Understanding nutrient solutions: N-P-K, micronutrients, and why your lettuce turned yellow

A hydroponic plant gets every mineral from the water. There's no soil buffer, no microbial breakdown of organic matter, no reserve pool of nutrients locked in clay particles. If a nutrient isn't dissolved in the solution, the plant doesn't have it.

This makes hydroponics both simpler and less forgiving than soil growing. Simpler because you control everything. Less forgiving because a mistake in the mix shows up as visible symptoms within days.

The macronutrients

Nitrogen (N). Drives leaf and stem growth. The nutrient plants consume in the largest quantity. In hydroponic solutions, nitrogen is supplied as nitrate (NO3-) or ammonium (NH4+). Most formulations use a mix of both, with nitrate dominant (70-90% of total N). Excess nitrogen produces lush green growth but delays flowering and fruiting. Deficiency: older leaves yellow from the tips while new growth stays green (nitrogen is mobile in the plant, so the plant cannibalizes old leaves to feed new ones).

Phosphorus (P). Critical for root development, energy transfer (ATP), and flowering. Listed on labels as P2O5 (phosphorus pentoxide) by convention, not as elemental P. To convert: P2O5 x 0.44 = elemental P. Deficiency: dark green or purplish leaves, stunted growth, poor root development.

Potassium (K). Regulates water movement within the plant, enzyme activation, and fruit quality. Listed on labels as K2O (potassium oxide). To convert: K2O x 0.83 = elemental K. Deficiency: leaf edge scorch (browning of margins) on older leaves. Potassium is mobile, so like nitrogen, deficiency shows on old growth first.

The N-P-K ratio shifts through the plant's life:

• Seedling stage. Roughly balanced, low concentration. N:P:K around 1:1:1 at EC 0.6-1.0.
• Vegetative growth (leafy greens). High nitrogen. N:P:K around 3:1:2 at EC 1.0-2.0.
• Flowering transition. Shift toward phosphorus and potassium. N:P:K around 2:2:3 at EC 2.0-2.5.
• Fruiting. Potassium dominant. N:P:K around 1:2:4 at EC 2.5-3.2.

Leafy crops stay in the vegetative stage forever (they're harvested before flowering), so they get high-nitrogen feed throughout. Fruiting crops (tomatoes, peppers, cucumbers) need the ratio shift as they transition from growing leaves to setting fruit.

The nutrient mix calculator computes these ratios per crop and growth stage using the data in the edible plant profiles.

The secondary nutrients

Calcium (Ca). Structural. Cell walls are built with calcium pectate. Deficiency causes blossom end rot in tomatoes (the black, sunken bottom of the fruit) and tip burn in lettuce (brown, papery leaf edges). Calcium is immobile in the plant; deficiency shows on new growth, not old. This is why tip burn hits the inner leaves of a lettuce head first.

Calcium is supplied separately from phosphorus and sulfate in two-part nutrient systems because calcium reacts with concentrated phosphate and sulfate to form insoluble precipitates (calcium phosphate, calcium sulfate / gypsum). Part A contains the calcium nitrate. Part B contains the phosphate, sulfate, and micronutrients. Never mix the concentrates together; mix each into water separately, then combine the dilute solutions.

Magnesium (Mg). The central atom of chlorophyll. Deficiency causes interveinal chlorosis on older leaves: the tissue between the leaf veins turns yellow while the veins stay green. Classic checkerboard pattern.

Sulfur (S). Component of amino acids. Deficiency is rare in hydroponics because sulfate is a common anion in most nutrient formulations (magnesium sulfate, potassium sulfate).

Micronutrients

Required in tiny amounts but critical. The main ones:

Iron (Fe). Chlorophyll synthesis. Deficiency: new leaves emerge pale yellow or white (iron is immobile). The most common micronutrient deficiency in hydroponics. Usually caused by pH above 6.5 locking out iron even when it's present in the solution. Chelated iron (Fe-DTPA or Fe-EDDHA) stays available at higher pH than non-chelated forms.

Manganese (Mn), Zinc (Zn), Copper (Cu), Boron (B), Molybdenum (Mo). All essential. All supplied in trace quantities by standard hydroponic micronutrient mixes. Individual deficiencies are uncommon when using a complete commercial nutrient; they appear when someone mixes from raw salts and omits a component.

pH and nutrient availability

This is the most important concept in hydroponic nutrition. Every nutrient has a pH range where it's soluble and available to roots. Outside that range, the nutrient precipitates out of solution or forms chemical compounds the roots can't absorb, even though a test would show it's "in the water."

The sweet spot is pH 5.5-6.5 for most crops. In that range, all macronutrients and micronutrients overlap in availability. Above pH 6.5, iron, manganese, zinc, and copper start locking out. Below pH 5.0, calcium, magnesium, and phosphorus become less available. This is why pH management matters more than getting the nutrient ratios perfect. A mediocre nutrient mix at pH 5.8 outperforms a perfect mix at pH 7.5.

Common deficiency symptoms

Here's a quick diagnostic by leaf position and color. New growth versus old growth matters because it tells you whether the missing nutrient is mobile (deficiency shows on old leaves as the plant reallocates to new growth) or immobile (deficiency shows on new leaves because the plant can't move it).

Old leaves yellowing, new growth green: Nitrogen deficiency. Most common. Increase N or overall EC.

Old leaf edges browning: Potassium deficiency. Increase K.

New leaves pale or yellow, veins green: Iron deficiency. Check pH first (above 6.5 locks out iron). If pH is fine, add chelated iron.

New growth distorted, tip burn on lettuce: Calcium deficiency. Often a humidity/transpiration issue rather than a solution issue; calcium moves into the plant only via transpiration. Low airflow or very high humidity reduces transpiration and causes calcium deficiency symptoms even when the solution has plenty of calcium.

Old leaves with interveinal yellowing (veins green, tissue yellow): Magnesium deficiency. Supplement with Epsom salt (magnesium sulfate), 1-2 g/L.

Purplish leaves, stunted growth: Phosphorus deficiency. Less common in hydroponics because most formulations supply adequate P.

The nutrient mix calculator and the individual edible plant profiles list the stage-specific nutrient demand for each crop.