Nutrients and pH: Feeding Your Plants Correctly

Cannabis plants are heavy feeders with specific nutritional requirements that shift significantly between vegetative and flowering stages. Providing the right nutrients at the right concentrations in the right pH range is one of the most important skills in cultivation — and the source of most identifiable grow problems.

THE MACRONUTRIENTS

Nitrogen (N), Phosphorus (P), and Potassium (K) are the three primary macronutrients displayed on all commercial fertiliser products as an NPK ratio.

Nitrogen drives vegetative growth — leaf size, stem thickness, and overall plant vigour. Cannabis requires high nitrogen in the vegetative stage and moderate nitrogen in early flowering, reducing to near-zero in the final 2 weeks (the "flush" or "fade" period). Nitrogen excess produces dark green, claw-shaped leaves; nitrogen deficiency begins with yellowing of older, lower leaves and progresses upward.

Phosphorus is critical for root development and, most importantly, for flower formation, density, and resin production. Phosphorus requirements increase significantly once flowering begins, peaking at weeks 3–5 of flower. A classic flowering nutrient switch involves increasing phosphorus and reducing nitrogen.

Potassium supports overall plant health, water uptake, disease resistance, and enzymatic function. It remains important throughout the plant's lifecycle, with increased requirements during heavy flowering.

SECONDARY MACRONUTRIENTS

Calcium (Ca), Magnesium (Mg), and Sulphur (S) are required in smaller but still significant quantities. Calcium and magnesium deficiencies are among the most common problems in cannabis cultivation, particularly in coco and hydroponic grows, or when using soft water (low mineral content). Cal-Mag supplements are standard additions to most nutrient programmes.

pH: THE MASTER VARIABLE

pH management is arguably more important than any specific nutrient application. Cannabis absorbs different nutrients effectively only within a specific pH range: 6.0–7.0 in soil, 5.5–6.2 in coco and hydroponics. Outside these ranges, nutrients are "locked out" — present in the root zone but chemically unavailable to the plant — causing deficiency symptoms that look identical to actual nutrient deficiencies.

Always adjust pH after mixing your nutrient solution and before feeding. Use a calibrated pH meter (not strips — strips are insufficiently accurate). Adjust with pH Up (potassium hydroxide or calcium hydroxide) or pH Down (phosphoric acid) in small increments.