Feeding, And When Soil Runs Out

Feeding is where beginners do the most damage while trying the hardest. The bottle says to use 10ml per gallon, the plant looks a little pale, and it seems obvious that more food equals more bud.

It does not. Cannabis in soil is easier to overfeed than to underfeed, and an overfed plant is harder to fix than a hungry one. A hungry plant recovers in about a week. A plant with salt buildup in the root zone takes longer and may never fully catch up.

So the approach here is deliberately conservative. Feed later than you think, at half the strength the label says, and let the plant tell you when it wants more.

Your soil is already feeding it#

If you followed step one and bought pre-amended organic potting soil, there is roughly three to four weeks of food already mixed in. Fox Farm Ocean Forest, Roots Organics and similar bags are built exactly for this.

That means no nutrients at all for the first three to four weeks. None. Plain pH adjusted water.

Adding fertilizer on top of amended soil in week two is the single most common way people burn a seedling, and because burn and deficiency look similar to an untrained eye, they often respond by adding more.

The signal to start feeding#

Watch the lower leaves. When the oldest, bottom-most leaves start to fade from deep green to light green to yellow, and it is happening evenly across the whole leaf rather than in spots, the plant is out of nitrogen and pulling it from old growth to feed new growth. That is your cue.

This usually shows up somewhere between week three and week five, depending on plant size and how fast it is drinking.

Two things to confirm before you reach for a bottle:

• Is it actually the bottom leaves? Yellowing that starts at the top or in the middle is a different problem, and it is usually pH, not food.
• Are the cotyledons just dying? Those first two round leaves yellow and fall off on schedule. That is not a deficiency.

How to start#

Buy two bottles: a grow formula and a bloom formula. That is a complete beginner lineup, twenty five to forty dollars total. Ignore the eight bottle systems. They exist to solve problems you do not have and to sell you eight bottles.

Read the feeding chart on the bottle, then use half of what it says, or a quarter if you want to be careful. Manufacturer charts are written for large plants in inert media by people who sell nutrients. They are not lying, they are just not describing your situation.

Feed every second or third watering, not every watering. Plain water in between lets the soil flush and gives the roots a break from salts.

Increase slowly. If the plant handled half strength for two weeks with no burnt tips and the lower leaves greened up, go to three quarters. If it looks perfect, leave it alone.

Grow versus bloom#

The three numbers on a nutrient bottle are N-P-K: nitrogen, phosphorus and potassium.

Nitrogen builds leaves and stems. A vegetative plant wants a lot of it, which is why grow formulas are nitrogen heavy, something like 6-4-4.

Phosphorus and potassium support flower development. Bloom formulas flip the ratio, more like 2-8-4.

The transition is simple. Run grow through veg. When you flip a photoperiod to 12/12, or when an autoflower starts showing pistils, switch to bloom over a week or two rather than all at once. A common approach is to run both at half rate for the first two weeks of flower, then drop the grow formula entirely.

Stage	What to feed	Strength
Weeks 1 to 3	Plain water only	None
Weeks 3 to 4	Grow, if lower leaves are fading	1/4 to 1/2
Veg through the flip	Grow	1/2 to 3/4
Flower weeks 1 to 2	Grow and bloom, both light	1/2 each
Flower weeks 3 to 7	Bloom	1/2 to 3/4
Final 1 to 2 weeks	Plain water	None

Calcium and magnesium#

One supplement is genuinely worth adding for most indoor LED growers: cal-mag, around fifteen dollars a bottle.

Two reasons. LEDs drive faster growth than the old lighting these nutrient lines were formulated around, and heavy feeders run through calcium and magnesium quickly. And soft or filtered water strips out the calcium and magnesium that tap water normally supplies.

Magnesium deficiency looks distinctive: yellowing between the veins on middle and lower leaves while the veins themselves stay green. Calcium deficiency shows as brown spots and distorted new growth up top.

If you are on ordinary tap water in soil, you may not need it. If you are on reverse osmosis or softened water, you almost certainly do. Add it at half the label rate when you feed.

Reading the mistakes#

Nutrient burn starts at the very tips of the leaves and works inward. The tips go yellow, then brown and crispy, usually on the upper leaves that are growing fastest. A dark, almost glossy green plant with clawing leaf tips is being fed too much nitrogen.

The fix is to flush the pot with three times its volume in plain pH adjusted water, then cut your next few feeds in half. Burnt tips do not heal, but new growth will come in clean.

Deficiency is more uniform. Whole leaves fade rather than just tips, and it usually starts at the bottom.

The flush debate#

You will see advice to flush with plain water for the last one to two weeks before harvest, to improve taste. You will also see growers say the lab evidence for this is weak and that a proper dry and cure does all the work.

Both camps have a point. The call: run plain water for the last one to two weeks. It costs you nothing, the plant is barely eating by then anyway, and if it turns out the taste benefit is real, you got it for free.

Organic versus bottled#

If you want to simplify further, there is a whole approach where you build living soil with compost, worm castings and dry amendments, and then only ever add water. It genuinely works and it makes very good weed.

It also takes weeks of soil prep before you plant anything, and if the mix is off you have no fast way to correct it. Pre-amended bagged soil plus two bottles is the faster path to a finished harvest, which is the actual goal of grow one.

Notice how many times pH came up on this page. It is the thing that sits underneath feeding, watering and half the symptoms people misdiagnose, and it is worth understanding properly.