Chemical fertilizers can supply the nitrogen necessary for quick plant growth, but the best organic nitrogen fertilizers add micronutrients and organic matter to the soil in addition to nitrogen, with less chance of nitrogen 'burn' or the problem of excess nitrogen leaching into water bodies. Seed meal, fish emulsion, and composted manure are each excellent organic nitrogen fertilizers for use in different applications.
Seed meals are the by-product of the seed oil pressing industry. Seed meal, including canola, sunflower, cottonseed, and flaxseed, is available in bulk very inexpensively from farm supply depots, where it is sold as animal feed rather than fertilizer. Homesteader and organic garden book author Steve Solomon, writing in Mother Earth News, recommends using certified organic seed meals which do not contain chemical herbicide or pesticide residues, and are not genetically modified.
Seed meals contain an NPK analysis of about 6-4-2, and provide a significant amount of organic matter to the soil to improve tilth, soil structure, and drainage, making it the best organic fertilizer for turning into the soil in bulk before planting.
Liquid Fish Emulsion
Liquid fish emulsion's NPK analysis runs about 5-2-2. Liquid fish emulsion also contains significant quantities of micronutrients like iron, magnesium, and boron which play a critical role in many plants' growth. Liquid fish emulsion is easily soluble and can be diluted for small, consistent doses, readily absorbed by small seedlings and in other applications where bulk dry fertilizers would be difficult to work with. Liquid fish emulsion is the ideal organic nitrogen fertilizer for houseplants, containers, and seed trays.
Composted manure is high in nitrogen, but according to the Oregon State University Extension Service Gardening and Water Quality Protection publication, composted manure does not wash out of the soil, thus reducing its potential to create a hazard to nearby aquatic environments.
Composted manure releases its nitrogen slowly over a long period of time, supporting strong, healthful plant growth. Composted manure's high organic material content makes it an ideal mulch or top dressing. Worms and other micro-organisms will break the compost down further from below, pulling its nutrients down to the plant root zone. Meanwhile the composted manure mulch will keep weeds down and insulate plant roots from swift changes in temperature or moisture levels, reducing plant stress. Composted manure is the ideal multi-purpose organic nitrogen fertilizer.