Nutrient deficiency symptoms can be read like a map if you know one key principle: whether a nutrient is mobile or immobile in the plant determines which leaves show symptoms first.

Mobile nutrients (nitrogen, phosphorus, potassium, magnesium) can be relocated from older tissue to newer growth when supplies are limited. The plant salvages these nutrients from expendable older leaves and moves them to the active growing tip. Result: symptoms appear on the oldest, lowest leaves first.

Immobile nutrients (calcium, iron, boron, and most micronutrients) cannot be relocated once deposited in leaf tissue. When supply is inadequate, deficiency appears in the newest growth - the young leaves and growing tips that are developing right now.

This mobile vs. immobile distinction is the primary diagnostic principle. Before trying to identify the specific deficiency, establish whether old leaves or new leaves are affected first. This halves your diagnostic possibilities immediately.

1. Nitrogen Deficiency

Mobility: mobile
Symptom location: oldest, lowest leaves first
What you see: uniform yellowing of older leaves, progressing upward toward the growing tip as deficiency deepens. The entire leaf yellows - not just the margins or between veins. The plant overall has a pale, washed-out green appearance. New growth at the top is the last to yellow.

Distinguish from natural senescence: the lower leaves of tomatoes, peppers, and squash naturally yellow and drop as they age and are shaded out by upper canopy. This is normal and concentrated on the very oldest leaves near soil level. Nitrogen deficiency produces more widespread yellowing across multiple leaf layers, and the yellowing is accompanied by overall reduced plant vigor (slow growth, small leaves, reduced fruit set).

Why it happens: nitrogen is the most rapidly depleted soil nutrient, especially in sandy soils, raised beds with fresh (undecomposed) compost, and container mixes that have been growing actively for several weeks. Heavy rainfall leaches nitrogen out of the root zone faster than most other nutrients.

Quick fix: fish emulsion (5-1-1 or similar) or liquid kelp applied as a foliar spray and soil drench provides bioavailable nitrogen within 3-5 days. Side-dress with blood meal (12-0-0) around the base of the plant and water in for a longer-lasting fix. Granular balanced fertilizer (10-10-10 or similar) applied to the soil surface is slower-acting but lasts longer.

Long-term prevention: adequate compost at planting; mid-season side-dressing for heavy feeders (tomatoes, corn, squash); avoiding nutrient leaching through mulching.

2. Phosphorus Deficiency

Mobility: mobile
Symptom location: older leaves first
What you see: purple-red to bronze discoloration on leaf undersides; dark green or blue-green on the upper surface. Overall, the plant looks unusually dark green rather than pale. The purple pigment (anthocyanin) accumulates in leaves where phosphorus metabolism is disrupted.

The cold soil mimicry: this is the most commonly misdiagnosed deficiency in spring gardens. Cold soil (below 55°F) blocks phosphorus uptake even when adequate phosphorus is present. Many gardeners in spring see purple-tinged leaves and assume deficiency when the actual problem is simply soil temperature. Before applying any phosphorus amendment, check soil temperature at 4-inch depth with a soil thermometer.

If soil temperature is below 55°F: wait. The discoloration usually disappears within 7-10 days as soil warms. If soil is above 60°F and discoloration persists: phosphorus deficiency is likely.

Why it happens: in addition to cold soil, phosphorus is unavailable in very acidic soils (pH below 5.5) and very alkaline soils (pH above 7.5) even when present. Soil pH in the 6.0-7.0 range provides maximum phosphorus availability.

Quick fix: bone meal (3-15-0) worked into the soil or liquid phosphoric acid products. Mycorrhizal fungi inoculants (applied at transplanting) significantly improve phosphorus uptake by expanding the effective root surface area.

3. Potassium Deficiency

Mobility: mobile
Symptom location: older leaves first
What you see: brown or tan scorching at the leaf margins and tips, progressing inward. The interveinal tissue often stays green while the margins brown. Affected leaves may curl inward. The scorching starts on the outermost leaf edges because potassium moves to the margins last when supplies are limited.

Distinguish from windburn and salt damage: windburn also causes margin browning but typically affects exposed plants after high-wind events and develops quickly rather than progressively. Salt damage from over-fertilization causes similar margin burn but is accompanied by other symptoms (stunted growth, white salt crust on soil surface) and is concentrated at the outer margins of the root zone.

Why it happens: potassium is relatively abundant in most garden soils but can be deficient in sandy soils, highly leached soils, or beds that have been heavily cropped for several years. High levels of calcium or magnesium in soil can antagonize potassium uptake.

Quick fix: kelp meal or potassium sulfate (0-0-50) applied to the soil and watered in. Wood ash is a traditional potassium source (approximately 5-8% K) but also raises pH significantly; use it only in acidic soils.

4. Calcium Deficiency

Mobility: immobile
Symptom location: newest growth and developing fruits
What you see: tip burn in lettuce heads and brassicas (brown, papery edges on innermost leaves that never fully opened); blossom end rot in tomatoes, peppers, and squash (dark, sunken lesion at the blossom end of fruits); hollow heart in broccoli and cauliflower; bud death in the growing tip of plants.

The watering connection: calcium deficiency symptoms almost always indicate a calcium delivery problem rather than a soil calcium shortage. Calcium is abundant in most US garden soils. The issue is that calcium moves exclusively with the transpiration stream - it can only reach developing tissue if water is flowing steadily from soil through roots through the plant. Inconsistent watering, drought stress, or anything that interrupts this water flow interrupts calcium delivery to the fastest-developing tissue (the newest leaves and developing fruits).

For detailed blossom end rot coverage, see the Blossom End Rot guide.

True soil calcium deficiency occurs in very acidic soils (pH below 5.5), very sandy soils with low cation exchange capacity, and soils that have never been limed. Agricultural lime (calcium carbonate) or gypsum (calcium sulfate) corrects these conditions. Confirm with a soil test before amending.

5. Iron Deficiency (Interveinal Chlorosis)

Mobility: immobile
Symptom location: newest leaves first
What you see: young leaves at the growing tip turn yellow while the veins remain distinctly green, creating a net-like pattern called interveinal chlorosis. The vein pattern is sharp and clear - green lines on a yellow or pale background. Older leaves below the growing tip often look normal.

The pH lock explanation: this is almost always a soil pH problem, not an iron soil shortage. Iron becomes unavailable to plants in alkaline soils (pH above 7.0-7.5). The iron is physically present in the soil; the chemistry at that pH prevents root uptake. Adding iron to the soil in an alkaline soil does nothing - the chemistry re-locks it immediately.

The fix: lower soil pH to 6.0-6.5, where iron availability is high. Sulfur (elemental sulfur) or acidifying fertilizers (ammonium sulfate) lower pH over weeks to months. Iron chelate drench provides a temporary fix that bypasses the soil pH issue - chelated iron remains available across a wider pH range and corrects the chlorosis while you work on the underlying pH problem.

Crops most affected: blueberries, raspberries, and acid-loving ornamentals grown in neutral or alkaline soil; vegetables grown in raised beds with compost-heavy mixes that have drifted alkaline; any crops in soils with pH above 7.2.

6. Magnesium Deficiency

Mobility: mobile
Symptom location: older leaves first
What you see: interveinal chlorosis (yellowing between veins while veins stay green) on older, lower leaves. The pattern often shows a distinctive V-shape or arrowhead of yellowing pointing toward the leaf tip from the base between major veins. The leaf edges may stay greener longer than the center, giving an “inside-out” yellowing pattern compared to potassium deficiency, which scorches the edges.

Why it happens: magnesium is a mobile nutrient, so older leaves show symptoms first - this distinguishes magnesium from iron deficiency, which shows on new growth. Magnesium deficiency is common in acidic, sandy soils with high rainfall. Heavy potassium fertilization antagonizes magnesium uptake; over-applying potassium can induce magnesium deficiency even in soils with adequate magnesium.

Tomatoes are particularly prone to magnesium deficiency. The symptoms often appear mid-season when fruit load is high and the plant is prioritizing fruit development over lower leaf maintenance.

Quick fix: Epsom salt (magnesium sulfate) - 1 tablespoon dissolved in 1 gallon of water applied as a foliar spray and soil drench. Results visible within 7-10 days. Epsom salt is not a fertilizer and does not replace a deficient soil; it’s a temporary correction. For persistent deficiency, apply dolomitic limestone (which contains both calcium carbonate and magnesium carbonate) at the next soil amendment opportunity.

Nutrient Interactions: Why Adding More Can Make Things Worse

Nutrients interact in the soil and in plant uptake. Adding too much of one nutrient can block the uptake of another - called antagonism. The most practically important antagonisms:

  • High potassium blocks magnesium uptake. If you’ve been applying potassium-heavy fertilizers (wood ash, kelp meal heavily) and see magnesium deficiency symptoms, adding more potassium won’t help - reducing it and adding Epsom salt will.
  • High calcium blocks potassium uptake. Soils amended heavily with lime or gypsum can be potassium-deficient despite adequate potassium application.
  • High phosphorus blocks iron and zinc. Over-application of phosphorus (common in older heavily amended garden beds) can induce iron deficiency symptoms even in properly pH-balanced soil.
  • High pH blocks iron, manganese, zinc, and copper simultaneously. A single pH correction resolves what looks like multiple deficiencies.

This is why a soil test before adding amendments is more useful than visual diagnosis alone. Adding a missing nutrient without knowing the existing levels can worsen antagonisms.

The Diagnostic Sequence

When you see a leaf problem that might be a deficiency:

  1. Identify old vs. new growth affected. Old leaves: think mobile nutrients (N, P, K, Mg). New growth: think immobile nutrients (Ca, Fe, B).

  2. Describe the yellowing pattern. Uniform yellow: likely N. Yellow with green veins: likely Fe or Mg (which one depends on step 1). Margin scorch: likely K. Purple underside: likely P.

  3. Check soil temperature. Below 55°F? Cold soil mimics P and sometimes K deficiency without actual deficiency.

  4. Check soil pH. High pH (above 7.0)? Fe and Mn are locked up regardless of soil content. Low pH (below 5.5)? Ca and P availability drops.

  5. Confirm with a soil test before adding amendments. A $15-25 test from your state cooperative extension laboratory tells you exactly what’s deficient and what’s not - more reliable than visual diagnosis alone and prevents the common error of adding nutrients that are already adequate (Cornell Cooperative Extension, Soil Testing for Home Grounds, 2022).

Related reading: Soil pH by Crop - pH requirements and how to hit them; Soil Test Decision Chain - when to test and how to interpret results; Soil Test ROI - the financial case for testing before amending