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Nutrient deficiencies in plants: a visual guide and how to correct them

A nutrient-deficient plant is sending you a message. The problem is that message—written in yellow leaves, purple spots, or burnt tips—can be difficult to read if you don't know how to interpret it. And in Costa Rica, where the tropical climate adds its own layer of complexity, correct diagnosis is even more important.This comprehensive visual guide teaches you how to identify the most common deficiencies, understand their true causes (which are not always a lack of the nutrient in question), and correct them efficiently without damaging the plant.Before Diagnosing: The Most Common MistakeThe most frequent mistake we see among growers in CR is adding more nutrients when the real problem is pH. Most deficiencies are not due to a lack of the nutrient in the solution—they are because the pH is outside the optimal range and the available nutrients cannot be absorbed by the roots.Before adding any corrector, always check: pH of irrigation water (target: 5.8-6.0 in coco, 6.0-6.5 in soil) EC of runoff (if much higher than input EC, there is salt accumulation) pH of runoff (if too high or low from target, substrate needs correction) The Nutrient Availability Map by pHThis is key to understanding why pH matters so much. Each nutrient has an optimal pH window where it can be absorbed by the roots: Nutrient Best availability Blocked below Blocked above Nitrogen (N) 5.5 - 7.0 4.5 8.0 Phosphorus (P) 5.5 - 7.0 5.0 7.5 Potassium (K) 5.5 - 7.5 4.5 8.0 Calcium (Ca) 6.0 - 8.0 5.0 — Magnesium (Mg) 5.5 - 7.5 5.0 8.0 Iron (Fe) 5.0 - 6.5 — 6.5 Zinc (Zn) 5.0 - 6.5 — 7.0 Manganese (Mn) 5.0 - 6.5 — 7.0 The 9 Most Common Deficiencies: Diagnostic Guide1. Nitrogen (N) Deficiency 🟡How it looks: Uniform yellowing starting on older (lower) leaves and progressing upwards. Yellow leaves have no spots—the color is uniform and pale. Affected leaves eventually fall off on their own.Why it occurs: Nitrogen is a mobile nutrient—when scarce, the plant moves it from old leaves to new tissues. This is why symptoms first appear at the bottom.Causes in CR: pH too high (above 7.0 in coco), under-dosing of nutrients, or over-watering that washes nutrients from the substrate.Correction: Check and adjust pH. Increase nitrogen nutrient dosage (Pro Grow, Terra Vega). In urgent cases, foliar spray with diluted ammonium sulfate.2. Phosphorus (P) Deficiency 🟣How it looks: Purple or reddish hues on the underside of leaves, especially on stems and veins. Leaves may become darker than usual, with blue-green reflections. New shoots may be affected in severe cases.Why it occurs: Phosphorus is mobile. The plant redistributes it from old leaves to new ones and reproductive organs.Causes in CR: Low substrate temperature (below 15°C in mountainous areas), pH above 7.0 or below 5.0, or excess zinc competing with phosphorus.Correction: Adjust pH to optimal range. Ensure substrate temperature is not too low. Apply soluble phosphorus (PK 13/14, Bloom) in the irrigation solution.3. Potassium (K) Deficiency 🟠How it looks: Burnt leaf edges and tips (marginal necrosis), starting on middle and old leaves. Edges dry out and curl upwards. Brown spots on the edges are characteristic.Why it occurs: Potassium regulates osmoregulation and water transport in the plant. Without sufficient K, edge tissues dry out first.Causes in CR: Excess calcium or magnesium competing with potassium, high pH, or under-dosing during the flowering stage.Correction: Increase Bloom or Stack dosage (rich in K). Check for excessive CalMag that might be blocking absorption.4. Calcium (Ca) Deficiency 🟤How it looks: Brown or necrotic spots on new leaves (calcium is immobile, so symptoms appear in young tissues). Leaves may distort or curl. New shoots may be affected first.Why it occurs: Calcium is immobile in the plant. It cannot be redistributed from old to new tissues. Therefore, if there is a lack of available calcium in the solution, symptoms appear in the newest tissues.Causes in CR: Low pH (below 5.0 in coco), high humidity reducing transpiration (calcium moves with water flow), or coco substrate without adequate CalMag.Correction: Add Pro Core or CalMag to the solution. Ensure pH is above 5.5. Improve air circulation to increase transpiration.5. Magnesium (Mg) Deficiency 🟢How it looks: Interveinal chlorosis—leaf veins remain green while the tissue between them turns yellow or pale. Starts on old or middle leaves and progresses upwards. In severe cases, yellow areas turn brown and dry.Why it occurs: Magnesium is the central atom of the chlorophyll molecule. Without sufficient Mg, chlorophyll cannot form correctly, affecting photosynthesis and giving the characteristic interveinal pattern.Causes in CR: Coco naturally competes with magnesium. In areas with very alkaline water, excess calcium can inhibit magnesium absorption. In the rainy season with high humidity and lower transpiration, absorption also decreases.Correction: Foliar spray of magnesium sulfate (epsom salt) at 0.2-0.5% for quick correction. Increase CalMag dosage in irrigation. Check pH.6. Iron (Fe) Deficiency 🟡How it looks: Interveinal chlorosis similar to magnesium, but appearing on the newest shoots and leaves (iron is immobile). Young leaves look pale or almost white while old ones remain green.Causes in CR: High pH (above 6.5 in coco, above 7.0 in soil). Iron precipitates and becomes inaccessible to roots when pH is high.Correction: Lower irrigation pH to 5.8-6.0. In urgent cases, foliar spray with iron chelate (FeSO4 or Fe-EDTA at 0.1%). Iron chelates are absorbed directly through leaves regardless of substrate pH.7. Zinc (Zn) DeficiencyHow it looks: Small, deformed new leaves, with reduced internodal distance. Interveinal chlorosis on new leaves similar to iron. New shoots may appear mottled or with asymmetrical leaves.Causes in CR: High pH (above 6.5), excess phosphorus competing with zinc, or very alkaline substrates.Correction: Adjust pH. Zinc chelates (Zn-EDTA) are the most effective and rapid correction. Many quality nutrients already include chelated zinc.8. Manganese (Mn) DeficiencyHow it looks: Interveinal chlorosis on young leaves (similar to iron and zinc), but spots may have a more grayish or silvery tone instead of bright yellow. Progresses from middle to new leaves.Causes in CR: High pH, excess iron or calcium, or old, depleted substrates with little available manganese.Correction: Adjust pH. Apply diluted manganese sulfate via foliar. Complete micronutrients (trace elements) containing Mn are a good preventive option.9. Sulfur (S) DeficiencyHow it looks: Uniform yellowing of new leaves (similar to nitrogen but in young tissues). Stems may turn purple.Causes: Uncommon with quality nutrients, as most fertilizers contain sulfates as carriers for other nutrients. Can occur with very pure water (reverse osmosis without remineralization).Correction: Magnesium sulfate (epsom salt) provides both magnesium and sulfur. Most complete nutrients have sufficient sulfur.Toxicities: When the Problem Is ExcessToxicities are as dangerous as deficiencies, and among enthusiastic growers—who think more nutrients = better results—they are very common. Toxicity Symptoms Most common cause Nitrogen Extremely dark leaves, tips curling downwards (claw) Veg overdose during flowering Calcium Irregular brown spots, blocking of Mg and K Excess CalMag + alkaline water Iron Brown spots on old leaves, bronze discoloration Very low pH (below 5.0) Excess salts Widespread tip burn, stressed plants EC too high, lack of flush Step-by-Step Diagnostic ChecklistWhen you see something unusual in your plants, follow this process before adding anything: Where on the plant do symptoms appear? (old leaves = mobile nutrient; new leaves = immobile nutrient) Measure the pH of irrigation water and runoff Measure the EC of irrigation and runoff (very high runoff = salt accumulation) Check substrate temperature (low temperature blocks absorption) Check substrate moisture (over-watering reduces root oxygen) Compare with images and descriptions in this guide Act on pH first if it is out of range Essential Diagnostic Tools pH meter: Absolutely indispensable. Basic pH pens work, but calibrating them regularly with buffer solutions is critical for accurate readings. EC/TDS meter: To measure the total salt concentration in your solution and runoff. Infrared thermometer: To measure substrate temperature without introducing contamination. Loupe or pocket microscope: Useful for distinguishing deficiencies from small pests (spider mites, etc.). Frequently Asked Questions How to tell if it's a deficiency or a pest? Deficiencies produce uniform patterns—yellowing follows a defined pattern (interveinal, marginal, tip). Pests produce irregular spots, dots, bites, or visible traces of excrement. A 30x loupe resolves the doubt in seconds. How long does it take to correct a deficiency? Depends on the nutrient and correction method. Foliar sprays of micronutrients (iron, zinc, magnesium) show results in 48-72 hours. Corrections via irrigation take 5-10 days. Damaged leaves do not recover—the indicator of success is that new tissues grow healthy. Is it normal for lower leaves to turn yellow at the end of flowering? Yes, it is completely normal. The plant redirects nitrogen and other mobile nutrients from old leaves to developing fruits and flowers. Some yellow and falling leaves at the end of flowering are a sign of a plant working correctly, not a disease. Can I use foliar spray during flowering? With care. Foliar sprays in late flowering can cause excessive humidity in flowers and favor Botrytis. If you need to foliar spray during flowering, do it early in the morning with good air circulation and only in the first few weeks (up to week 3). After that, correct only via irrigation. ConclusionDiagnosing deficiencies is a skill that improves with practice. At first, every yellow-leaved plant can seem like a mystery. Over time, you'll be able to identify most problems in seconds just by seeing the pattern of symptoms. Most importantly: always check pH before adding more nutrients. 80% of deficiencies in indoor crops in Costa Rica are due to incorrect pH, not an actual lack of the nutrient in question.