Onions are supposed to like phosphorus. Not all plants have the same requirements so what worked for tomatoes may not be good for onions. Go to the Dixondale site and read what they have to say about onions

Here is a link that might be useful: onions

Nate, Onions love, love, love good soil fertility. I add lots of compost and blood meal to the bed before I plant them. Some years I also use Dixondale's fertilizer, which is a 10-20-10 compost fertilizer, which contains not only the N-P-K but also humic acids from the compost, and also all the micronutrients onions need for steady growth and proper leaf development. As long as you have good healthy green leaves and lot of them at the time the onions begin to bulb up, Candy will give you the largest, prettiest onions you've ever seen. (The micronutrients in Dixondale's compost fertilizer are iron, copper, magnesium, boron, manganese, zinc, and molydebnum.) In the years when I don't use Dixondale's own fertilizer, I look for an organic fertilizer, like something from the Espoma line, for example, that contains the micronutrients.

As early as it is in the year, if you think that your onion plants look a bit off-color, that could be related to the incredibly cold nights we've had sporadically during the last few weeks. It dropped down to 14 degrees at our house either last week or the week before, and my onions have just recovered their good green color in the last day or two. All it took was a couple of nice warm days with plenty of sunshine.

Sometimes they are off-color because you have thrips, so check them for thrip damage.

I often use a foliar spray of liquid seaweed, liquid fish or Garrett Juice (an enhanced compost tea) for the onions, but normally not until April. Occasionally I use alfalfa tea as a foliar spray or a drench.

All,
many thanks for all the responses.