Richest source of nitrogen is

Nitrogen is the main component of our atmosphere and is required as a micronutrient by plants for their growth, development and also for survival.
The richest source of nitrogen out of all given options is dried blood. Adding dried blood to the soil will increase nitrogen levels to improve the growth of strong and healthy plants.
A blood meal is a dry, inert powder made from blood and one of the highest non-synthetic sources of nitrogen. Its nitrogen content is about 13.25%.