Much of Florida has calcerous soil, with visible chips of broken seashell mixed in. But Florida is a big state, and perhaps you are located somewhere in the Northern portion? A soil with pH = 4 is pretty acidic, and I would suspect that many plants, including some weeds, would not be able to grow in it. Is this native soil, from your yard, or is it soil that you purchased? If there is moisture present, the lime that you applied will have some effect within an hour or two, although it will take several months for all of the lime to dissolve. If I were in your situation, I would go ahead and plant the sweet potato slips.

How much organic matter is in that soil?
Sweet potatoes prefer growing in soils that are much warmer then most other plants like with a more acidic reaction, 5.8 to 6.2, which is one reason they do much better in the south than in the north. But higher levels of organic matter help buffer soil pH that is out of a plants usually preferred range.

Thanks for your responses and insight.

A little more info - I'm in Seminole, Pinellas County, Florida about 3 miles inland from the Gulf of Mexico between St. Pete Beach and Clearwater Beach.

Regarding this plot of soil - I added a 50 pound bag of Black Kow to that 50 sq ft area and tilled it in right before I planted my onions back in mid-November. So that's the only additive that I started with on top of the 4.0 pH soil. Over the next 3 months, I added a total of 12 pounds of Ammonium Sulfate as recommended for the onions by Dixondale. However, as mentioned I still have only trace amounts of nitrogen in the soil after I pulled up the bulbs.

And yes, I think I will plant the sweet potato slips soon, but it's my understanding I really can't do anything with adding nitrogen now because it's not good to mix lime with fertilizer?? Not sure how long I need to wait before I can mix some in.

The 50 lb bag of "Black Kow" followed by 12 lb of ammonium sulfate, taken together, are more fertilizer than I have ever applied to any garden plot. We have blueberry shrubs, and I give them Schultz Plant Food, dissolved in water, every spring. Each shrub gets one tablespoon of fertilizer in March, April, May, and June. That works out to about 1/8 lb of concentrated fertilizer, per shrub, per year. I realize that this does not directly compare to your garden plot, but it does give some perspective on how to apply concentrated fertilizers, and get good results. If you were to pick up some composted tree leaves, and mix this in with your garden soil, it would dilute the fertilizers that you have applied, and it might move the soil back toward normal. My guess is that at this point in time, your plants are suffering from too much fertilizer.

I guess I'm still confused since the tests showed only trace amounts of nitrogen. I realize fertilizer comes with a variety of compositions of nitrogen, phosphorous and potash, but the Black Kow is .5-.5-.5 and I used less than the recommend amount for a new garden bed.

But ... in thinking out loud.... the soil tests were done at a depth of 6-10 inches. But the onion roots never got that deep. I'm wondering if the Black Kow never made it that deep when tilled. And I will also say that amount of ammonium sulfate is exactly what Dixondale recommended for the area size and frequency, although it too was applied on the surface in trenches. Not sure how much made it to the depth of the soil test.

Hmmm.... if I hadn't already applied the lime, I'd be interested to redo the test closer to the surface. Ahhhh, the complexities of a gardening for an OCD newbie.

One last note - that patch only gets about 3 hours of direct sunlight and another 3 of filtered sunlight. I think the onions may have preferred more direct sunlight during the plant growth stage since they never got more than 6-7 leaves compared to the 12-13 they were supposed to get. And I believe I read the # of leaves relate to the number of rings in the onion, explaining why mine were so small. I also planted mid-November which was recommended for my area. But I think next year I will wait till late January, early February.

The nitrogen was used up or leached away rapidly, so it's not surprising there wasn't much there. Ammonium sulfate will lower pH though! I think that's what happened here. What was your pH before you added that? Unless it was way too alkaline for onions, thus requiring acid, it was not the best choice for a nitrogen fertilizer.

To answer your original question, there's no reason to wait as far as lime goes, if you used regular ground limestone (pH 8.3) and not the super alkaline ag lime (pH 12+). Your worst problem may be that the pH won't go up fast *enough* from 4.0. Yikes.

This is all really good information and I can't thank everyone enough for posting. I'm new at gardening and this site has been invaluable. Unfortunately, there's just a multitude of information out there, but most of it is from suppliers or from those who benefit by selling you a product. As a newbie, I'm anxious to learn, and learn from my mistakes, so the advice from experienced gardeners has been very helpful. Thank you.

Happy gardening, come back and let us know how things go!