I have been feeding it 8-4-4 and recently changed to MG 9-4-9 Liquafeed Tomato, Fruit, & Vegetable Food.
Do I need more Phosphorus? If so, what can I do to give it a phosphorus boost? Would Flower Fertilizer 0-10-10 work?
My impression is that if the flowers are growing then falling off, it's not a deficiency. I think 9-4-9 is balanced enough to provide for whatever the plant wants to do. I've always used 9:3:6 and once they were ready, mine set fruit as much as they wanted (even despite being neglected in poor cultural conditions for awhile).
Is this a new plant from this year? How long has it been dropping flowers?
More likely it just isn't quite mature enough to bloom and start fruiting. Many plants will go through a phase of dropping flowers before they actually start to stick and do something with them--I've definitely had most of my peppers start off by dropping flowers. Waiting solved it for me.
So you might be best served by waiting, though I'm not sure what you should expect from Jalapenos in the Chicago area this time of year.
You might also consider asking in the Hot Peppers forum, if you haven't already--probably lots of good advice in there.
(On a side note: I'm not *good* at growing peppers as a crop so far, but I have spent a lot of time with the plants!).
Nr2g: What kind of potting mix are you using? How old are the plants? How concentrated are you making the fertilizer? How often are you fertilizing? I'm not growing jalapenos this year, but I'm growing several other kinds of peppers. Mine were put in pots with 5-1-1 including some controlled release fertilizer at half strength on May 14. A couple of my peppers already have fruits and others dont even have blooms yet. Hot peppers take longer to produce fruits. I don't plan to add additional fertilizer for a few more weeks. And then I will only fertilize at half strength every couple weeks. Of all the vegetables I grow in containers, peppers seem to need the least amount of fertilizer.
Don't be so quick to diagnose every problem as a nutrient deficiency. Blossom drop is more likely to be caused by other things, like high or low temperatures, too much water or even overfertilizing. Be patient. Summer hasn't even arrived in your zone yet. Let mother nature do her thing.
Chances are, the plant just needs some time. I agree with Ohio...let Mother Nature do her thing.
Causes of flower drop, paraphrased from Willard at the Hot Pepper Forum.....
Hot day temps, cool night temps
Too humid, too dry
Too much Nitrogen, too little Nitrogen
Too much water, too little water
lack of pollinators
Too much grower attention ;-)