Short answer
Cannabis plants take 13 mineral elements from fertilizer or from the growing medium. Six matter most in a first grow: nitrogen, phosphorus, potassium, calcium, magnesium and sulfur. Seven more are micronutrients: iron, manganese, zinc, copper, boron, molybdenum and chlorine. One base nutrient, either one part or two parts, covers all 13. Bloom boosters, amino acid bottles and microbial products are not required in a first grow.
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What the plant uses
- Nitrogen (N): leaves, stems, proteins. Highest demand during vegetative growth.
- Phosphorus (P): roots, energy transfer, flower development.
- Potassium (K): water movement, enzyme activation, bud mass.
- Calcium (Ca) and magnesium (Mg): cell walls and chlorophyll. Coco coir and reverse osmosis water need extra.
- Sulfur (S): oils and aroma compounds.
- Micronutrients: iron, manganese, zinc, copper, boron, molybdenum, chlorine. Needed in parts per million.
N-P-K ratios on the label
The three numbers on a fertilizer label are the percent by weight of nitrogen, phosphorus (listed as P2O5) and potassium (listed as K2O). A 3-1-2 ratio fits vegetative growth. A 1-3-2 ratio fits flowering. Nutrient brands print feed charts, but plants respond to light, temperature and root volume as much as to the chart.
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pH
Soil: 6.0 to 7.0. Coco coir and peat: 5.5 to 6.5. Hydroponic water: 5.5 to 6.5. Outside that window, iron, calcium, magnesium and manganese stop moving into the roots even when they sit in the water. A pH meter with calibration solution costs about $40 to $80. Calibrate before the first feed and again each month.
EC and ppm
EC measures dissolved salts in the nutrient water. Common ranges: 1.2 to 1.8 mS/cm in vegetative growth, 1.8 to 2.4 mS/cm in flower. ppm readings depend on the meter conversion factor, 500 or 700, so compare EC numbers instead. Start at one quarter of the label strength and raise it over two weeks.
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Feed schedule
Weeks 1 to 2, seedling or clone
Plain water or one quarter strength base nutrient. Roots are small. A full dose burns them.
Weeks 3 to 6, vegetative
Base nutrient at half to full strength, rising as the plant fills the pot. Nitrogen demand peaks here.
Flowering
Switch to a bloom ratio or keep the base and cut nitrogen. Phosphorus and potassium demand rises. Calcium and magnesium demand stays high through week 5.
Final week
Some growers feed plain water for 7 to 14 days before harvest. Published research on flushing is thin. Test on one plant, not the whole room.
Common first-grow mistakes
- Full strength feed on day one. Nutrient burn shows as brown leaf tips and crisp edges.
- No pH meter. Most photos labeled nutrient deficiency are pH lockout.
- Adding boosters before the base nutrient is dialed in.
- Ignoring the water source. Hard tap water adds calcium and magnesium. Reverse osmosis water adds none.
- Confusing overwatering with overfeeding. Drooping leaves in wet soil point to water, not nutrients.
Legal note
Cannabis cultivation rules differ by US state. Some states allow home grows with plant count limits. Some allow none. Check your state law and local ordinance before you buy nutrients.