Sad Beets

For the past couple years, the beet crop has gotten more and more disappointing. The rows come up very sporadic and they don't grow to a very large size. Last year I thought it was caused by poor seeds, but this year we tried several different varieties and brands and yet they are still a poor showing. All the leafy greens are going really well but both the radishes, carrots and beets, all root-forming, are doing terrible. So Alex and I have concluded that we must be missing something in the soil that encourages root development, but we have yet to determine what that is. Nitrogen is good for leaf production, so we must have lots of that, as can be seen in the previous two postings.
Alex and I will have to research whether we need to add more phosphorus or potassium and how to go about doing it naturally.