Hydroponic Macronutrients — N, P, K, Ca, Mg, S

Quick answer

Plants require six macronutrients in concentrations above 0.1% of dry weight: nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur. Each plays a distinct role and each shows distinct deficiency symptoms. Calcium is the most frequently deficient in hydroponics; nitrogen the most over-dosed [OSU-NUT-01].

Nitrogen (N)

Role: protein synthesis, chlorophyll, vegetative leaf growth. The element plants use most by mass.

• Deficiency: uniform pale green to yellow on older leaves first. Plant looks "hungry" — pale, stunted, slow.
• Excess: dark green leaves, soft tissue, delayed flowering, increased susceptibility to pests and disease. Common in over-fed lettuce.
• Hydroponic form: primarily nitrate (NO3-) with a small fraction as ammonium (NH4+). Above 10% NH4+ of total N becomes toxic [OSU-NUT-01].

Nitrogen is mobile — the plant pulls it from old leaves to feed new growth, so symptoms appear bottom-up.

Phosphorus (P)

Role: energy transfer (ATP), root development, flower initiation, seed production.

• Deficiency: purple or red tinge on undersides of older leaves, stunted growth, weak root system. Cold root zones (below 15 °C) cause P deficiency even at adequate concentration because P uptake slows in cold solution [CORN-CEA-01].
• Excess: rare in hydroponics, but can lock out zinc, iron, copper.
• Hydroponic form: phosphate (H2PO4- or HPO4 2-), uptake highest at pH 6.0–6.5.

Phosphorus is mobile.

Potassium (K)

Role: enzyme activation, osmotic regulation, sugar transport, fruit quality, stress tolerance.

• Deficiency: marginal scorching (brown leaf edges) on older leaves, weak stems, poor fruit quality. Common in fruiting crops when K supplementation falls behind fruit load.
• Excess: competes with calcium and magnesium uptake. High-K "bloom boosters" routinely cause Ca and Mg deficiency in tomato and pepper [OSU-NUT-01].
• Hydroponic form: potassium ion (K+).

Potassium is mobile. Fruiting crops uptake roughly 2× more K than N at peak; leafy crops uptake equal or more N than K.

Calcium (Ca)

Role: cell wall structure, membrane stability, signaling. Cannot be substituted by any other nutrient.

• Deficiency: tip burn on new leaves, blossom end rot on fruit, hollow stem in brassicas. The most common deficiency in hydroponics [CORN-CEA-01].
• Calcium is immobile — once incorporated into a cell wall, it stays there. New growth requires continuous fresh calcium delivery.
• Calcium moves only with the transpiration stream. Low transpiration (high RH, low VPD) → low Ca delivery → tip burn even at correct solution concentration.
• Hydroponic form: calcium ion (Ca 2+), supplied as calcium nitrate.

This is the nutrient that fails most often in indoor hydroponics under LED [OSU-NUT-01].

Magnesium (Mg)

Role: central atom of chlorophyll, enzyme cofactor, sugar transport.

• Deficiency: interveinal chlorosis on older leaves — yellow between the veins, green veins remain. Lower canopy yellows first.
• Excess: competes with calcium uptake.
• Hydroponic form: magnesium ion (Mg 2+), supplied as magnesium sulfate (Epsom salt) or magnesium nitrate.

Magnesium is mobile. Symptoms appear bottom-up like nitrogen but with the distinct interveinal pattern [OSU-NUT-01].

Sulfur (S)

Role: amino acid synthesis (cysteine, methionine), enzyme function.

• Deficiency: rare in hydroponics because most formulas oversupply S via sulfate forms (magnesium sulfate, potassium sulfate). When it occurs, looks like nitrogen deficiency but on new growth first rather than old.
• Excess: can contribute to osmotic stress at high EC.
• Hydroponic form: sulfate (SO4 2-).

Sulfur is immobile. Deficiency symptoms appear top-down.

Reading deficiencies — mobile vs immobile

The single most useful diagnostic distinction:

• Mobile (N, P, K, Mg): Deficiency shows on old leaves first. The plant relocates the element to feed new growth, sacrificing old tissue.
• Immobile (Ca, S, and most micronutrients): Deficiency shows on new growth first. The plant can't relocate them, so old tissue stays fine while new tissue suffers.

When you see a deficiency, ask first: is this on new or old leaves? That answer narrows the suspect list to three or four elements before you check anything else [OSU-NUT-01].

Stage-dependent ratios

Plants need different macronutrient ratios at different stages:

• Seedling / propagation: balanced low-EC, roughly 1-1-1 NPK.
• Vegetative leafy growth: N-heavy, roughly 3-1-2 NPK.
• Flowering / fruit set: P-heavy at this stage only — roughly 1-2-2 for 2 weeks then transition.
• Fruit fill / ripening: K-heavy, roughly 1-1-3 NPK with high Ca [CORN-CEA-01].

Most commercial A+B formulas hold a fixed ratio. Stage-specific boosters (bloom, fruit) shift the ratio without complete formulation.

What we recommend

Use a single complete A+B formula with documented nutrient analysis as your base. Supplement Cal-Mag for any crop showing tip burn or blossom end rot. Don't chase deficiencies by adding individual salts unless you have an ICP test or clear visual diagnosis — random additions to a balanced solution cause lockouts faster than they cure deficiencies. The 14-day full reservoir reset solves more "deficiency" problems than any supplement does.