Library › Part E · Article 40 in the series · Level 2 · 11 min read

Diagnosing nutrient deficiencies and toxicities

Standfirst: After reading this article you will be able to look at a plant with yellow, brown or deformed leaves and narrow the cause to one or two elements by asking which leaves show it first and what the pattern is. You will then know how to confirm the diagnosis with a solution test, a tissue analysis and a corrective dose, and why in hydroponics the solution is often not short of the element at all.

Before you read: What plants are made of: the essential elements, How roots take up nutrients, pH, Managing a recirculating solution.

Why the position of the symptom names the element

A plant that runs short of an element keeps its growing tip supplied for as long as it can. It does this by moving the element out of mature leaves through the phloem, the tissue that carries sugars and dissolved nutrients from leaves to the places where the plant is growing. Some elements move easily in the phloem; others, once built into a leaf, stay there^2,6.

If an element is mobile, deficiency symptoms tend to appear first in older leaves; if it is immobile, they appear first in younger leaves². Taiz and Zeiger name nitrogen, phosphorus and potassium as elements that move readily from leaf to leaf, and boron, iron and calcium as relatively immobile in most species². Marschner’s diagnostic method starts from the same point¹: symptoms in young leaves and growing points mean the plant could not move enough of the element there⁶.

The second clue is the pattern. Chlorosis is yellow tissue and necrosis is brown tissue³; necrotic spots are spots of dead tissue². Interveinal chlorosis, yellow tissue between green veins, appears because the chlorophyll along the veins lasts longer than that between them². Marginal necrosis starts at the leaf edge and spreads inward³.

The third clue is that a nutrient disorder is symmetric. Leaves of the same age on a plant show nearly the same symptoms, with a gradation from old to young, while diseases and pests produce symptoms placed at random on the plant and in the crop¹. In hydroponics, Resh adds, a nutritional disorder usually appears on all plants at the same time³.

The key: leaf age, then pattern

The table combines Marschner’s principles of visual diagnosis¹ with Resh’s dichotomous key³ and Taiz and Zeiger’s descriptions². Marschner’s table puts some elements in brackets without a legend¹; we read them as the less likely candidates for that pattern.

The sources do not agree on every row, and the disagreements are worth knowing. Resh places zinc and molybdenum among the older-leaf deficiencies³, Taiz and Zeiger list zinc and molybdenum as mobile², while Marschner places zinc on young leaves¹. For sulfur, Taiz and Zeiger say chlorosis usually starts in mature and young leaves but in many species appears in all leaves at once or even in the older ones first². Manganese appears on younger or older leaves depending on species and growth rate². Treat the key as a short list of suspects, not a verdict.

Rule out the look-alikes first

Before using the key, Resh checks for causes other than nutrition: insects, diseases, pesticide damage, pollution, water stress, and light and temperature injury³. Herbicide drift such as 2,4-D deforms leaves, and excess sunlight or heat burns leaf margins³; some virus diseases also mimic deficiencies². Root rot is covered in Pests and diseases in soilless systems and light injury in Light. The oldest leaves of a healthy plant also yellow as it ages; in this article’s reading, one or two yellow leaves at the base with fully green leaves above them are not yet a disorder. Look at the roots before you look at the recipe.

Confirming the diagnosis

Visual diagnosis narrows the field but, except in cases such as iron chlorosis, is not a sound basis for deciding what to add¹; a shortage that reduces growth only slightly usually shows no specific symptom¹. Three checks turn a suspicion into a diagnosis.

The solution. Measure pH, EC, temperature and, if you can, dissolved oxygen (Meters and measurement). Morgan, writing in Mason’s book, recommends a full analysis of the solution or of the drainage: a low level of an element confirms that the supply is the problem; if the level is adequate, the deficiency is induced⁵.

The tissue. A laboratory reports each element per kilogram of leaf dry matter, read against a sufficiency range, the concentrations at which the element does not limit growth¹. For nitrogen, potassium and magnesium, which the plant moves out of old leaves, mature leaves are the better indicator; the youngest leaves suit immobile elements¹. Jones recommends the end leaflet of a recently mature leaf for tomato, sampled every 2 to 3 weeks⁴. Critical concentrations change with plant age, so use ranges rather than a single value¹.

Jones’s two tables disagree for tomato, most of all for potassium, and the Appendix B ranges change with the cluster sampled: nitrogen falls from 3.5 to 5.0% at the first cluster to 2.0 to 3.0% at the fifth and sixth⁴. Read a result against both before acting.

The response. Resh suggests raising the suspected element above its normal level, by up to 25% to 30%, then lowering it to 10% to 15% above the level at which the deficiency appeared as the plants recover; a response may take 7 to 10 days to show³. Judge recovery on new growth. Necrotic tissue is dead², so in this article’s reading a brown leaf edge will not recover whatever you add.

Induced deficiencies: the common case in hydroponics

Iron, calcium and magnesium deficiencies appear even when the solution holds more than enough of these elements⁵. The cause lies between the tank and the leaf.

High pH locks out iron and manganese. Above pH 7.0 a nutrient solution can produce deficiencies of iron, manganese and zinc⁴. Iron precipitates at high pH, which causes chlorosis of the upper leaves⁵. The chelate in the recipe sets how high the pH can go: Hershkowitz and co-workers state that Fe-EDTA binds iron up to pH 6.5, Fe-DTPA up to 7.5 and Fe-EDDHA up to 9, and in their 2025 trial in growing media all three chelates prevented chlorosis in Calibrachoa up to pH 6.5 but above pH 7.2 only Fe-EDDHA did (abstract only)¹². Chelates are covered in Micronutrients and chelates.

Calcium shortages are water-flow shortages. Calcium moves in the xylem with the transpiration stream and hardly at all in the phloem, so organs that transpire little, such as shoot tips, inner leaves and fruit, run short first⁹. Tipburn in lettuce and blossom-end rot in tomato and pepper are calcium transport problems induced by conditions such as high humidity, not a lack of calcium in the solution⁵. The mechanism is in Water in the plant and the climate settings in Temperature, humidity and VPD.

Cations compete. The uptake of magnesium can be strongly depressed by potassium, ammonium, calcium, manganese and low pH, and magnesium deficiency induced by competing cations is fairly widespread⁷. On tomato, high potassium uptake can induce magnesium deficiency⁵. Calcium deficiency is likewise induced by high concentrations of ammonium, potassium, magnesium or salts⁹. Jones advises that in a fruiting tomato the leaf contents of potassium and calcium should be about equal⁴.

Cold or airless roots stop uptake. Iron deficiency is common in cool conditions and where the roots are waterlogged or damaged⁵. High or low root-zone temperatures reduce nutrient and water uptake⁵; at low root temperatures phosphorus uptake usually falls more than that of other nutrients¹¹. Lack of oxygen is probably the main cause of root death in hydroponics⁵. See Dissolved oxygen and Root-zone temperature.

Toxicities

Deficiencies are far more common than toxicities in hydroponics^3,5, and toxicity symptoms are less specific¹.

Manganese. Many species show brown speckles on mature leaves, and excess manganese induces deficiencies of calcium, magnesium, iron and zinc⁸. The concentration that cuts dry matter by 10% ranges from 200 mg/kg in maize to 5,300 mg/kg in sunflower⁸; Jones gives a toxic level above 400 mg/kg⁴.

Boron. Excess boron shows as chlorosis and necrosis at the tips and margins of mature leaves, following the transpiration stream⁸. Critical leaf concentrations are about 100 mg/kg in maize, 400 in cucumber and 1,000 in squash⁸.

Sodium, chloride and total salts. Resh considers water with 50 ppm or more of sodium chloride unsuitable for optimum growth, and in a closed system these ions build up with every top-up³; see Water quality. An EC too high for the crop stunts the root system severely, more commonly in crops that prefer a lower EC, such as lettuce⁵; see EC and concentration. Ammonium excess is treated in Nitrogen form: nitrate and ammonium.

The correction path

1. Check the roots, then pH, EC, solution temperature and oxygen.
2. Correct the pH first. A high pH alone can make iron unavailable^4,5, and iron added to such a solution precipitates too unless it is held by a chelate stable at that pH^5,12.
3. If the solution test shows a real shortage, raise that element as described above³. If in doubt about the solution, replace it; see Managing a recirculating solution.
4. Use a foliar spray as a stop-gap only. Resh suggests 2% magnesium sulfate for magnesium and 0.02% to 0.05% iron chelate every 3 to 4 days for iron, tested on a few plants first³. Sprays suffer from poor wetting, run-off, low penetration and, for calcium and iron, poor movement out of the sprayed leaf¹⁰; the root zone must still supply the element⁵.

For toxicities, Resh’s remedy in a substrate is to flush with plain water for about a week³.

Worked example 1: iron lock-out in lettuce

A lettuce crop in a 100 L deep-water tank shows yellowing between the veins of the youngest leaves. The veins stay green, there are no dead spots, and every plant shows it. The meter reads pH 7.3 and EC 1.4 mS/cm.

Walk the key. Young leaves first, interveinal, no spots: iron^2,3, with sulfur as the alternative that Resh’s key separates by colour, light green rather than bright yellow³. Marschner’s table lists young-leaf interveinal chlorosis under zinc (manganese) and uniform young-leaf chlorosis under iron (sulfur)¹, so keep zinc and manganese in mind. Check the solution. Resh describes commercial lettuce raft systems kept at an EC of 1.2 to 1.3 mmho, the same as mS/cm³, so in our reading 1.4 mS/cm rules out a general shortage. The pH does not: Mason gives 6.0 to 6.2 as the optimum for lettuce⁵, and above 7.0 iron, manganese and zinc deficiencies can follow⁴. If the recipe uses Fe-EDTA, pH 7.3 is above the pH 6.5 to which that chelate holds iron¹². This is an induced deficiency.

Compute the acid. Suppose an alkalinity test of the tank reads 150 mg/L as CaCO₃, the upper limit for irrigation water in Whipker’s guideline quoted by Jones⁴; this is our illustrative input. With Resh’s atomic weights, CaCO₃ is 100.09 g/mol and one milliequivalent is 50.05 mg, so, counting all of it as bicarbonate, the tank holds 3.00 mmol/L (our conversion from³). Boyd gives the first dissociation constant of carbonic acid as 10⁻⁶·³⁵¹³; rearranged, pH = 6.35 + log(bicarbonate ÷ dissolved CO₂). The rest is our estimate from that equation. At pH 7.3 the ratio is 8.91, so the tank holds 0.34 mmol/L of CO₂. Added acid reacts with bicarbonate to produce more CO₂¹³, one millimole for one, and pH 6.0 needs a ratio of 0.447. Solving (3.00 − a) ÷ (0.34 + a) = 0.447 gives a = 1.97 mmol/L, or 0.197 mol for 100 L.

Nitric acid at 67% is close to the 68% constant-boiling acid, whose density is 1.41 g/mL at 20 °C¹⁴. Taking that density, each millilitre holds 1.41 × 0.67 ÷ 63.02 = 15.0 mmol of HNO₃ (molar mass from³). The dose is 197 ÷ 15.0 = 13.1 mL, which adds 1.97 × 14.01 = 27.6 mg/L of nitrate-nitrogen (our arithmetic). Add it in two or three parts, mixing in between.

What to watch. The reading of 6.0 holds only while the new CO₂ stays dissolved. If the CO₂ falls back to its pre-dose 0.34 mmol/L, the same equation gives pH 6.84 (our estimate), and aeration that strips the CO₂ further, towards equilibrium with air, would push the pH higher still (our reading). Recheck the next day and dose again; the lasting fix is to acidify the top-up water (Alkalinity and buffering). Resh expects a response to show within 7 to 10 days³; look for it in the new leaves. If none comes, test for manganese and look at the roots.

Worked example 2: magnesium on old tomato leaves

A tomato crop shows interveinal chlorosis on the lower leaves, with small dead spots, spreading upward. The key points to magnesium, with manganese as the bracketed alternative^1,3. Suppose the laboratory reports, for the leaf below the flower cluster, magnesium 0.20%, potassium 3.2% and calcium 1.4% of dry matter; these are our illustrative values. Against Jones’s Table 11.10, magnesium is below 0.35%, potassium above 2.50% and calcium below 1.50%⁴, and potassium is 2.3 times calcium where that table advises about equal⁴ (our ratio). Against Jones’s Appendix B ranges for the same leaf, potassium at 3.2% is normal and calcium at 1.4% is at the bottom of the first-cluster range, while magnesium is still below 0.3%⁴. The leaf test therefore confirms the magnesium shortage, and the solution analysis decides the cause: if it shows normal magnesium and high potassium, the reading is a magnesium deficiency induced by potassium^5,7: lower the potassium, keep magnesium supplied, and use a magnesium sulfate spray only to bridge the gap³.

Key numbers

Common mistakes

1. Adding the element that the key names before checking pH. Above pH 7.0 iron, manganese and zinc deficiencies follow⁴, Fe-EDTA holds iron only to pH 6.5¹², and unchelated iron precipitates⁵.
2. Reading only the young leaves. For mobile elements such as magnesium and potassium the plant protects the young leaves at the expense of the old, so the old leaves show the shortage¹.
3. Treating tipburn with more calcium. The shortage is in delivery by the transpiration stream, not in the tank^5,9.
4. Taking a random pattern for a nutrient disorder. Nutrient symptoms are symmetric by leaf age; random spots point to a pathogen or pest¹.
5. Expecting brown leaves to recover. Necrotic tissue is dead², so judge a correction on new growth.
6. Relying on foliar sprays. Calcium and iron hardly move out of the sprayed leaf¹⁰, and the root supply still has to be fixed⁵.

Summary

A mobile element runs short first in old leaves and an immobile one in young leaves, so ask which leaves are affected before anything else. The pattern then narrows the list, and the sources disagree on a few elements, notably zinc and sulfur. Rule out roots, pests, light and heat, then confirm with a solution test, a tissue analysis read against sufficiency ranges, and the response to a corrective dose. In hydroponics the solution often holds enough of the element: high pH, poor transpiration, competing cations and cold or airless roots induce the deficiency. Correct pH, temperature and oxygen before adding nutrients. Toxicities are rarer, less specific and treated by dilution or replacement.

Next in the series: Pests and diseases in soilless systems.

References

1. Römheld, V. (2012). Diagnosis of deficiency and toxicity of nutrients. In: Marschner, P. (ed.), Marschner's Mineral Nutrition of Higher Plants, 3rd ed. Academic Press, London, pp. 299–312. https://doi.org/10.1016/B978-0-12-384905-2.00011-X
2. Taiz, L. and Zeiger, E. (2002). Plant Physiology, 3rd ed. Sinauer Associates, Sunderland, MA. Chapter 5, Mineral Nutrition.
3. Resh, H. M. (2013). Hydroponic Food Production: A Definitive Guidebook for the Advanced Home Gardener and the Commercial Hydroponic Grower, 7th ed. CRC Press, Boca Raton. ISBN 978-1-4398-7869-9. Tables 2.3–2.6, 3.5.
4. Jones, J. B. Jr. (2005). Hydroponics: A Practical Guide for the Soilless Grower, 2nd ed. CRC Press, Boca Raton. Tables 7.6, 11.10 and Appendix B.
5. Mason, J. (n.d.). Commercial Hydroponics, 3rd ed. ACS Distance Education. ISBN 978-0-9871022-2-5. Chapter 17, Troubleshooting, by L. Morgan.
6. White, P. J. (2012). Long-distance transport in the xylem and phloem. In: Marschner, P. (ed.), Marschner's Mineral Nutrition of Higher Plants, 3rd ed. Academic Press, London. https://doi.org/10.1016/B978-0-12-384905-2.00003-0
7. Hawkesford, M., Horst, W., Kichey, T., Lambers, H., Schjoerring, J., Skrumsager Møller, I. and White, P. (2012). Functions of macronutrients. In: Marschner, P. (ed.), Marschner's Mineral Nutrition of Higher Plants, 3rd ed. Academic Press, London. https://doi.org/10.1016/B978-0-12-384905-2.00006-6
8. Broadley, M., Brown, P., Cakmak, I., Rengel, Z. and Zhao, F. (2012). Function of nutrients: micronutrients. In: Marschner, P. (ed.), Marschner's Mineral Nutrition of Higher Plants, 3rd ed. Academic Press, London. https://doi.org/10.1016/B978-0-12-384905-2.00007-8
9. Wiesler, F. (2012). Nutrition and quality. In: Marschner, P. (ed.), Marschner's Mineral Nutrition of Higher Plants, 3rd ed. Academic Press, London. https://doi.org/10.1016/B978-0-12-384905-2.00009-1
10. Eichert, T. and Fernández, V. (2012). Uptake and release of elements by leaves and other aerial plant parts. In: Marschner, P. (ed.), Marschner's Mineral Nutrition of Higher Plants, 3rd ed. Academic Press, London. https://doi.org/10.1016/B978-0-12-384905-2.00004-2
11. White, P. J. (2012). Ion uptake mechanisms of individual cells and roots: short-distance transport. In: Marschner, P. (ed.), Marschner's Mineral Nutrition of Higher Plants, 3rd ed. Academic Press, London. https://doi.org/10.1016/B978-0-12-384905-2.00002-9
12. Hershkowitz, J., Dey, S., Heins, R. and Bugbee, B. (2025). Fertigation with Fe-EDTA, Fe-DTPA, and Fe-EDDHA chelates to prevent iron chlorosis of sensitive species in high-pH soilless media. HortScience, 60(3), 404–410. https://doi.org/10.21273/HORTSCI17892-24 (abstract only)
13. Boyd, C. E. (2012). Water quality. In: Lucas, J. S. and Southgate, P. C. (eds), Aquaculture: Farming Aquatic Animals and Plants, 2nd ed. Wiley-Blackwell, Chichester. Section 3.2.10.
14. National Center for Biotechnology Information (2026). PubChem Compound Summary for CID 944, Nitric acid. Section "Other Experimental Properties", quoting the Merck Index via the Hazardous Substances Data Bank. https://pubchem.ncbi.nlm.nih.gov/compound/944

This article was drafted with the help of AI tools and reviewed and edited by PonicLabs. Sources are listed above; tell us if you find an error.