How much to feed in flower (and where it's wasted)

Nutrient Ratios That Actually Move the Dial

What You Need to Know

The conventional wisdom says PK boosters drive flower size. Buy the bottle with the big numbers, push it in weeks 3–6, and watch the buds pack on. Sounds good. Bevan’s team at the University of Guelph actually tested this. They ran response surface analysis across twenty different NPK combinations, measured yield, ran the numbers, and published findings that should shift how you think about nutrient ratios in flower.

Here’s what they found: potassium doesn’t matter. Not a little. Not conditionally. Across every tested concentration from 60 to 340 mg/L, potassium had zero effect on inflorescence yield. The response surface was flat. Meanwhile, nitrogen and phosphorus show clear, measurable response curves — there IS a sweet spot for each, and most growers are overshooting on phosphorus without knowing it.

This module breaks down exactly where those sweet spots are, what the underlying biology is doing, and what this means for how you mix your nutrient solution.

The Science

Every nutrient bottle has three numbers on the front: N-P-K. Nitrogen, phosphorus, potassium. In Level 1, we covered the basics. Now we’re looking at the data that defines optimal ranges during flower.

Bevan’s team grew 100 cannabis plants (cultivar ‘Gelato’, high THC) in deep-water culture — same genetics, same light (1,000W metal halide at 570 PPFD), same environment, same everything except the nutrient solution. They tested nitrogen from 70 to 290 mg/L, phosphorus from 20 to 100 mg/L, and potassium from 60 to 340 mg/L. Every plant got all three nutrients — the question was how much of each.

The response surface model — a statistical method that optimises multiple variables simultaneously — produced findings that re-define nutrient efficiency in flower.

Nitrogen: Yield responded quadratically to N. This means there’s a sweet spot. Too little N and yield suffers. Too much and it starts declining. The optimal concentration was predicted at 194 mg/L. The best yield range sat between 160 and 230 mg/L. Below 160, yields dropped markedly. This makes biological sense — nitrogen is a structural component of chlorophyll, amino acids, and proteins. Without enough N, the plant can’t build the photosynthetic machinery to convert light into bud. It’s the raw material for growth.

Phosphorus: Also quadratic, also has a sweet spot. Optimal concentration: 59 mg/L. Best range: 40–80 mg/L. Outside that window — especially above 80 — yield dropped. This one matters because most cannabis-specific nutrient brands recommend phosphorus concentrations above 100 mg/L during flower, and some push north of 200. Bevan’s data says the plant doesn’t want it. Worse, excess phosphorus runs off into the environment and contributes to waterway pollution. You’re paying to pollute.

Potassium: Across the entire tested range — 60 to 340 mg/L — potassium had no statistically significant effect on inflorescence yield. The response surface was flat. The line was flat. The plant took what it needed and ignored the rest. Some nutrient companies are recommending 300–400 mg/L of K during flower. The plants in this trial received up to 340 mg/L and it made no difference to yield compared to plants getting 60 mg/L.

The highest average yield predicted by the model was 144 g/plant at 194 mg/L N and 59 mg/L P. The K could be anywhere in range — didn’t matter.

And cannabinoids? No nutrient treatment affected them. THC, THCA, CBD — all statistically the same across every single treatment. You can’t feed your way to higher potency. The plant’s chemistry is set by genetics and other environmental factors, not by how much potassium you throw at it.

One more finding worth sitting with: yield correlated almost perfectly with overall plant size. The correlation between inflorescence yield and aboveground plant fresh weight was r = 0.98. Yield and root dry weight: r = 0.9. Bigger plant, more bud. It sounds obvious when you say it out loud, but it means the single best predictor of how much your plant yields is how big and healthy it is going into flower. Get the vegetative stage right — proper N, proper light, proper environment — and the flowers take care of themselves.

How To Apply This

The research gives you concrete anchors for formulating your flower nutrition:

• Target 160–230 mg/L nitrogen during the entire flower run. This range captures the sweet spot identified by response surface analysis. If you’re in coco or hydro and mixing from concentrates, this is achievable. If you’re in soil with slow-release amendments, the principle still holds: moderate N through flower.

• Target 40–80 mg/L phosphorus, with 59 mg/L as the predicted optimum. Check your nutrient brand’s recommended PPM/EC against these numbers. If their flowering schedule puts you above 100 mg/L phosphorus, you’re in excess. Feed charts are designed to sell product, not to optimise yield. The feed chart is the marketing department’s favourite tool.

• Potassium can sit in the 60–200 mg/L range. It doesn’t matter where within that window you land. Most complete nutrient solutions already provide enough. Unless you’re mixing from individual salts and deliberately removing potassium, you’re grand.

• Focus your veg stage on building a big, healthy plant with a strong root system. The r = 0.98 correlation between plant size and yield means the most impactful thing you can do for your harvest happens before you flip to flower. Vegetative nitrogen demand is higher — 180–220 mg/L is reasonable — because you’re building structure. In flower, dial it back to 160–230 mg/L.

• Stop buying PK boosters. Three years of testing across a range wider than most feed charts, and the yield line didn’t flinch. Save the €50 a cycle and spend it on equipment that actually tells you something true — a decent pH pen, a TDS meter, or tissue analysis.

Seb’s Corner (Level 2+)

The response surface methodology (RSM) used by Bevan’s team is worth understanding because it addresses a chronic limitation of cannabis nutrition research. Most prior studies varied one nutrient at a time while holding others constant, which misses the interaction effects between nutrients. RSM allows concurrent optimisation of multiple factors and can detect interactions — like whether high N compensates for low P, or whether K only matters when N is deficient. In this trial, no significant N×K, P×K, or N×P×K interactions were found. Potassium was genuinely inert within the tested range. However, two caveats warrant attention. First, this was a single cultivar (Gelato) in deep-water culture with weekly solution replacement. Substrate-based systems with less precise nutrient control may show different K dynamics, particularly in coco coir where cation exchange can temporarily sequester potassium. Second, the K range tested (60–340 mg/L) may not have gone low enough to identify a true deficiency threshold. At 60 mg/L K, no deficiency symptoms were observed — but that doesn’t mean 30 mg/L would be sufficient. The safe takeaway: most growers are providing K well above the minimum required, and increasing it further does nothing for yield.

Watch Out For

• Phosphorus excess. Most growers overshoot P, and excess P drives down yield while creating runoff problems. The plant’s window is narrow: 40–80 mg/L. Check your actual concentrations.

• Assuming PK boosters work because you can’t see the alternative. This is selection bias — you’d need a side-by-side comparison to know if the boost actually happened. Bevan’s team ran that experiment and found nothing.

• Cannabinoid chasing via nutrient ratios. No nutrient ratio in this trial altered THC, THCA, or CBD. Genetics and environment set the ceiling. Nutrition sets whether you reach it.

• Overfeeding vegetatively then expecting flower to fix itself. A stunted or weak plant going into flower will produce weak flowers. The veg stage is where yield is decided — not by any miracle of flower nutrition, but by building the infrastructure.

• Missing the r = 0.98 signal. Plant size predicts yield almost perfectly. If your plant is small at flip, no nutrient ratio in flower will recover that yield loss. This is the highest-value insight in the module.

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