Q.Root Maggots in Turnips and Radishes
Will sprinkling ashes on top of the row where I have planted turnip seeds keep the maggot flies from laying their eggs there? Will the ashes harm the plants or the soil?

I would not recommend using ashes to get rid of them as the ashes, depending upon what all is in the mix of ash, can cause soils problems that will take some time to correct. Here is a link to a great article on controlling root maggots for you:
https://www.gardeningknowhow.com/plant-problems/pests/insects/root-maggots.htm
Stan