Composted cow manure.
It is available in bags at the garden center for a couple bucks. One bag covers about a 10 x 10 foot area, so 3 or 4 bags raked around on the soil should be great. By adding a large volume of organic matter, you will increase the biologic action in the soil, which helps more than just putting nitrogen down, plus, it won't burn if you apply it wrong.
Each application should last about a year, so it's slow and steady.
At the recommended rate, it can even be applied over grass, just rake it around, so if you just can't give up the old lawn area, it will still work.