Ok, so you have some of the Black Prairie soil !
It may naturally grow good cole crops easier than some other soils !
I believe you said you had a small area of about 15 sq feet which is like 3 feet wide and 5 feet long ?
So this may be easy to take care of to grow some good food !
Try to find the Limestone and the Bone meal and some gypsum to apply this fall yet !
Since it is already getting cold up there, if you can loosen the topsoil some and then put the bonemeal down first and then the limestone and then the Gypsum, then you need to find some alfalfa hay from a feed supply house and get one square bale to spread over the garden soil for mulch this winter to help feed the soil Life and stop the ground from freezing and then water it down with some Sulfered Blackstrap Molasses , if you can find it and this may help the soil Life to start working !
Then in the spring time you can just rake back the mulch in different areas and plant your plants or seeds and as they start to grow add some more mulch around to stop any weeds from growing !
This should work very good for your cole crops and celery and carrots etc !
Divide your sq feet into the sq feet of what I had made suggestion, I believe it was for 250 sq feet, earlier, and as you may see it may not take very much Lime or Bonemeal or Gypsum, so the cost may not be very much !
The hay may cost the most ?
If you can not find the bale of hay, then you may be able to find alfalfa meal 50# to spread over the garden and then put oak or maple leaves or grass clippings over this for more mulch ?
Smile Tis your choice.