Have you done a soil test? Did the test indicate that you needed to add nutrients to your garden soil?
If you are doing just a general soil amendment to your garden soil, you can work in some compost in the spring. This can be tilled in before you plant.
Your County Extension Office can help you with a soil test for just a few dollars and spring is a great time to do this testing.
Here is a link to help you locate your nearest office.
Doing a soil test will help you make the right decisions on garden amendments and will save you time and money in the process.