When we fertilize our gardens we are adding needed nutrients.    Nitrogen is the primary ingredient, provided to assist plants in the production of chlorophyll to support photosynthesis, allowing plant cells to build.   Nitrogen is needed to produce green leaves, buds, and fruit.   In addition, nitrogen works with the other needed ingredients, notably phosphorus and potassium.    Nitrogen may be present in the soil, but it can be depleted by plants and leaching, and erosion.

We can dig in organic material to enrich the soil.  I dig holes between plants in both the vegetable and the flower gardens and put leaves and other vegetative material in them through the growing season and more in fall before freeze-up.  Material from the compost will also help.

Nitrogen-fixing plants are different in that they can take nitrogen from the air in a symbiotic relationship with nitrogen-fixing bacteria in their roots.    Nitrogen is provided for these plants, but also added to the soil.

The legume family are nitrogen-fixers.  For most home gardeners, this will mean peas and beans, but also lupines.   Some shrubs and trees are also nitrogen fixing, but may not be practical additions (and may be invasive).

So, for the home garden, we grow peas, beans, and lupines, then we dig in the plants after seed production (fruiting).   Some gardeners may add a fall crop but the research is not conclusive about the benefits, especially if the growing season is short, as it is in the north.