[djf]

What does the soil need if radishes and carrots grow (actually, quite vigorously), but they don’t make any radishes and the carrots are barely as big as a ping pong ball?

I think it’s potash but not sure...

[hoosiermama]

FWIW Was never able to grown any of the root veggies till I made raised sand beds....

[Red_Devil 232]

For your radishs they do need some space to develop the root 1 to 3 inches. They also need sun light. Hot weather can cause them not to develop also.

[Diana in Wisconsin]

You need to cut back on the nitrogen. Completely. I get this question all the time from customers at work. They're too NICE to their root crops; you have to be MEAN to carrots, LOL!

Wisconsin grows potatoes only second to Idaho. Carrots and onions, too. BUT...they grow best north of here where the glaciers turned the stone to sandy soil as they were passing through, so loose soil is a must.

This sounds like a good root crop fertilizer mix to me:

2 cups alfalfa meal, 1/2 cup kelp meal, 1/4 cup rock phosphate, 1/4 cup bone meal

Blend and use the same day.

http://www.gardenwiseonline.ca/gw/plants/2007/03/01/growing-root-vegetable-crops