A bloom booster is a flowering-stage supplement with a lot of phosphorus, and usually potassium, compared with its nitrogen. It is added on top of the regular base nutrient once plants start to flower; it is not a replacement for that base feed. The name is marketing: the promise is that extra phosphorus means more or bigger flowers. The evidence supports a narrower conclusion. In cannabis trials where the base feed already supplied enough phosphorus, adding more did not raise flower yield or cannabinoid levels; it mostly sent more phosphorus out in the runoff. Extension horticulturists call “more phosphorus, more flowers” a consumer myth. The same trials do not show that a PK supplement is always useless: they did not test weak or unbalanced base feeds, real shortages of phosphorus or potassium, or specific commercial products. Flowering cannabis needs complete, balanced nutrition, not a phosphorus surplus.
What a bloom booster is
“Bloom booster” is a marketing name, not a technical category. The products vary, but most share one feature: the middle number on the label is much higher than the first. North Dakota State University Extension notes that in high-phosphorus fertilizers the ratio can be one part nitrogen to as much as five parts phosphorus (expressed as phosphate). Some products also add potassium, micronutrients or other ingredients.
How to read an N-P-K label
The three numbers on a fertilizer label are percentages by weight. Clemson University Extension explains that they give nitrogen (N), phosphorus expressed as phosphate (P2O5), and potassium expressed as potash (K2O). So the middle and last numbers are not pure phosphorus and potassium.
To convert, the University of Massachusetts soil guide gives factors of 0.44 for P2O5 to elemental phosphorus and 0.83 for K2O to elemental potassium. A label reading 10-30-20 therefore contains about 13% phosphorus and about 17% potassium by weight. That is still a lot of phosphorus, but less than the label suggests at first glance.
Two more points from Clemson are worth knowing. A “balanced” fertilizer, such as 10-10-10, has equal amounts of nitrogen, phosphate and potash. But plants do not use the three in equal amounts: nitrogen and potassium are needed in higher amounts than phosphorus.
What the research says
Commercial practice already disagrees with the myth. NDSU’s horticulturist notes that commercial greenhouses often apply fertilizers with a nitrogen-to-phosphorus ratio of 3 to 1, or even 5 to 1. The reason given: studies show that high phosphorus rates do not increase the number of flowers. Even a balanced 10-10-10 is considered a waste of phosphorus in that context.
Cannabis-specific evidence. A 2022 study in Frontiers in Plant Science fed one high-CBD cannabis variety with 25, 50 or 75 mg of phosphorus per litre through flowering, in an otherwise complete nutrient solution. The authors found no significant effect of phosphorus on flower yield or cannabinoid concentration, and concluded that 25 mg per litre was enough for maximum yield under their conditions. What did change was waste: leachate phosphorus rose 12-fold in response to the 3-fold increase in input. Phosphorus use efficiency rose from 31% to 80% as the input dropped from 75 to 25 mg per litre. The authors also point out that they studied a single cultivar, so the result cannot simply be extended to every variety.
The same paper reviews earlier work, and later trials point the same way. A study on the high-CBD hemp cultivar ‘BaOx’ compared 15, 60, 120 and 180 mg of phosphorus per litre; no rate above 15 mg gave more bud than 15 mg did. Another study found that extra superphosphate on top of normal liquid feeding did not raise flower yield or the concentration of major cannabinoids. A 2025 study from the same laboratory as the 2022 paper raised phosphorus from 15 to 90 mg per litre: phosphorus in the leaves doubled, but yield and quality did not change, and doubling the strength of the whole nutrient solution did not raise them either. The 2022 authors also note that it is common commercial practice to apply more than 100 mg per litre, even though few studies have tested whether such high rates help.
Where more phosphorus did help. Very low phosphorus does limit cannabis. An Israeli study grew two drug-type cultivars, one high in THC and one with balanced THC and CBD, at 5 to 90 mg of phosphorus per litre. At 15 mg per litre or less, photosynthesis and growth fell; the optimal range was 30 to 90 mg per litre, and 30 mg per litre already gave about 80% of the maximum yield. A Canadian study on a high-THC cultivar in deep-water culture predicted the highest flower yield at about 59 mg of phosphorus per litre, although the 2022 authors note that this design had less statistical power than the other trials. The studies therefore disagree on exactly where the ceiling lies, but none of them found a yield gain above 90 mg per litre.
What flowering plants actually need
Phosphorus is essential. It is one of the macronutrients, and a real deficiency will hurt growth. The point is that “enough” is much lower than bloom-booster marketing implies. The Frontiers paper notes that phosphorus in continuous liquid feed is considered adequate at 10 to 20 mg per litre for most species. Cannabis appears to need more in flower than in veg: about 15 mg per litre has been found adequate for vegetative growth in greenhouse conditions, one study found 30 mg per litre was needed for maximum dry mass in the vegetative stage, and several studies, including the 2022 one, indicate that flowering needs higher levels than that.
In practice, work out how much phosphorus your base feed already delivers at the strength you actually mix it, using the label conversion above, before adding more. For example, 1 g of a 10-30-20 powder in a litre of water supplies about 130 mg of phosphorus per litre, more than any rate that raised yield in the trials above. What makes buds is healthy leaves, strong light, a healthy root zone and steady feeding, not a spike in one element. The same Frontiers team notes that plants can take up nutrients beyond what they need, which they call “luxury uptake”. Higher phosphorus in the tissue does not mean more flowers.
Why too much phosphorus causes problems
Micronutrient antagonism. Penn State Extension lists phosphorus as antagonistic to zinc: an excess of one reduces uptake of the other. The Alabama Cooperative Extension System warns that excessive phosphorus, particularly with a high soil pH (above 6.5), can induce zinc and iron deficiencies. Colorado State University notes that high phosphorus levels may make zinc deficiency more severe. The Frontiers authors also point out that excessive phosphorus in the root zone can interfere with uptake of other essential nutrients.
Runoff and pollution. NDSU notes that excess phosphorus in soil can leach into lakes and rivers and promote algae growth. In a container or hydroponic grow, the surplus leaves with the runoff, as the 12-fold rise in leachate phosphorus shows.
Build-up in soil. In garden beds, the problem lasts. NDSU writes that phosphorus levels fall very slowly, and that it may take years for excessive levels to drop. It advises that unless a soil test shows otherwise, there is no need to add high-phosphorus fertilizers.
Living soil. The Vietnamese edition of True Living Organics warns that phosphorus-rich liquid bloom fertilizers can kill or upset mycorrhizal fungi. The grower can then see phosphorus and potassium deficiency signs and be forced back onto liquid feeding to finish the crop.
When a PK supplement can make a difference
The trials above added phosphorus to a feed that was already complete and strong enough. Outside those conditions a PK supplement can have a real job, although the evidence is thinner:
- A weak or unbalanced base feed. In one study with a fairly lean base feed (about 17 mg of phosphorus and 90 mg of potassium per litre), raising nitrogen, phosphorus and potassium together by about 15% increased fresh shoot weight by 41%, mostly in flowers and stems, while extra phosphorus alone did not raise flower yield. If your base feed is mixed below label strength or was designed for the vegetative stage, bring the whole feed up to strength first rather than adding phosphorus alone.
- A real shortage of phosphorus or potassium. In cannabis trials, 15 mg of phosphorus per litre or less reduced photosynthesis and growth, and in the vegetative stage 15 mg of potassium per litre was too little for normal growth and caused visible deficiency symptoms. A shortage that a tissue test or a feed calculation confirms is a reason to add the missing nutrient.
- Alkaline water or growing media. Here the problem is usually availability, not supply. At alkaline pH, iron, manganese, zinc and boron become unavailable to roots, and extra phosphorus at high pH makes zinc and iron deficiency more likely. Correct the pH first; a high-phosphorus product is the wrong fix.
- Potassium during bud development. Growers commonly report that flowering plants draw more potassium. The trial data are mixed: in an aquaponic system with a low-nitrogen solution, flower yield rose steadily as potassium went from 15 to 150 mg per litre, but in a deep-water hydroponic trial that varied nitrogen, phosphorus and potassium together, yield did not respond to potassium anywhere between 60 and 340 mg per litre.
For how one commercial PK supplement is labelled and dosed, see this product guide.
What the evidence does not cover yet
Most of these trials used a single variety each, grown under controlled conditions, so several questions remain open:
- The potassium half of PK. Only two flowering-stage trials that varied potassium were found for this article, in different growing systems and with different results, so how extra potassium in flower affects yield is still unclear.
- Weak or deficient base feeds. Most trials fed complete, adequate solutions. There is little data on what a PK supplement adds to a feed that is diluted, poorly balanced or short of phosphorus or potassium.
- High-THC cultivars under very strong light. Only a few high-THC cultivars have been tested, under light of about 570 to 980 µmol per square metre per second. Results for other cultivars, or for rooms lit more strongly than that, are not known.
- Specific commercial products. No published trial of a branded PK or bloom-booster product on drug-type cannabis was found for this article. Yield claims on the bottle come from the manufacturer, not from trials.
Choosing a flowering feed
- Use a complete base nutrient made for the flowering stage, at the recommended strength.
- Measure the EC of your feed and runoff instead of adding products “just in case”.
- In soil beds, test before adding phosphorus.
- If your water or medium runs alkaline, be extra careful with high-phosphorus products, because of the zinc and iron risk.
- Watch the plant, not the label. Healthy, green leaves through mid-flower are a better sign than any bottle promise.
Common mistakes
- Stacking a bloom booster on top of a complete bloom base that already supplies enough phosphorus. This raises salt levels (EC) and the risk of nutrient burn, and in the trials above extra phosphorus on a sufficient feed did not add yield.
- Reading P2O5 as pure phosphorus. Elemental phosphorus is only about 44% of the middle number on the label.
- Treating phosphorus as the “flower nutrient”. Nitrogen and potassium are needed in larger amounts.
- Blaming the wrong thing. Poor flowering has many possible causes. Virginia Tech’s diagnostic guide notes that flowering is reduced, delayed or absent under low light. Stress and pH problems also play a part.
- Ignoring zinc and iron. Yellowing new growth after heavy bloom-booster use can be antagonism, not a new deficiency to feed.