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The uselessness of high phosphorus levels

Posted by penfold2 4a, MN (My Page) on
Fri, Mar 25, 11 at 13:08

A recent thread (and many others) prompted me to write this, but I figured it would be more appropriate in its own thread.

So I see these high phosphorus "bloom booster" formulas used and recommended pretty frequently, and it bugs me a bit because I see no justification for it. I thought I'd toss out some data and see what everyone thinks. Here are a couple of threads on the use of high P fertilizers on plants in general.

The uselessness of high P fertilizers (I found the email from Dyna-Gro's own CEO to be particularly enlightening.)

Bloom Boosters - How Much P is Enough?

And here's an article I found which lists the tissue composition of several orchids. Apparently the information was taken from a book which stated that tissue composition was found to be very similar among different species of orchids.

Care & Feeding of Orchids

And the data:


If we average the values for leaves and roots, we get an N-P-K ratio of 1.9-0.25-3.2. If we then compensate for the P2O5 and K2O units that fertilizers are measured in, we get 1.9-0.57-3.86. And that is about equivalent to a 10-3-20 ratio. This matches the data presented in the first two links pretty closely, except for a considerably higher potassium requirement. I see no data anywhere however, that suggests that plants use the crazy amounts of phosphorus present in bloom booster formulas. When using these fertilizers to supply the proper amount of N and K, the excess P just raises the TDS (total dissolved solids) to higher concentrations which could potentially burn roots.

I know there will be people who say "Well I use such-and-such fertilizer with great results," but the problem only occurs when we are pushing the limits of fertilizing. High fertilizer use during peak growth, and plants that are very sensitive to high TDS levels are the problem areas. High P fertilizers may work with many species at low to moderate concentrations, but there's no sense in creating a potential problem by supplying many times more P than a plant will ever use.

FWIW, I use Dyna-Grow 9-3-6, and have never had trouble with blooms. I am however, now considering adding some Pro-Tekt 0-0-3 after seeing how much K was found in the above study.

Follow-Up Postings:

RE: The uselessness of high phosphorus levels

I think the term "bloom booster" is a misnomer. It stimulates the growth of new roots, and used every third fertilizing will initite blooming if it is TIME for the plant to bloom. It is used judiciously by some growers preparing for sales in shows, or specific holiday sales.

RE: The uselessness of high phosphorus levels

It has been shown that reducing nitrogen while maintaining other nutrient levels can initiate blooming, but I've never seen any evidence that excess phosphorus stimulates anything in plants. This seems to be a long held myth among growers. In the first link I provided, a fertilizer company's own CEO stated that their bloom booster formula is made because of consumer demand for it, not because of any scientific evidence of its usefulness. "...It is simpler to give the market what they think they need than to try to reeducate it."

Of course people are free to use whatever they feel is best. I just like to see people looking at the evidence rather than looking at what others do. That's how myths get perpetuated.

RE: The uselessness of high phosphorus levels

As long as there are threads like this one with information from knowledgeable growers, perhaps someday the public will become re-educated. Much food for thought. Many thanks.

RE: The uselessness of high phosphorus levels

"It has been shown that reducing nitrogen while maintaining other nutrient levels can initiate blooming"

Your sentence proves that there is something not quite right with your theory. You reduce Nitrogen of 20-20-20 fertilizer (high phosphorus) to 6.6-20-20 to initiate blooming. Your fertilizer is almost the same with commercial 10-30-30 fertilizer (high phosphorus, blooming fertilizer).

RE: The uselessness of high phosphorus levels

I was talking about starting with a fertilizer like the 9-3-6 that I use, and switching to something like a 6-3-6. The N is reduced to 6 while maintaining P and K at 3 and 6.

The exact numbers will vary depending on what fertilizer you use. My point was just that I would always keep the middle number the smallest.

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