Soil fertility is a measure of how well soil can hold onto nutrients and provide them to plants. A healthy vegetable garden depends on soil fertility to keep carrots, beans, and tomatoes flourishing.
There are many conditions that are necessary for soil fertility, including adequate water; plenty of organic matter, such as compost; and aeration that allows air, nutrients, and water to move through the soil. Fertile soil is full of the specific nutrients that a particular plant needs, and it's capable of delivering those nutrients to the plant's roots.