Nitrogen (N) is the engine of vegetative growth. It forms the backbone of chlorophyll, amino acids, and proteins. In hydroponics, nitrogen is typically supplied as nitrate (NO₃⁻) or ammonium (NH₄⁺). A ratio of 90% nitrate to 10% ammonium is ideal for most species; excess ammonium causes root zone acidification and ammonium toxicity (curled leaves, dark green foliage). Nitrogen deficiency shows as uniform yellowing of older leaves. Excess N leads to lush, weak stems, delayed flowering, and increased susceptibility to pests.

Phosphorus (P) drives energy transfer (ATP), root development, and flowering. It is critical during transplanting and bloom initiation. In nutrient solutions, phosphorus is available as H₂PO₄⁻ or HPO₄²⁻ depending on pH. The optimal pH for P availability is 6.0-6.5. Phosphorus deficiency manifests as dark purplish discoloration on leaf undersides and slow growth. Overdosing phosphorus can lock out iron and zinc, causing interveinal chlorosis.

Potassium (K) regulates osmotic balance, enzyme activation, and stomatal movement. It is the most abundant cation in plant tissues. Potassium deficiency leads to scorched leaf margins and weak stems. During fruiting, K demand spikes: tomatoes absorb nearly twice as much K as N during ripening. In hydroponics, K is supplied as potassium nitrate, potassium sulfate, or potassium silicate. High K levels can antagonize magnesium and calcium, so balanced formulations are crucial.

Understanding the oxide conversion: N-P-K values on labels represent elemental N, but P as P₂O₅ and K as K₂O. To get actual elemental P, multiply P₂O₅ by 0.436; for K, multiply K₂O by 0.83. For example, a 10-10-10 fertilizer contains 10% N, 4.36% elemental P, and 8.3% elemental K. Many hydro calculators adjust for this automatically.

While N-P-K grab headlines, hydroponic success hinges on calcium (Ca), magnesium (Mg), sulfur (S), and seven trace elements. Deficiencies in these are common in reverse osmosis (RO) water systems.

Many growers use RO water which lacks buffering capacity and essential minerals. Always add a Cal-Mag supplement if using RO (<0.1 EC). Tap water (EC <0.4) often provides adequate Ca and Mg but check local water reports for chlorine/chloramine.

While pre-mixed liquid nutrients are convenient, mixing your own from dry salts (calcium nitrate, potassium nitrate, magnesium sulfate, monopotassium phosphate, and a micronutrient blend) saves 50-70% on cost. Here is a general-purpose lettuce formula (target N 150 ppm, P 50 ppm, K 200 ppm, Ca 120 ppm, Mg 40 ppm) for 100L of water:

Critical order of mixing: Always dissolve calcium nitrate first in a separate container, then add other salts (except magnesium sulfate mixed separately) to avoid precipitation of calcium phosphate or calcium sulfate. For stock solutions, use two concentrated tanks: Tank A (calcium nitrate + iron chelate) and Tank B (all other salts). Use a dosing pump for automation.