dsws

One of the more experienced people here can tell you the specific numbers, but I'm pretty sure 15-15-15 isn't what you want.

Also, I recall someone with poorly-drained soil getting told it can be better to make a mound of good soil on top of the impervious soil, instead of digging out a hole. A hole in poorly-drained soil is like a pot with no hole in the bottom: it fills with water, which has nowhere to go.