Getting your EC right is one of the highest-leverage decisions in any cannabis grow. Too low and the plant starves; too high and you trigger nutrient burn or lockout. This guide covers exact mS/cm and PPM targets for every stage from seedling to flush, full NPK ratios, cal-mag requirements, medium-specific adjustments, and a complete deficiency diagnosis table — so you can feed with confidence every single feed.
Use this quick-reference table for target EC ranges at each growth stage. PPM values use the 500 scale (EC multiplied by 500). For the 700 scale multiply EC by 700.
| Growth Stage | EC (mS/cm) | PPM (500 scale) | Status |
|---|---|---|---|
| Seedling (0–2 weeks) | 0.8–1.2 | 400–600 | Sensitive — go low |
| Early Veg (weeks 2–4) | 1.2–1.8 | 600–900 | Ramp up gradually |
| Late Veg (weeks 4–6) | 1.8–2.4 | 900–1200 | Peak veg feeding |
| Early Flower (weeks 1–4) | 2.0–2.6 | 1000–1300 | Transition to bloom |
| Mid/Late Flower (weeks 4–8) | 2.2–3.0 | 1100–1500 | Monitor daily |
| Pre-Harvest Flush | 0.0–0.4 | 0–200 | Plain water only |
EC — electrical conductivity — measures the total concentration of dissolved mineral salts in your nutrient solution. Cannabis roots absorb water and ions simultaneously; the osmotic gradient between the root zone solution and plant cells drives nutrient uptake. When EC is correctly dialed in, the plant draws water and minerals in proportion to its metabolic needs. When EC drifts out of the target range in either direction, cascading problems follow quickly.
Excessive EC creates an osmotic imbalance where water is actually drawn out of root cells rather than into them — sometimes called reverse osmosis at the root level. The first visible symptom is brown, crispy leaf tips (nutrient burn), appearing at the very tip of fan leaves first. As EC continues rising unchecked, you will see widespread tip burn, unusually dark-green foliage, and eventually wilting even though the reservoir is full. In severe cases, root tissue becomes permanently damaged, reducing the plant's ability to absorb water and nutrients even after EC is corrected. Dilute with plain pH-adjusted water rather than doing a complete reservoir swap to avoid shocking the root zone.
Low EC means insufficient dissolved minerals available for uptake. The plant will show pale, light-green coloration as nitrogen runs short, then progressively develop stage-specific deficiency symptoms. Yellowing of lower and older leaves usually signals nitrogen deficiency first. Slow growth, thin stems, small internodal spacing, and reduced lateral branching are common low-EC symptoms that appear before visible leaf changes. In recirculating DWC systems, plants actively feeding on a properly sized reservoir will naturally lower EC over time — check daily and top off with fresh nutrient solution to maintain your target EC.
Freshly germinated cannabis seedlings have tiny, undeveloped root systems with minimal surface area. The seed's stored endosperm nutrients sustain the plant for the first 3–5 days post-germination. Begin adding nutrients at EC 0.8 when the seedling has produced its first set of true leaves. Many experienced hydro growers start even lower — EC 0.5–0.6 in pure RO water for the first week — relying on cotyledon reserves before introducing any feed. Overfeeding seedlings is the single most common beginner mistake: at EC 2.0+ mS/cm, seedling roots will experience burn within 24 hours. Increase EC by only 0.2 mS/cm increments as roots develop and plants respond positively.
During vegetative growth, cannabis rapidly expands its root mass, stem girth, and leaf canopy. Nitrogen demand reaches its peak in this phase. Begin early veg at EC 1.2–1.4 and increase by 0.2–0.3 mS/cm per week as the plant establishes itself. By late veg with a well-rooted plant, most indica and hybrid strains handle EC 2.0–2.4 comfortably. Sativa-dominant cultivars often prefer slightly lower maximum EC — keep them at 1.6–2.0 in veg as they are more prone to tip burn. Autoflowering strains are generally more sensitive overall; limit veg EC to 1.0–1.8 mS/cm throughout their abbreviated life cycle and never push to the top of the range.
At the flip to 12/12 photoperiod — or when autoflowers initiate flower spontaneously — nutrient demand shifts from nitrogen-heavy to phosphorus- and potassium-heavy. The plant's nitrogen uptake decreases while demand for P and K spikes to support bud site development, terpene synthesis, and resin production. EC can be maintained at late-veg levels or slightly increased. During peak flowering (weeks 4–8 of flower), many growers using quality nutrient lines push EC to 2.5–3.0 mS/cm successfully. Monitor leaf tips daily as a warning indicator — tip burn signals you have exceeded the plant's current tolerance. In the final two weeks before harvest, EC is intentionally dropped to zero during the flush period.
| Stage | EC (mS/cm) | PPM (500 scale) | PPM (700 scale) |
|---|---|---|---|
| Seedling | 0.8–1.2 | 400–600 | 560–840 |
| Early Veg | 1.2–1.8 | 600–900 | 840–1260 |
| Late Veg | 1.8–2.4 | 900–1200 | 1260–1680 |
| Early Flower | 2.0–2.6 | 1000–1300 | 1400–1820 |
| Peak Flower | 2.4–3.0 | 1200–1500 | 1680–2100 |
| Pre-Harvest Flush | 0.0–0.4 | 0–200 | 0–280 |
NPK describes the ratio of nitrogen (N), phosphorus (P), and potassium (K) in your nutrient formula. Getting the right NPK ratio for each stage is as important as hitting the correct EC — feeding a flowering-stage formula during veg will stunt growth, and a veg-stage formula during flower produces airy, nitrogen-saturated buds with poor potency and density.
Nitrogen is the primary driver of vegetative growth. It is the central atom in chlorophyll molecules, the building block of amino acids and proteins, and essential for rapid leaf and stem expansion. During veg, your nutrient formula should provide roughly 3 parts nitrogen to 1 part phosphorus to 2 parts potassium — a 3-1-2 or similar N-dominant ratio. Most commercial "Grow" or "Veg" formulas are N-dominant. Avoid using bloom formulas during veg — the low nitrogen will cause pale, stunted growth and slow internodal development significantly.
In the first 1–2 weeks of flower, gradually transition from your veg formula to a bloom formula by mixing both in decreasing/increasing proportions. By week 3 of flower, shift entirely to a bloom formula with elevated P and K. Phosphorus supports bud site formation and root activity; potassium drives sugar and carbohydrate transport, resin gland production, and overall bud density. In the final 2 weeks, some growers use a dedicated "finishing" or "ripening" supplement that provides zero nitrogen with very high potassium to encourage final swell, trichome development, and harvest-ready maturation.
| Stage | NPK Ratio | Primary Focus | Example Formula Type |
|---|---|---|---|
| Seedling | 2-1-2 | Balanced, low strength | Seedling/starter formula |
| Vegetative | 3-1-2 | Nitrogen for leaf & stem | "Grow" or "Veg" base |
| Early Flower | 2-2-3 | Transition — reduce N | Bloom transition mix |
| Peak Flower | 1-3-2 | P/K for bud development | "Bloom" base formula |
| Late Flower / Ripening | 0-3-3 | No N, max K | "PK Booster" / finisher |
| Flush | 0-0-0 | Clear residual salts | Plain water or flushing agent |
Calcium and magnesium are often called "secondary" macronutrients, but in cannabis cultivation — especially in hydro systems running RO or soft water — they are just as critical as NPK. Both are consumed in large quantities throughout the plant's life cycle and are frequently the limiting factor when deficiency symptoms appear despite a correct base-nutrient EC.
Calcium is immobile in the plant, meaning deficiencies manifest in new growth rather than old leaves — the opposite pattern from nitrogen or magnesium. Calcium is essential for cell wall integrity, root tip development, and the transport of other nutrients across membranes. Deficiency produces brown spots and patches on newer leaves, distorted or cupped new growth at the growing tips, and eventual necrosis of shoot apices in severe cases. Target 150–200 ppm calcium in your final nutrient solution. Most dedicated Cal-Mag supplements provide 3–5% Ca by weight; dose at 3–5 ml per gallon as part of your base mix.
Magnesium is the central atom of every chlorophyll molecule — without adequate Mg, photosynthesis collapses. It is a mobile nutrient, so deficiency appears first on older, lower leaves as interveinal chlorosis: the leaf veins stay green while the tissue between them turns yellow or lime-green. Heavy LED growers and coco coir users are especially prone to magnesium issues because LEDs increase the photosynthetic demand for Mg, and coco can bind it. Target 50–75 ppm magnesium in solution. Epsom salt (magnesium sulfate) at 1 teaspoon per gallon is a reliable standalone Mg supplement when Cal-Mag is unavailable.
| Nutrient | Target in Solution | Deficiency Signs | Note |
|---|---|---|---|
| Calcium (Ca) | 150–200 ppm | Brown spots on new leaves, distorted growing tips | Immobile — new growth affected |
| Magnesium (Mg) | 50–75 ppm | Interveinal yellowing on older leaves | Mobile — old leaves affected first |
| Sulfur (S) | 50–100 ppm | Uniform yellowing of new growth, similar to N deficiency | Usually sufficient in base nutes |
| Iron (Fe) | 2–5 ppm | Bright yellow new growth with green veins | pH-sensitive — check pH first |
EC targets are not universal — the growing medium significantly affects how plants experience the nutrient solution. Adjust your targets based on medium characteristics to avoid chronic over- or under-feeding even when measuring solution EC correctly.
| Medium | EC Adjustment vs. Baseline | Reason |
|---|---|---|
| DWC / Recirculating Hydro | Use targets as listed | Direct root-to-solution contact — this is the baseline reference system |
| Coco Coir | Run +0.2–0.4 higher | Coco buffers and cation-exchanges Ca and Mg; slightly higher EC compensates for binding sites |
| Rockwool / Grodan | Match DWC targets | Fully inert medium; nutrient availability mirrors solution EC closely |
| Perlite / Leca | Match DWC targets | Inert, fast-draining; roots access available solution directly without buffering |
| Amended Potting Soil | Run 0.5–1.0 lower | Soil already contains buffered organic nutrients; avoid overloading cation exchange capacity |
EC problems manifest differently depending on whether the root cause is nutrient burn, pH-induced nutrient lockout, or a true nutrient deficiency. Use this reference table to narrow down the cause before adjusting your reservoir — making the wrong correction can worsen the problem.
| Problem | EC Reading | Visual Symptoms | First Action |
|---|---|---|---|
| Nutrient Burn | Too high (>3.2) | Brown crispy tips on fan leaves, dark over-green foliage | Dilute reservoir with plain water |
| Nutrient Lockout | Normal or elevated | Deficiency symptoms despite correct EC reading | Check and correct pH first |
| Nitrogen Deficiency | Too low (<1.0) | Yellowing starts on lower/older leaves, overall light green | Increase EC; use N-dominant formula |
| Phosphorus Deficiency | Low or pH out of range | Purple/reddish stems and leaf undersides | Check pH (target 5.5–6.5); adjust |
| Iron Deficiency | Usually pH-related | Bright yellow new growth with green veins (interveinal chlorosis on new leaves) | Lower pH to 5.8–6.0 immediately |
| Magnesium Deficiency | Low Cal-Mag | Interveinal chlorosis on older leaves; veins stay green | Add Cal-Mag supplement 5 ml/gal |
| Calcium Deficiency | Low Cal-Mag or pH high | Brown spots on new growth, distorted growing tips | Add Cal-Mag; check pH is not above 6.5 |
Flushing is the practice of running plain, pH-adjusted water — with no added nutrients — through the root zone in the final days or weeks before harvest. The goal is to stop nutrient loading into plant tissue and encourage the plant to metabolize its stored mineral reserves. Many experienced growers report that properly flushed cannabis burns more cleanly, produces whiter ash, and has improved flavor compared to unflushed flower with high residual mineral salt content.
Begin flushing when the plant reaches its target harvest window — typically when 70–90% of trichomes have turned milky white with few to no clear trichomes remaining, and 5–15% have begun to turn amber depending on your desired effect profile. In calendar terms, this is usually the final 7–14 days of the expected flowering period. Match flush duration to your medium: DWC and recirculating NFT require only 3–5 days; coco coir and rockwool benefit from 5–7 days; organic soil may need 10–14 days to fully clear buffered nutrients.
During the flush, use plain water adjusted to pH 5.8–6.2 with EC measuring 0.0–0.4 mS/cm at most. Some growers use commercial flushing agents such as Florakleen, Sledgehammer, or Final Phase — these chelating agents help break down and mobilize mineral salt deposits from growing media and root surfaces. Even with a flushing agent, EC should remain below 0.5 mS/cm. A successful flush is visually confirmed by fan leaves progressively yellowing and the plant entering a natural senescence process as it consumes its stored nitrogen reserves — this color fade is expected and desired in the final days before cutting.
GrowAI tracks pH, EC, temperature, humidity, VPD and CO₂ — and alerts you before problems hurt your yield.
Start Free TrialCannabis seedlings are highly sensitive to nutrient strength and should be started at EC 0.8–1.2 mS/cm (400–600 ppm on the 500 scale). Seedlings have very small root systems and cannot handle concentrated nutrients — over-feeding at this stage is one of the most common mistakes growers make. Some growers start even lower at EC 0.6 for the first few days after sprouting, particularly in DWC. Gradually increase EC as the first true leaves develop and roots become visible, usually after 7–10 days. Always use pH-adjusted water (pH 5.8–6.2) even at low EC during seedling stage.
The 500 scale (also called TDS-500) multiplies EC (in mS/cm) by 500 to get PPM, while the 700 scale (TDS-700) multiplies by 700. So 2.0 mS/cm equals 1000 ppm on the 500 scale but 1400 ppm on the 700 scale. Most North American cannabis nutrient brands (General Hydroponics, Advanced Nutrients, Fox Farm) use the 500 scale in their feeding charts. European brands typically reference EC in mS/cm directly. Always confirm which scale your meter uses before following a nutrient schedule — mixing up scales leads to under- or over-feeding.
Nutrient burn in cannabis appears as brown, crispy tips on the leaves — beginning at the very tip of the blade and working inward as severity increases. It is caused by EC being too high, which draws water out of leaf cells through osmotic pressure. Affected leaves feel dry and brittle at the tips while the rest of the leaf remains green. Nutrient burn cannot be reversed on affected tissue, but reducing EC to the correct target range will stop the progression. Do not confuse nutrient burn with wind burn (which causes clawing) or potassium deficiency (which also causes brown edges but starts at leaf margins, not tips).
Calcium and magnesium are secondary macronutrients that cannabis consumes in large quantities, particularly during vegetative growth and mid-flower. In hydroponic systems using RO (reverse osmosis) or very soft municipal water, these minerals are absent from the water baseline and must be supplemented. Calcium deficiency shows as brown spots, necrotic patches, and distorted new growth. Magnesium deficiency presents as interveinal chlorosis — yellowing between leaf veins — starting on older leaves. Most growers add 5 ml/gallon of a dedicated Cal-Mag supplement to their base nutrient mix, or use tap water with moderate hardness (150–200 ppm) as a natural source.
Flushing cannabis is the practice of running plain, pH-adjusted water through the root zone in the final 1–2 weeks before harvest to clear residual nutrients from the plant and growing medium. The flush water should have an EC of 0.0–0.4 mS/cm (just pH-adjusted water, possibly with a flushing agent). In hydro systems, a 3–5 day flush is typically sufficient; in coco, 7 days; in soil, 7–14 days. Signs that the flush is working include older leaves yellowing and the plant beginning to cannibalize stored nutrients — this is normal and desired. Flushing aims to improve the final taste and burn quality of the dried flower.