KNOWLEDGE · DIAGNOSIS
Diagnose nutrient deficiency by leaf position
Most field diagnoses go wrong at the first step. Before asking which nutrient is missing, ask where the symptom appears. Nutrients that the plant can move out of old tissue show up on lower leaves; nutrients it cannot move appear on new growth. That single question eliminates about half the possibilities.
Mobile versus immobile nutrients
Nitrogen, phosphorus, potassium and magnesium are mobile: when supply runs short, the plant relocates them from older leaves to growing points. Sulphur, iron, manganese, zinc and copper are largely immobile, so deficiency shows on the youngest tissue first. Calcium and boron behave similarly, affecting growing points and fruit.
Symptoms on old leaves
| Nutrient | Symptom | Confusion risk |
|---|---|---|
| Nitrogen | Uniform yellowing of whole older leaves, plant stunted | Looks like sulphur at a glance |
| Phosphorus | Dark green to purple leaves, thin stems, delayed maturity | Cold stress produces the same colour |
| Potassium | Margin and tip scorch, weak stems, poor fruit colour | Salinity damage looks similar |
| Magnesium | Interveinal yellowing on older leaves, veins stay green | Can be mistaken for virus symptoms |
Symptoms on new leaves
| Nutrient | Symptom | Confusion risk |
|---|---|---|
| Sulphur | Uniform yellowing of young leaves, normal leaf shape | Most often misdiagnosed as nitrogen deficiency |
| Iron | Interveinal yellowing, young leaves almost white in severe cases | Common on high-pH soils |
| Zinc | Short internodes, small leaves, mottled pattern | Sometimes confused with herbicide injury |
| Calcium | Distorted or hooked new leaves, blossom-end rot in fruit | Not a soil supply problem in most cases |
Three checks before you buy fertilizer
Deficiency diagnosis by eye is unreliable on its own. Confirm with:
- Soil pH — above 7.5 locks iron, zinc and manganese; below 5.5 locks phosphorus
- Leaf tissue analysis — the only way to confirm a specific deficiency
- Pattern in the field — deficiency follows soil type and drainage, disease clusters around inoculum sources
Why phosphorus deficiency deserves special attention
Phosphorus is the deficiency most often noticed too late, because the visible symptom — purple leaves — appears after root and tiller number are already set. By the time the colour shows, the yield potential has been reduced. The practical answer is to check the water-soluble phosphorus content of your basal NPK rather than waiting for symptoms.
Frequently asked
Can a plant show several deficiencies at once?
Yes, and that is common on sandy or eroded soils. Tissue analysis is the only practical way to separate them.
Is yellowing always a nutrient problem?
No. Waterlogging, root disease and herbicide residues all produce yellowing. Check the drainage and root health first.
How quickly do foliar corrections work?
Iron and magnesium sprays can show colour recovery in 5–7 days. Nitrogen is similar. Phosphorus and potassium deficiency is better corrected at the root.