppm of Si as elemental Si you might end up with precipitated and unavailable Si forms.

Why would you use acid-stabilized Si products?

Acid stabilized silicon sources are not more plant available. However, their starting pH is usually low and their mineral composition can also be minimal (depending on the preparation process). This means they can lower the need for acid additions and can help lower the pH of hard water sources when used. They can also contain stabilizing agents that could be beneficial for plants. However, the exact stabilizers used and the exact mineral composition used will vary substantially by product, since there are a wide array of choices available to manufacturers.

In the end, at the pH where plants are fed, acid stabilized Si and potassium silicate sources generate the exact same monosilicic acid. Plant availability is not an advantage of using this sort of product.

Why would you use non-aqueous Si products?

These products can be much more highly concentrated than either basic silicon or acid stabilized liquid silicon products by mass. This is because they are made from Si forms that are highly stable under water-free conditions. This means you can buy a small amount and add a small amount to your reservoir per gallon of solution prepared. Another advantage is that they are pH neutral and do not alter the pH of nutrient solutions at all. The formation of the silicic acid from these products requires only reactions with water, so no mineral addition, stabilizer additions or pH modifications happen.

A significant point however is that the reaction of a product like TEOS with water releases other substances into solution. For each 10 ppm of Si as elemental Si that you add from TEOS you will in fact be adding ~66pm of ethanol to your solution. These alcohols can be very detrimental for root and plant growth, reason why the use of these non-aqueous Si products needs to be carefully considered. When using a product containing non-aqueous Si sources, it’s important to consider that these substances can accumulate in your root zone and may cause problems. Which organics are present and whether they will cause problems will depend on the exact formulation. When using these organosilicon sources, passing the nutrient solution through a carbon filter to remove these organics before contact with plant roots would be ideal.

Is the final Si in solution from any product type more stable?

No, all three types of products, when used properly, will end up as stable monosilicic acid in your solution. The stabilizing agents in acid-stabilized products will be so dilute that any additional stabilizing effect will be relatively non-existent. If Si is dilute enough (<20ppm of Si as elemental Si), then it will be stable in solution indefinitely (I measured 5 weeks with no changes in concentration). At higher Si concentration, the Si will tend to polymerize (no matter which source it comes from) which will create problems with stability. To have stable Si in solution make sure that you prepare it properly and that you keep the concentrations low enough.

If they are mostly the same in terms of Si availability, why do I see differences between different products at an equivalent Si application rate?

Despite all of the different Si products leading to the same form of Si in the final solution, acid-stabilized Si products will contain a wide array of additional substances that are going to be active nutritionally. For example, Boron and Molybdenum are very commonly used stabilizing agents. Products, like PowerSi bloom, also contain “exotic plant extracts” (according to their website). Commonly used stabilizing agents include glycerol, carnitine, choline and sorbitol. All of these could potentially have an effect on the plants at the concentrations added with these products. Some of these stabilizing agents are usually added at 10-50x the amount of Si present by mass, meaning that your Si supplement might be adding way more of these stabilizing agents than what you’re adding in terms of Si.

What product is more cost effective per delivered mole of monosilicic acid?

There is a lot of space in labeling regulations to allow fertilizer manufacturers to trick people into believing a product might be more concentrated or dilute than another. First of all, labeling a product as “% of monosilicic acid” does not mean that the product contains that percentage of monosilicic acid, it means that the product contains Si, such that if that silicon was all converted to mono-silicic acid, it would give that percent. The only products that contain monosilicic acid in its actual form from the start are acid-stabilized Si containing products, which are usually limited to low concentrations due to the reactivity of this molecule when present.

Both non-aqueous silicon products and soluble potassium silicate products contain precursors to monosilicic acid. One in the form of organosilicon compounds and the other in the form of silicate chains. As mentioned above, both precursors can lead to very high conversions to mono-silicic acid when properly used.

To compare the actual concentration of products, it is best to always convert the amounts to elemental Si percentage values. To convert monosilicic acid % values to Si, multiply the value by 0.2922, to convert SiO₂ values to Si, multiply the value by 0.4674. For example, 40% Si as monosilicic acid is equivalent to 11.68% Si as elemental Si. Soluble potassium silicates like AgSil 16H can be around ~24% Si as elemental Si by mass, making them the most highly concentrated and lowest cost form of bioavailable silicon when used properly. More highly soluble potassium silicates than AgSil16H will usually be lower in Si, as higher K proportions lead to better solubility and a lesser need to add KOH when preparing stock solution. The table above, showcases the price differences per gram of Silicon of different products as of Jan 2023. When purchased in bulk (50 lbs) AgSil16H can be up to two orders of magnitude lower cost than other alternatives.

I have done lab tests measuring molybdenum reactive Si that show all the Si in AgSil16H can be quantitatively converted to monosilicic acid when following the preparation guidelines mentioned in this post.

What is your recommendation?

After studying the subject for years, using different products with different growers and testing the chemistry myself (preparing stabilized silicic acids and measuring active Si concentrations). Given the price of Si products and the chemistry involved, I would suggest anyone interested in Si supplementation in nutrient solutions to use a potassium silicate solid product. I would suggest to prepare a suitable stock with potassium silicate and potassium hydroxide to increase pH and stability and then prepare their nutrient solutions from dilutions of this stock. If a solid product like AgSil 16H is not available, then using a basic silicate concentrate product would be the next best choice. Usually preparing a more dilute stock from these products is recommended to ensure the stock already contains monomeric silicate.

I don’t think acid-stabilized silicon products or non-aqueous Si products are worth the price premium. If you’re having better results with a non-potassium silicate product compared to potassium silicate, bear in mind that this is likely because either the potassium silicate stock preparation and dilution were not done correctly or the product you’re using contains a substance different from Si that is giving you those effects. The stabilizing agents themselves are going to be much lower cost, so testing the eliciting effects of these agents might be more economical for you than using these expensive products long term.

In cases where mixing stocks and handling basic reagents is problematic or there is limited availability to adjust pH, then the use of non-aqueous Silicon reagents might be desirable. Non-aqueous silicon forms are also the most robust to mixing errors – wrong mixing order, mixing at variable pH, etc – because the hydrolysis reactions happen readily under a wide variety of conditions. However, my recommendation is to always couple these with carbon filtration to avoid potential issues from their organic side-products.

If you have issues with the use of soluble silicon sources – because of your initial water composition, injector limitations, cost, etc – and your media supports amending, I would also suggest considering using solid amendments to supplement Si (watch this video I made for more information). Amending can be a great choice, much more economical than soluble Si supplementation.

Do you have any questions about Si in nutrient solutions not addressed above? Feel free to leave a comment and I might also add it to the post!

I have discussed moisture content sensors extensively in the past. I have written posts about the use of capacitive moisture sensors to measure volumetric moisture content, including how to create sensor stations and how to calibrate them. However, while capacitive moisture content sensors can be a low cost alternative for low resolution monitoring of moisture content, more precise applications require the use of higher accuracy sensors, such as Time Domain Reflectometry (TDR) sensors. In this post I am going to show you how to connect a low cost microcontroller (NodeMCUv3) to a low cost TDR moisture content sensor. Note, some of the product links below are amazon affiliate links, which help support this blog at no additional cost to you.

While popular sensors like Teros-12 sensors cost hundreds of dollars, lower cost alternatives have been created by Chinese manufacturers. Using this github repository by git user Kromadg, I have been able to interface some of these low cost TDR sensors with a NodeMCUv3. The NodeMCUv3 is a very low cost microcontroller unit that you can get for less than 5 USD a piece. It is also WiFi enabled, so this project can be expanded to send data through Wifi to use in datalogging or control applications. For this project you will need the following things:

1. Micro USB cable
2. NodeMCUv3
3. THC-S RS485 sensor (Make sure to get the THC-S model)
4. TTL to RS485 communication board
5. Breadboard and jumper cables to make connections or cables and a soldering kit to make final connections.

The above diagram shows you how to connect the sensor, TTL-to-RS485 communication board and the NodeMCUv3. You will also want to make sure you install the ESP Software serial library in your Arduino IDE, as the normal Software Serial library won’t work. You can do this by downloading the zipped library from github and then using the Sketch->Include Library menu option. Once you do so, you can upload the following code into your NodeMCUv3.

#include <SoftwareSerial.h>
#include <Wire.h>

// This code is a modification of the code found here (https://github.com/kromadg/soil-sensor)

#define RE D2
#define DE D3

const byte hum_temp_ec[8] = {0x01, 0x03, 0x00, 0x00, 0x00, 0x03, 0x05, 0xCB};
byte sensorResponse[12] = {0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00};
byte sensor_values[11];

SoftwareSerial mod(D6, D5); // RX, TX

void setup() {
    Serial.begin(115200);
    pinMode(RE, OUTPUT);
    pinMode(DE, OUTPUT);
    digitalWrite(RE, LOW);
    digitalWrite(DE, LOW);
    delay(1000);
    mod.begin(4800);
    delay(100);
}

void loop() {
    /************** Soil EC Reading *******************/
    digitalWrite(DE, HIGH);
    digitalWrite(RE, HIGH);
    memset(sensor_values, 0, sizeof(sensor_values));
    delay(100);
    
    if (mod.write(hum_temp_ec, sizeof(hum_temp_ec)) == 8) {
        digitalWrite(DE, LOW);
        digitalWrite(RE, LOW);
        for (byte i = 0; i < 12; i++) {
            sensorResponse[i] = mod.read();
            yield();
        }
    }

    delay(250);

    // get sensor response data
    float soil_hum = 0.1 * int(sensorResponse[3] << 8 | sensorResponse[4]);
    float soil_temp = 0.1 * int(sensorResponse[5] << 8 | sensorResponse[6]);
    int soil_ec = int(sensorResponse[7] << 8 | sensorResponse[8]);

    /************* Calculations and sensor corrections *************/

    float as_read_ec = soil_ec;

    // This equation was obtained from calibration using distilled water and a 1.1178mS/cm solution.
    soil_ec = 1.93*soil_ec - 270.8;
    soil_ec = soil_ec/(1.0+0.019*(soil_temp-25));

    // soil_temp was left the same because the Teros and chinese sensor values are similar

    // quadratic aproximation
    // the teros bulk_permittivity was calculated from the teros temperature, teros bulk ec and teros pwec by Hilhorst 2000 model
    float soil_apparent_dieletric_constant = 1.3088 + 0.1439 * soil_hum + 0.0076 * soil_hum * soil_hum;

    float soil_bulk_permittivity = soil_apparent_dieletric_constant;  /// Hammed 2015 (apparent_dieletric_constant is the real part of permittivity)
    float soil_pore_permittivity = 80.3 - 0.37 * (soil_temp - 20); /// same as water 80.3 and corrected for temperature

    // converting bulk EC to pore water EC
    float soil_pw_ec;
    if (soil_bulk_permittivity > 4.1)
        soil_pw_ec = ((soil_pore_permittivity * soil_ec) / (soil_bulk_permittivity - 4.1) / 1000); /// from Hilhorst 2000.
    else
        soil_pw_ec = 0;

    Serial.print("Humidity:");
    Serial.print(soil_hum);
    Serial.print(",");
    Serial.print("Temperature:");
    Serial.print(soil_temp);
    Serial.print(",");
    Serial.print("EC:");
    Serial.print(soil_ec);
    Serial.print(",");
    Serial.print("READEC:");
    Serial.print(as_read_ec);
    Serial.print(",");
    Serial.print("pwEC:");
    Serial.print(soil_pw_ec);
    Serial.print(",");
    Serial.print("soil_bulk_permittivity:");
    Serial.println(soil_bulk_permittivity);
    delay(5000);
}

Note that RE and DE are not placed on digital pins 2 and 3, as other pins in the NodeMCUv3 carry out other functions and the board will not initialize if it has the RS485-to-TTL communicator connected through those pins. The R0 and RI pins are connected to digital pins D5 and D6, this is because in the NodeMCUv3 pins D7 and D8 are used in serial communication by the Serial swap command and therefore create conflicts if you use them with SoftwareSerial. The above digital pin distribution is one of the few that works well. Note that connecting RE or DE to digital pin 4 also works, but this means the blue LED on the NodeMCUv3 is powered on every time there is serial communication, a potentially undesirable effect if you’re interested in battery powering the device.

The board should now be printing all the measurements on your serial connection, so you should be able to see the readings through the Serial Monitor in the Arduino IDE. In the future I will be sharing how to expand this code to include WiFi and MQTT communication with a MyCodo server.

If you use this code please share your experience in the comments below!

During the past couple of years, cleaning products based on hypochlorous acid derived from electrolysis have become popular in the hydroponic industry. This is because, in the USA – per 40 CFR § 180.940 – hypochlorous acid products containing less than 200 ppm of active chlorine are exempted from many manufacturing and handling requirements and are therefore easy to produce and dispense to hydroponic growers. While more dilute, the formulations produced can often be much more stable than more concentrated products and still provide satisfactory cleaning results in a hydroponic reservoir. However, the products carry a lot of additional cost compared to traditional sodium hypochlorite based cleaning products. This is because more needs to be used – as they are more dilute – and the products themselves are often much more expensive.

In this post, I want to help you create a solution analogous to many commercially available, electrolytically derived hypochlorous acid cleaners, using products that are easily available and low cost. The resulting solution is – for all intents and purposes I can think of – equivalent to electrochemically derived hypochlorous acid, since the hypochlorite ion becomes protonated at low pH, generating the required substance during the preparation process. To create this formulation, I relied on the following documents and the scientific literature they referenced (1, 2, 3).

Important note. Hypochlorous acid is unstable in highly concentrated solutions. Increasing the concentration of the formulation below significantly can lead to potentially dangerous releases of chlorine gas when the pH is lowered. Work in a well ventilated area and do not exceed the concentration amounts recommended in this preparation. Work responsibly and make sure to read all the MSDS of the substances used and use appropriate personal protection equipment.

These are the things you will need for the preparation :

1. Freshly bought Clorox (7.4%). The solution should not be older than one week.
2. A 20 mL syringe.
3. Monopotassium Phosphate (MKP).
4. Sodium Chloride (table salt will do).
5. Magnesium Sulfate.
6. Sodium Tripolyphosphate.
7. A calibrated pH meter.
8. A scale to weigh salts, +/-0.1g.
9. A scale to weigh water +/-0.1kg
10. Distilled or RO water (tap water will not work). Distilled is preferable.
11. Clean plastic, air-tight container (at least 1gal) to store the resulting solution. The container should be opaque.

This is the procedure you should follow for the preparation of the hypochlorous acid solution (values for ~1.2 gallon, can be scaled up for larger amounts):

1. Calibrate your pH meter using fresh pH 4 and pH 7 buffer solutions.
2. Fill the container with 3.6 kg of distilled water, this will be referred to as the solution.
3. Weigh and add 0.5g of Sodium Chloride to the solution.
4. Stir until fully mixed.
5. Weigh and add 0.1g of Sodium tripolyphosphate to the solution.
6. Stir until fully mixed.
7. Measure 11mL of Clorox and add it to the solution. If you’re working with a bleach solution with concentration other than 7.4%, multiply 11mL by 7.4 and divide by your concentration to obtain the amount you should use in mL (for example, if using a 6% bleach solution, you would require 11*7.4/6 = 13.56mL).
8. Stir until fully mixed.
9. Weigh 0.5g of Monopotassium phosphate and add to the solution.
10. Stir until fully mixed.
11. Measure the pH of the mix. If the pH is >7 slowly add and fully mix small portions (~0.1g) of monopotassium phosphate until the pH is in the 6.5-7 range. Take at least 1 minute between additions to ensure the pH has stabilized before adding more.
12. Weigh and add 3.5g of Magnesium sulfate to the solution
13. Stir until fully mixed.
14. Add 0.9kg of water.
15. Confirm final pH is in the 6-7 range, you can add more monopotassium phosphate if needed to drop the pH.

This should provide you with a solution that is stable in the medium term and has the active chlorine concentration of a formulation similar to products like Athena Cleanse. The expected concentration of hypochlorous acid should be around 0.02% (200ppm). It can be used from 2 to 10mL/gal of hydroponic nutrient solution, depending on the severity of the problems that need to be solved. For overall maintenance and the solution of minor infections, dosages of 5mL/gal should be more than adequate. The Magnesium Sulfate and Sodium Chloride are added as stabilizing agents, while the mono potassium phosphate is added as a pH buffering agent and the sodium tripolyphosphate is a cleaning agent meant to keep irrigation lines clean (it can be omitted if this is not a concern). Note that the contributions of the mineral ions to a formulations nutrition at the applied concentrations are negligible. 

Please do let me know if you have any questions about the above preparation. If you have prepared it, please let us know how it went in the comments below!