Download Full Text (878 KB)

Publication Date:



Nitrogen (N) is an essential nutrient for animals and plants. For cereal crops, it is often the most limiting nutrient and therefore important in regard to fertilization and management. Nitrogen is abundant in nature; air is 78% N; rocks of the earth’s crust have 50 times more N than the atmosphere; and the surface layer of most cultivated soils contains 1,200 to 6,000 lb N/acre, with more than 90% in organic forms. However, the majority of this N is not in a form that plants can take up, and must be converted to plant available ammonium (NH4) or nitrate (NO3), or supplied from atmospheric N2 fixation by plant/microbe symbiosis or industrial fertilizer manufacture. Nitrogen is very reactive in that it can change among many forms: organic like amino acids, proteins, and chlorophyll; gasses like ammonia (NH3), dinitrogen (N2), and nitrous oxide (N2O); and ions like NH4 +, nitrite (NO2 -), and NO3 -. Conversion from one form to another occurs by many chemical and biological processes, which are highly influenced by environmental conditions, especially temperature and moisture. The overall interaction between soil, air, microbes, plants, animals, and humans is called the N cycle (see figure). In soils, plants and microbes interact with all components of the cycle, with many processes occurring simultaneously and all having potential influence on the fate of N. A major factor complicating N management for crop production, and the importance of climate, is that the soil is an open system – meaning than N can move out of the soil (“be lost”) to the atmosphere or to ground and surface waters. If such movement did not occur, then N management would be much less complicated. Research continues to better understand the intricacies of the soil N cycle and the influence from climate, with the goal to provide management options in order to enhance N use by crops and therefore improve agronomic efficiency, economic profitability, and environmental quality.


Agricultural Science | Agronomy and Crop Sciences | Climate


Iowa State University Extension and Outreach publications in the Iowa State University Digital Repository are made available for historical purposes only. Users are hereby notified that the content may be inaccurate, out of date, incomplete and/or may not meet the needs and requirements of the user. Users should make their own assessment of the information and whether it is suitable for their intended purpose. For current publications and information from Iowa State University Extension and Outreach, please visit

Soil Nitrogen Cycle