The uselessness of high phosphorus levels
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.)
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.
And the data:
TYPICAL COMPOSITION OF DRY ORCHID TISSUE BY PERCENT
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.