[header] How to make nitrogen fertilizer [title] Use compost. [step] Compost is nothing more than decomposed organic matter. The average compost pile contains a plethora of beneficial nutrients, including potassium, phosphorus, and nitrogen. 
Regarding nitrogen, the bacteria in compost break matter down into ammonium, which is then naturally converted by other bacteria into nitrates that plants can absorb through their roots. Compost high in nitrogen materials, including moist greens, fruits, and vegetables, tends to provide the highest content of nitrogen to the soil bed it is applied to.