Soil organic matter (OM) releases plant-available nutrients slowly during the growing season.  Your reliance on organic or synthetic fertilizers will probably decrease as your organic matter content increases.

• Aim for a soil organic matter content of 5-10% (it’s measured by weight by soil testing labs). Soils in this range are fertile, easy to work and have a large number of earthworms.
• For each 1% of OM, about 0.4 lbs. of nitrogen/1,000 sq. ft. is available for plants (conservative estimate). A soil with a 5% OM level would release about 2 lbs. of nitrogen/1,000 sq. ft. which is a typical nitrogen recommendation for vegetable gardens.
• Soil organic matter may not supply sufficient nutrients at particular times of the season and at particular stages of plant development. For example, in the early spring and when fruits start to form. The peak for nitrogen release typically occurs in July if soils have adequate moisture.
• Nevertheless, many people with well-established, high organic matter gardens, achieve large harvests without using fertilizers.