Building healthy soil is an ongoing process, because plants can deplete the nutrients in the soil as they grow. Adding organic matter will help replenish or "feed the soil". Organic fertilizers, made from plant, animal or mineral sources, release their nutrients slowly, which means that plants can feed as they need to and there is no sudden change in the makeup of the soil which might harm the microbial activity.
Soil is full of living organisms which help to maintain the balance needed to maintain soil health. Along with fertility, a good garden soil will have a friable texture that allows it to hold and drain water and a good soil pH.. By making healthy soil a focus at the start of making a garden, you will have a head start on creating a sustainable organic garden.
Soil fertility is hard to measure, but you can let your plants tell you if your soil is at least in a good range. Healthy, lush, pest free plants are happy with their soil fertility. Plants that are struggling, lanky, discolored or stunted are asking for better soil fertility.