Apply no phosphorus without a soil test showing it is needed. You can't get rid of an overdose without digging out and replacing the treated soil, since phosphorus leaches very slowly. And you can produce a toxic condition with overfertilization, including and especially with phosphorus (which is commonly applied to home gardens in high amounts).

Strictly speaking it is always the case that it is not known if and how much fertilizer is called for without soil testing or some other more or less reliable indicator. Soils very in how much nutrients they are providing to plants, both in time and in location. The same lot may have markedly different soil conditions between one part and another.

The points raised are noteworthy and to be considered...although what is 'toxic' for some organisms may be helpful to others...it is relative to the organism and desired results...and plant response can be a reliable indicator...

The Ipomoea Morning Glories often flower more profusely,flower longer and set better seed when a phosphorus based bloom booster is used and the additional phosphorus is doing something other than simply correcting a 'generalized' phosphorus deficiency by some standard other than what produces the best flowering results in Ipomoea Morning Glories...

The amounts of phosphorus present in the soil and as may be present around the rhizosphere per se is not tantamount to phosphorus levels as may be actually absorbed by the plant...the actual bioavailability of the phosphorus is the key factor and increasing phosphorus bioavailability is the crux of the issue...

The asagao that are grown in Japan receive phosphorus supplements and the plants that don't will not yield show quality plants...because beneficial organisms are particularly difficult to maintain in small containers a 'brute force' application of phosphorus may sometimes be the only way to insure that the plants absorb the phosphorus for show quality plants...

The rhizosphere organisms greatly assist in the absorption of various nutrients and phosphorus levels that are excessive are generally inhibiting to the rhizosphere organisms...if the rhizosphere organisms are tuned just right the roots will absorb phosphorus much more efficiently than if the rhizosphere organisms are somehow lacking(e.g.,rhizosphere carboxylates and other organic acids vary with soil properties and enhance phosphorus uptake). ...the catch-22 is that the rhizosphere is very often less than optimal for ideal phosphorus absorption and therefore application of high phosphorus is often required to insure high phosphorus intake by the plant...ideal rhizosphere environments are particularly difficult to achieve in containers,more particulatly in small containers...

I generally advocate trying to encourage and maintain soil mycorrhizal relationships and mycorrhization helper bacteria (MHB),but as this is particulary difficult to achieve in containers,and application of phosphorus produces results unless you are willing to spend a lifetime of experimentation trying to fine tune your rhizosphere for maximum phosphorus absorption / utilization...different forms of phosphorus can react very differently relative to the overall soil environment (including valency of various ions)

http://en.wikipedia.org/wiki/Phosphorus

http://www.google.com/search?hl=en&as_q=what+are+the+different+forms+of+phosphorus&as_epq=&as_oq=&as_eq=&num=100&lr=lang_en&as_filetype=&ft=i&as_sitesearch=&as_qdr=all&as_rights=&as_occt=any&cr=&as_nlo=&as_nhi=&safe=images

http://www3.interscience.wiley.com/journal/119621707/abstract?CRETRY=1&SRETRY=0#ss1
[Superphosphate, granular and powdered basic slag, and normally and very finely ground Gafsa rock phosphate were compared as sources of phosphorus for established grassland under a wide range of conditions in England and Wales. Superphosphate and both forms of basic slag were about equally effective in all cases. Gafsa rock phosphate was a useful form of phosphate for grassland where the soil pH value was below about 6.0, but it was much less effective and slower acting on neutral soils or where lime had recently been applied. Very finely ground rock phosphate was only slightly more effective than the normally ground form, and their value as phosphatic fertilizers depended primarily on soil pH rather than on rainfall.]

The benefits and drawbacks of phosphorus application are something for each individual to decide for themselves...try some different experiments in your local conditions and note plant response...experience is the best teacher...

I shared on the rhizosphere in the thread here
http://forums.gardenweb.com/forums/load/vines/msg040710098176.html

You can focus on increasing the beneficial micro-organisms by using compost and micro-organism stimulating substances such as humic substances [humin, humic acid, and fulvic acid resistance to microbial decomposition in the order humic acid > fulvic acid > humin (Qualls 2004).
a clear distinction must be made between products containing humic substances and those products that are humic (or fulvic) acids extracted from the primary sources listed. Differences in microbial response were related to the molecular weight of the humic acids, with the lower weight fractions, typical of composts, causing greater microbial stimulation than the higher molecular weight fractions extracted from brown coal.
Inoculation with natural or genetically engineered microbial formulations can be broadly categorized according to whether they are intended to (a) exist on their own in the bulk soil, (b) populate the rhizosphere, (c) form symbiotic associations with plants, or (d) promote microbial activity on leaf or straw surfaces. they will also affect beneficial soil fungi and other soil organisms.]

phosphorus bioavailability
http://library.wur.nl/frontis/gene-plant-crop/19_lambers.pdf

Soil Syrup
http://www.algoflash.com/SoilSyrup.htm
Actino-Iron
http://www.surefill.com/products/iron.asp

Mighty Myco
http://homeharvest.com/beneficialmicroorganisms.htm

Beneficial microbes
http://www.advancednutrients.com/landing_pages/piranha_landing.html
http://www.advancednutrients.com/landing_pages/tarantula_landing.html

Soil that is alive and rich in thriving beneficial organisms is the Greenest way to go...

Hope the aforementioned contributes to food for additional thought...

Regards...

Ron

P.S. - The former Genus of Mina has been absorbed into Ipomoea as a subgenus and therefore the current accepted botanical name is Ipomoea lobata.

The judicious application of Nitrogen after (!) the plants have initiated flowering will often increase the degree of flowering response...

Thanks for the advice guys. I'm wondering if a water soluble 15-30-15 diluted to half the recommended strength will be good for the vines, or if it will still be too much nitrogen and give me leaves and no blooms.
Also, Ron since you've been on this thread...
What do you think would be the best choice for the situation I describe above: ipomoea lobata, ipomoea obscura, impomoea nil (cameo elegance), or ipomoea tricolor.
Thanks
George

Hi George - The fertilizer NPK ratio you mentioned would usually work out well for many MG's...

My thoughts would be that the Ipomoea nil and Ipomoea tricolor would be more reliable choices that the others that you mentioned...

I'd also like to suggest the following for consideration

Ipomoea cristulata - US native
http://davesgarden.com/guides/pf/showimage/216053/

Ipomoea lindheimeri - US native -
http://davesgarden.com/guides/pf/showimage/127323/
caudex forming perennial root can be overwintered indoors

Ipomoea pandurata - US cold hardy perennial
http://davesgarden.com/guides/pf/showimage/207097/

Ipomoea purpurea - flaked strain
http://davesgarden.com/guides/pf/showimage/150686/
http://davesgarden.com/guides/pf/showimage/165547/
http://davesgarden.com/guides/pf/showimage/65373/

Ipomoea nil x purpurea

Yaguruma
http://davesgarden.com/guides/pf/showimage/174762/

Maisugata
http://davesgarden.com/guides/pf/showimage/89520/

Ipomoea setosa - unusual soft spines and maple shaped leaves make for an exotic look
http://davesgarden.com/guides/pf/showimage/170281/
http://davesgarden.com/guides/pf/showimage/86512/
http://davesgarden.com/guides/pf/showimage/88134/

Hope that helps...

Regards,...

Ron