Short answer: Trace elements (micronutrients) are nutrients your plant needs in very small amounts: iron, zinc, manganese, boron, copper and molybdenum. A shortage shows first on the youngest leaves, because the plant cannot move these elements from old leaves to new growth. Before you add anything, check your pH and your runoff EC. Most "deficiencies" are lockout, not a real lack. If the pH is right and you reuse soil or use very pure water, a trace element mix such as Bionova Micromix is the fix.
What are trace elements in plants?
The trace elements plants need are the nutrients that work in tiny amounts but cannot be skipped. They are part of enzymes, chlorophyll and hormone processes. Without them, the big nutrients (N, P, K) cannot do their job.
You do not need a lot. That is also why mistakes are easy: a small shortage causes visible damage, and a small overdose can cause burn.
Why deficiencies show on new growth
Nitrogen and magnesium are mobile. When they run short, the plant takes them from old leaves, so old leaves yellow first. Most trace elements are not mobile. The plant cannot recycle them, so the youngest leaves and the growing tips show the problem first.
This is your first clue. Yellow lower leaves usually point to a mobile nutrient such as nitrogen. Pale, spotted or distorted new leaves point to a trace element or to lockout. If the leaves are pale with green veins, read our guide on calcium and other mineral deficiencies first.
Trace element deficiency symptoms: how to tell them apart
The symptoms below are typical patterns. Real plants often show a mix, so use them as a guide, not as a lab test.
| Element | Typical symptom | Where it shows |
|---|---|---|
| Iron (Fe) | Pale yellow leaf tissue, veins stay green | Youngest leaves |
| Zinc (Zn) | Small leaves, short gaps between nodes, pale patches between veins, sometimes twisted new growth | New growth and tips |
| Manganese (Mn) | Pale tissue between veins, sometimes with small dark spots | Young to middle leaves |
| Boron (B) | Thick, brittle or distorted growing tips, slow tip growth | Growing tips |
| Copper (Cu) | Pale, limp new leaves that may droop | New growth |
| Molybdenum (Mo) | Pale leaves, curled leaf edges; rare | Mostly older to middle leaves |
Note that two of these overlap with other problems. Pale tissue with green veins can also be magnesium or iron lockout. Distorted tips can also be heat or light stress. This is why you check the causes below before you treat.
Possible causes, from most to least likely
- pH out of range. This is the most common cause. When the pH of your root zone drifts, iron, zinc and manganese become unavailable even when they are present. Test your input water and your runoff.
- Overfeeding or salt buildup. Too much of one nutrient can block others. A high runoff EC with pale new leaves points to this.
- Reused soil or substrate. Used substrate is often low in trace elements after a full grow. Upgrading the trace elements is advised when you reuse soil.
- Very pure water. Reverse osmosis or very soft water carries almost no minerals, so the plant gets only what you add.
- A real lack. A true shortage is the least likely cause when you feed with a complete main fertilizer on schedule, but it is possible.
How to tell lockout from a real lack
Follow these checks in order. They take ten minutes.
- Measure the pH of your feed water and of the runoff. Compare both with the pH range in your feeding chart.
- Measure the EC of the runoff. If it is clearly higher than what you feed, salts are building up.
- Look at where the symptoms start. New leaves only means a trace element or lockout. Old leaves first means a mobile nutrient.
- Check your history. Did you reuse soil? Did you switch to pure water? Did you change your main fertilizer?
How to fix a trace element deficiency
Step 1: Correct the pH first. If the pH was wrong, the plant may recover without any extra product. If you suspect salt buildup, do a light flush with correctly pH-adjusted water before you add anything.
Step 2: Add a trace element mix. Bionova Micromix contains iron (Fe, 0.36% HEEDTA), manganese (Mn, 0.08% EDTA), boron (B, 0.4%), zinc (Zn, 0.09% EDTA) and molybdenum (Mo, 0.06%). The elements are chelated, so the plant can absorb them across the pH range of your irrigation water. For a deficiency, add 2.5 to 5 ml per 10 liters of water. Micromix can also be applied on the leaf. It mixes with all Bionova fertilizers, stimulators, minerals, universals and substrates.
Step 3: Watch the new growth. Old damaged leaves will not turn green again. Judge the result on the leaves that grow after the treatment. If new leaves come in healthy after a few days, the fix worked.
Note that the Micromix label does not list copper. If you suspect a copper shortage, check your pH first and look at the full analysis of your main fertilizer in the feeding chart.
Not sure what is wrong? The SOS Yellow Leaves Pack combines N 27, MgO 10 and Micromix, so you can treat the three most common causes of yellow leaves.
How to prevent trace element problems
Keep your pH in range and feed according to the feeding chart for your substrate. Do not add extra products "just in case". When you reuse soil, upgrade the trace elements before the next grow. When you grow with very pure water, check that your main fertilizer program covers the micronutrients, and add Micromix when you see signs of a shortage.
Browse all single-element products in the Minerals collection.
FAQ
How do I recognise zinc deficiency in plants?
Typical signs are small new leaves, short gaps between nodes and pale patches between the veins. The symptoms appear on new growth. Check pH before you treat, because zinc lockout is more common than a real zinc shortage.
Which trace elements are in Micromix?
Micromix contains iron, manganese, boron, zinc and molybdenum. It does not list copper. See the Micromix product page for the full composition.
How much Micromix do I add?
In case of a deficiency, add 2.5 to 5 ml per 10 liters of water. For a full week-by-week program, see the feeding chart for your substrate.
Do I always need extra trace elements?
No. Use a trace element mix when a deficiency occurs and when you reuse soil. Otherwise follow your feeding chart and keep the pH in range.
