Micronutrient Deficiencies: A Field Guide to Getting the Diagnosis Right

Micronutrients are needed in tiny quantities — grams per hectare in some cases — but a shortfall in any one of them limits the crop just as effectively as a shortage of nitrogen. The difficulty is that their visual symptoms overlap heavily, and treating the wrong one costs money and a spray window without fixing anything.

This guide works through the six micronutrients you are most likely to encounter, what distinguishes each, and how to confirm before you buy.

The single most useful diagnostic clue: leaf position

Before looking at the symptom itself, look at which leaves show it. This one observation eliminates about half the possibilities immediately.

Nutrients differ in whether the plant can move them around once they have been incorporated into tissue:

• Mobile nutrients (nitrogen, phosphorus, potassium, magnesium) are withdrawn from old leaves and sent to new growth when supply is short. Symptoms therefore appear on older, lower leaves first.
• Immobile nutrients (iron, boron, calcium, copper, and manganese and zinc to a large degree) stay where they were deposited. Symptoms appear on young, upper leaves and growing points.

So: yellowing at the bottom of the plant points towards magnesium, nitrogen or potassium. Yellowing at the top points towards iron, manganese, zinc or sulphur.

Iron (Fe)

Where: Youngest leaves, from the top down.
Symptom: Sharp interveinal chlorosis — pale yellow tissue with a fine, distinctly green vein network. In severe cases leaves go almost white and margins scorch.
Typical situations: Calcareous soils, high-bicarbonate irrigation water, waterlogging, cold wet spring soil, over-liming.
Most often confused with: Manganese deficiency, and with root damage of any kind.

Iron deficiency is nearly always an availability problem rather than a supply problem — the soil has iron, the plant cannot reach it. Correction depends on root-zone pH: Fe-EDDHA above pH 7.5, Fe-EDTA below 6.5. We cover this in the iron chlorosis guide.

Manganese (Mn)

Where: Middle-aged leaves, sometimes younger ones.
Symptom: Interveinal chlorosis with a broader green band along the veins than iron produces — the pattern looks coarser and less sharply defined. Grey or brown necrotic flecks may develop in cereals and legumes.
Typical situations: Over-limed soils, naturally calcareous ground, high-pH organic soils, dry seasons.
Most often confused with: Iron deficiency.

The practical difficulty is that soil-applied manganese oxidises rapidly at high pH into forms roots cannot use, sometimes within days. Foliar application with Mn-EDTA bypasses the soil chemistry entirely and is usually the more reliable route.

Zinc (Zn)

Where: Young leaves and growing points.
Symptom: Shortened internodes producing rosetted or bunched growth; small, narrow leaves; interveinal chlorosis. In maize, broad pale bands either side of the midrib on young leaves. In citrus, a distinctive mottled small-leaf pattern.
Typical situations: Sandy soils, high-pH soils, soils where topsoil has been removed by levelling, and after heavy phosphate applications.
Most often confused with: Herbicide injury, virus symptoms.

The reduced leaf size and shortened internodes are the reliable clue — chlorosis alone could be several things, but chlorosis plus stunted, bunched growth points to zinc. Correct with Zn-EDTA, generally foliar in the early vegetative period.

Boron (B)

Where: Growing points, flowers and fruit.
Symptom: Death of growing points; brittle, thickened, distorted young leaves; poor flowering and fruit set; hollow or brown-hearted stems in brassicas and beet; corky tissue and cracking in apples and other fruit.
Typical situations: Sandy soils with low organic matter, high-pH soils, dry seasons, high-demand crops such as oilseed rape, sunflower, brassicas and beet.
Most often confused with: Calcium disorders, physical damage to growing points.

A serious caution: boron has by far the narrowest safe range of any nutrient. The concentration that corrects a deficiency and the concentration that damages the crop are close together, and sensitivity varies widely between species. Never apply boron speculatively, and never exceed the label rate. Confirm with a soil or tissue test first, then use soluble boron at the recommended rate.

Copper (Cu)

Where: Young leaves and shoot tips.
Symptom: Wilting or twisting of young leaves that are not short of water; blue-green colouration before chlorosis; dieback of shoot tips; in cereals, white or twisted leaf tips and poor grain fill.
Typical situations: Peat and organic soils, sandy soils, heavily limed soils, ground that has received large nitrogen applications.
Most often confused with: Drought stress, frost damage.

Copper deficiency is less common than the others but is persistent on organic soils. It is also easy to overdo — copper accumulates in soil and is toxic at elevated concentrations, particularly where copper-based fungicides have been used for many years.

Molybdenum (Mo)

Where: Older and middle leaves first.
Symptom: General pale yellowing resembling nitrogen deficiency, sometimes with cupped or narrow distorted leaves (“whiptail” in cauliflower is the classic case). In legumes, poor nodulation and nitrogen deficiency symptoms.
Typical situations: Acidic soils — molybdenum is the one micronutrient whose availability increases with pH.
Most often confused with: Nitrogen deficiency.

Because availability rises with pH, liming an acidic soil often corrects molybdenum deficiency without any molybdenum being applied. Requirements are extremely small.

Quick reference

Antagonisms: when the soil test looks fine

Nutrients compete for uptake. A soil test can show adequate levels of an element while the plant is still deficient, because something else is blocking it:

• High potassium suppresses magnesium and calcium uptake. This is the most common cause of magnesium deficiency in intensively fertigated crops.
• High phosphate reduces zinc and iron availability.
• High calcium competes with magnesium, potassium and several micronutrients.
• Excess of any one micronutrient can induce deficiency of another — copper, manganese, zinc and iron all interact.
• High nitrogen drives rapid growth that can dilute micronutrient concentrations below sufficiency.

This is why a nutrition programme should be considered as a whole rather than nutrient by nutrient. Our fertigation guide works through building a balanced solution.

Confirm before you spend

Visual diagnosis narrows the field; it rarely settles the question. Before buying:

1. Take a tissue analysis. Sample recently matured leaves following the laboratory’s protocol for your crop and growth stage. Sample the affected area and an unaffected one for comparison.
2. Take a soil test including pH. pH explains most micronutrient availability problems on its own.
3. Get an irrigation water analysis. Bicarbonate, chloride, sodium and existing nutrient content all affect the programme.
4. Check the roots. Poor root health mimics almost every deficiency, because the plant cannot take up anything properly.

Test costs are trivial compared with a wasted micronutrient programme, and far smaller than the yield lost to an untreated deficiency.

Correcting efficiently

If tissue analysis shows one element limiting, treat that element specifically. If you are building a complete nutrient solution and no single deficiency dominates, a chelated micronutrient mix covers the requirement in one product and is usually the more economical purchase.

Send us your tissue or water analysis on WhatsApp and we will tell you which products address what the numbers actually show — and which ones you do not need.