What a plant eats
The plant makes its own food with light, water and air (photosynthesis), but it needs minerals from the soil to build itself. The ones it uses most are three, the ones written on every fertilizer as n-p-k:
N, nitrogen: leaves and stems. What it asks for most in vegetative.
P, phosphorus: root and flowers.
K, potassium: flowers, resistance and water transport.
The others
Calcium, magnesium and sulfur are needed in smaller amounts but are key (magnesium sits at the center of chlorophyll). Iron, zinc, manganese and others are needed in tiny amounts: they are the micronutrients.
How to give it
In veg, a fertilizer with more nitrogen; in flowering, one with more phosphorus and potassium. It dissolves in the watering water and you start with less than the label says (half is a good start): it is easier to fix a deficiency than an excess. Good fresh soil already has food for the first 2 to 4 weeks.
Deficiency and excess
Lack of nitrogen: the lower leaves turn yellow and the yellow climbs up.
Excess: the leaf tips burn (brown, crispy) and the leaves turn very dark, shiny green, with the tips pointing down.
Careful: many times the "lack" isn't a lack of feed but a pH out of range, which stops the root from taking it up.
🎮 Practice it in UnderGrow
The nutrients bar is like a battery. There are two fertilizers and the game uses the right one: grow in veg, bloom from pre-flower to harvest. You give them by tapping the plant › feed: no need to water or mix anything. Each dose adds 50 % to the bar: with the bar at 50 % one dose fills it (if it dropped to 0 %, two). With the bar above 100 % you went too far, and from 125 % the tips burn. Without an EC meter you only see low, good or high. In flowering, every hour with the bar between 35 % and 100 % counts towards a bonus of up to +12 % harvest grams. Cal-mag is something else: the cure for calcium or magnesium deficiency.
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