There isn't really anything to add to the soil to destroy any pests and if you add a fertilizer it will make your beets fork next season. If you could find an different area to grow them it would be much better.

Thanks Seaburngirl!

Yes, ideally I would like another spot but we don't have one. I could get some fertiliser then.

Seaburngirl was warning you against using fertilizer!
I'd go back to containers next year to give the ground a rest. Grow peas or beans on your plot next year and ideally brassicas the year after, and then go back to beetroot. You could try putting beets on half the plot and peas on the other half to see what happens but don't put roots on the same plot two years running if you want them to stay healthy.

Cheers!

A few things I've learned about growing beets; always use fresh new seeds - seed viability declines very quickly compared to other vegetables; they are a cool weather crop, best grown in early Spring or Autumn - avoid hot weather. They are heavy feeders so lay down an inch or two of compost between plantings. Don't use chemical fertilizer at all; The trace mineral Boron must be present for proper beet development. Soil PH is very important. Never let the soil dry out. Harvest when beets are about 2-3 inches in diameter. Beet leaves make an excellent spinach substitute.

Thanks as usual Bathgate!

I am thinking of putting onions where the beets were last year and finding a spot somewhere else for the beets, maybe between shrubs where there is a reasonable gap.

Also thanks again to Steragram and Seaburngirl.

You're welcome: Beets & Onions - 2 of my faves!

Onions are still root crops Joan. Better than repeating beets but best to change from roots to legumes if you can.