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Hydroponics EC, TDS & pH: Targets, Meters, and Management (2026)

In soil-based agriculture, the soil matrix buffers pH shifts, holds nutrients in reserve, and forgives inconsistent management over weeks. Hydroponic systems have none of that. pH can shift a full unit overnight. A salt buildup that would take months to cause problems in field soil can damage roots in days in a recirculating reservoir. EC, TDS, and pH are the three measurements that tell you whether your nutrient solution is feeding your plants or harming them — and getting them right is the whole game in soilless production.

This guide synthesizes research and practitioner guidance from six university extension programs — OSU, UF/IFAS, Penn State, Cornell CEA, Purdue, and Virginia Tech — plus a 2026 peer-reviewed automation study. Where sources disagree on specific targets, we note it.

What EC Actually Measures

Electrical conductivity (EC) measures how readily a solution allows an electric current to pass through it. When fertilizer salts dissolve in water, they dissociate into charged ions — potassium nitrate (KNO₃) becomes K⁺ and NO₃⁻, for example. These ions carry electric current. More ions present = higher conductivity. EC is therefore a direct proxy for total dissolved fertilizer concentration — the more fertilizer in the water, the higher the EC.

This makes EC the fastest and cheapest way to know whether your nutrient solution is concentrated enough or too dilute. But EC has a critical limitation every grower needs to understand: it measures total ions, not the right ions. As plants selectively absorb nutrients at different rates — rapidly taking up nitrogen, potassium, phosphorus, and manganese; more slowly absorbing calcium, magnesium, and sulfur — the ionic composition of a recirculating solution diverges from the original formula. A solution maintained at target EC by topping up with premixed fertilizer can have very different elemental ratios from a fresh batch, especially after 1–2 weeks. This is why periodic complete replacement of nutrient solution is necessary regardless of EC readings.

EC vs. TDS vs. ppm — The Conversion Problem

This is the single most common source of confusion among new hydroponic growers. TDS meters and EC meters both measure the same thing — electrical conductivity — but display the result differently. EC meters show the actual conductivity in mS/cm or µS/cm. TDS meters apply a conversion factor and display the result in ppm (parts per million), as if it were a direct weight-based dissolved solids measurement. It isn't — it's a calculated approximation.

The problem is that three different conversion scales are in common use, and your meter's documentation may not clearly state which one it uses:

EC Units and Conversion

The CF (Conductivity Factor) scale multiplies mS/cm by 10 and is common in Australia and some European grower communities. CF 15 = 1.5 mS/cm. Published targets from US university extension sources use mS/cm or dS/m (identical values) — convert CF to mS/cm by dividing by 10.

Crop-Specific EC and pH Targets

The following tables are drawn from OSU Extension HLA-6722 (Dunn & Singh, 2017), supplemented with Cornell CEA Lettuce Handbook data and Virginia Tech Extension SPES-464. Seedling and germination stages require 50–70% of the production EC target — osmotic stress from high EC is a leading cause of poor germination and seedling damping-off.

Leafy Greens and Herbs

Fruiting Vegetables

Root Crops, Brassicas & Others

Growth Stage EC Adjustments

Water Source Assessment

Before adding any fertilizer, measure and record your source water EC and pH. Natural water contains dissolved minerals — calcium, magnesium, sodium, bicarbonates, chlorides, sulfates — that contribute to EC and affect pH. This baseline EC must be subtracted to understand how much of your nutrient solution's total EC is actually fertilizer.

EC Measurement and Management Protocol

pH and Nutrient Availability

pH is the most powerful regulator of nutrient availability to plant roots in hydroponic systems. Penn State Extension identifies the optimal range as 5.5–6.5, within the broader acceptable range of 5.0–7.0. Outside 5.5–6.5, specific nutrients lock into chemical forms that plant roots cannot absorb — even when they're present in the solution in adequate quantities.

Alkalinity — The Hidden pH Driver

Alkalinity is one of the most misunderstood water quality parameters in hydroponics. It is not the same as pH. Alkalinity measures the capacity of the water to resist acidification — its buffering power — primarily from dissolved bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻) ions.

High alkalinity water pushes nutrient solution pH steadily upward, requiring repeated acid additions to maintain the target range. More dangerously: a water source can have a moderately acceptable pH (say, 7.2) but very high alkalinity (200+ ppm bicarbonate), meaning it will continuously drive pH up in a recirculating system regardless of initial adjustment. Testing only pH and not alkalinity misses this driver entirely.

pH Measurement and Correction Protocol

When to Replace Nutrient Solution

This is one of the most economically significant decisions in recirculating hydroponics. Two university sources give slightly different guidance:

• Purdue Extension (HO-286-W): replace every 7–10 days during active production in recirculating systems.
• OSU Extension (HLA-6722): complete replacement every two weeks as a general practice.

The reason for replacement is not EC drift — it's compositional drift. As plants absorb nutrients selectively, the ratios of ions in the recycled solution change. A solution held for two weeks and continuously replenished to maintain EC with premixed fertilizer may have elevated sodium, chloride, sulfate, or calcium relative to the original formula, while being depleted in the specific micronutrients the crop is actively pulling. The UF/IFAS 2025 study found that only a small fraction of most elements added were actually assimilated by the lettuce plants — the majority accumulated or were lost — suggesting that periodic fresh-batch replacement is more effective than indefinite topping-up.

EC-Related Nutrient Problems

Choosing an EC/TDS/pH Meter for Hydroponics

For most growers, a combination EC + pH meter is the most practical choice — it reduces the number of instruments to calibrate, carry, and maintain. A few practical criteria:

The Apera PC60 measures EC, TDS, pH, salinity, and temperature in one probe and is well-suited to hydroponic use. The Myron L Ultrameter II is the professional-grade choice with the 442™ Natural Water standard, four-electrode conductivity cell, and User mode for custom TDS conversion ratios — important for hydroponic nutrient solution chemistry that differs from natural water.

UF/IFAS Study: Does Higher EC Grow More Lettuce?

A 2025 University of Florida IFAS study (Vought, Bayabil, and Martin-Ryals, EDIS Publication AE610) directly tested whether maintaining a higher EC in NFT lettuce production improves growth and nutrient use efficiency. The results challenge the common assumption that more fertilizer leads to more growth.

The study compared EC treatments of 1.2 dS/m (low fertilizer rate) and 1.6 dS/m (high fertilizer rate) in a controlled NFT system over 35 days. The fertilizer was a standard commercial premixed formula (Chem Gro 8-15-36 + calcium nitrate + magnesium sulfate). When EC fell below the set point, nutrient solution was added to restore the target level.

The key finding: the effect of EC level on lettuce biomass was minimal. The 1.6 dS/m treatment did not enhance lettuce growth or fresh/dry weight compared to 1.2 dS/m. More fertilizer did not produce more lettuce.

More significantly, the researchers found that only small fractions of most added nutrients were assimilated into plant biomass. The majority either accumulated in the recirculating solution or were lost to other pathways. This directly implicates premixed fertilizers with fixed nutrient ratios: they add all elements in set proportions regardless of what the crop actually needs at each growth stage, leading to systematic overloading of some nutrients and accumulation in the solution.

Automation and Smart Control

Manual EC and pH monitoring is feasible for small operations but becomes increasingly labor-intensive at commercial scale. OSU Extension noted in 2017 that automated monitoring systems cost $500–$4,000, with costs continuing to fall. Commercial options include Autogrow, Intellidose, Hanna Instruments GroLine, and CropKing Fertroller.

A 2026 peer-reviewed study at Institut Teknologi Sepuluh Nopember (ITS, Indonesia) demonstrated a fuzzy logic controller for simultaneous regulation of pH, TDS, and LED light intensity in a compact urban hydroponic system. The system grew lettuce, pak choi, and spinach. Compared to uncontrolled growing conditions, the fuzzy logic system improved plant growth by 35–50% and reduced LED energy consumption by 20% via PWM dimming. After a simulated disturbance, the system recovered to target ranges within 10–15 minutes. Crucially, the system applied independent corrections — if only pH was out of range, only the pH solenoid activated, without disturbing TDS or light intensity, preventing the cascade errors common in simpler binary control systems.

FAQ

What EC should my hydroponics nutrient solution be?

Lettuce and leafy greens: 1.2–2.0 mS/cm (Cornell CEA targets 1.5–1.8 mS/cm for floating raft lettuce production). Fruiting vegetables (tomatoes, peppers): 2.0–3.5 mS/cm. Herbs: 1.0–1.6 mS/cm. Seedlings: reduce to 50–70% of the production target. See the crop-specific tables above for 25+ crops. Always reduce EC during heat stress.

What is the difference between EC and TDS in hydroponics?

Both measure dissolved ion concentration, but TDS meters convert the EC reading to ppm using one of three different scales (500, 640, or 700 — producing a 40% range from the same sample). EC meters display the actual conductivity in mS/cm, which is unambiguous. For hydroponics, use mS/cm if possible. If your meter only shows ppm, check which conversion scale it uses before comparing with crop target tables. Full explanation above.

What pH should hydroponic water be?

5.5–6.5 for most crops. This is where all nutrients are most available. Below 5.5, calcium and magnesium availability drops. Above 6.5, iron, manganese, and micronutrients lock up. Most nutrient deficiency symptoms in hydroponics are pH-driven unavailability, not true absence from solution — check pH before adding more nutrients.

Can I use a TDS meter for hydroponics instead of an EC meter?

Yes, but identify which ppm scale your meter uses (500, 640, or 700). Most budget pens use the 500 scale. Crop guidelines are published in mS/cm — to convert your ppm to mS/cm: divide by 500 (if using 500 scale), 640, or 700. An EC meter displaying mS/cm eliminates the ambiguity entirely.

How often should I change hydroponics nutrient solution?

Every 7–10 days (Purdue Extension) to every 2 weeks (OSU Extension) in recirculating systems. The reason is compositional drift, not just EC drift — selective plant uptake changes nutrient ratios over time. A 2025 UF/IFAS study found most added nutrients accumulated in the solution rather than being assimilated by lettuce, underscoring the need for periodic fresh-batch replacement.