What plants eat
Beyond light, water, and air, plants need nutrients: mainly nitrogen, phosphorus, and potassium (the N-P-K on the bottle), plus calcium, magnesium, and trace elements. Veg growth uses more nitrogen; flowering uses more phosphorus and potassium. Most nutrient lines split this into a grow bottle, a bloom bottle, and a cal-mag supplement.
In rich living soil you may need little or no added feed early on, since the soil supplies it. In coco and hydro, you provide everything.
Start low
Overfeeding is a common early mistake. Start at roughly half the label's recommended dose and increase only if the plant calls for it, such as pale color or slow growth. Burnt leaf tips usually point to too much rather than too little.
pH: the variable that gates the rest
Roots can only take up nutrients within a pH window. Outside it, the nutrients are present but unavailable, which shows up as "nutrient lockout" that resembles a deficiency. Check and adjust the pH of your water or feed after mixing in nutrients.
- Soil: aim for pH 6.2 to 6.8
- Coco and hydro: aim for pH 5.5 to 6.2
PPM / EC: how strong
PPM (or EC) measures how concentrated your feed is. A TDS meter lets you feed by measured numbers instead of estimates, and comparing your input PPM with the runoff from the pot indicates whether the plant is taking up nutrients or salts are accumulating.