Most gardeners know that many perennial herbs such as rosemary, thyme and sage are most flavorful when grown in lean soil without a lot of fertility. But more fertile soil has often been recommended for annual herbs such as basil.
In some interesting new research done at Southwestern University, nitrogen fertilization was found to have a significant effect on the levels of phenolic compounds in both 'Genovese' and 'Dark Opal' basil. These are the compounds that give this herb its healthful antioxidant properties. In the case of both basil cultivars, the level of phenolic compounds was highest when nutrient availability was lowest. So, although higher rates of fertilization may result in lusher growth, for the highest antioxidant activity and nutritional value, go easy when adding nitrogen fertilizer to your basil crop.
For more information on this research, go to: Effects of Nitrogen Fertilization on Basil.