Our recommendation is to fertilize at planting time and then again when the plants begin to set fruit or later in the season depending on what you are growing.
It would be best to get your soil tested, to get a baseline read on the nutrients, organic matter, and pH (which can affect nutrient uptake). The results of the test will inform you if you need to add specific amendments. Take a look at our soil testing page for information on how and where to submit a soil sample: https://extension.umd.edu/hgic/soils/soil-testing
If you add compost to your soil on a regular basis, it provides a good source of nitrogen and you can reduce supplemental fertilizing. Look at our website for more information http://extension.umd.edu/hgic/plants/caring-your-garden and fertilizing the crops that you want to grow. http://extension.umd.edu/hgic/plants/vegetable-crops