The Nitrogen Cycle Simplified

//The Nitrogen Cycle Simplified

by: John Deibel, Freelance Author

Nitrogen is the foundation of all living things on our planet, comprising 80% of the atmosphere as N 2 Gas. In the process of utilizing Nitrogen for creating and sustaining life, it undergoes several complex processes involving biology, chemistry, and actually, physics.

In Agriculture, Nitrogen is by far the most important macro element to producing crops. It is Nitrogen, after all, that comprises the proteins which plant and animal life both rely upon and produce, and then contribute back as by-products and their ultimate decay once their life cycle is complete.

Understanding the different sources, factors, forms and fates of Nitrogen we either apply or encourage in our crop production systems is important. Let’s review some of these:

Soil Type and Texture

Regardless of where you are farming, knowing the nature of your soil is the most significant contributor to understanding the fate of any Nitrogen source you are managing. The conversion of Nitrogen in the soil to plant available forms is heavily dependent on how well the soil can support aerobic bacterial action. When the soil is heavy or water laden and soggy, nitrates (the form taken up by plants) are ‘denitrified’ and converted to N 2 gas which is no longer available to the plant. The bacteria and processes that make nitrogen available to plants also require moisture for their processes, so on the opposite end of the spectrum, an excessively dry soil or extended periods of drought will slow or stop the process.

Green Manures, Cover Crops, Companion Crops

This group of Nitrogen contributors are the cornerstone of calculating your Nitrogen requirements in crops. When considering their value to ‘this’ year’s crop, several factors come into play. There are many generalizations made regarding cover crops that should be understood. Just because you’re plowing down a nice, green, thick cover or sod, doesn’t necessarily mean it is contributing Nitrogen to the crop that follows.

Grasses that have no legume component actually initially ROB the soil of nitrogen and associated bacteria during their breakdown; and their ability to contribute to the organic Nitrogen pool in the soil may take months or up to a year; depending on their nature and how you destroyed them. The carbon:nitrogen ratio of the biomass being incorporated drives this availability. Tall, old, thick grasses are higher in cellulose and carbon. This carbon needs to break down in the soil using a wider spectrum of microbes and soil fauna; which takes more time. In the end, they might contribute a larger amount of total organic matter, but its availability is much slower. Conversely, a young, short, fresh, green, lush seeding of a small grain like oats, rye, or wheat might have a lot of nitrogen available, but very little carbon; so it provides a miniscule amount of nitrogen and virtually no organic matter. So.. the moral of the story is to not assume immediate contribution from these sources toward your nitrogen calculation. A general guide is about 30-60 lbs/acre or 70-150 lbs/hectare of actual N. from a grass sod, mineralized over the course of the season.

Legumes like clover, alfalfa, on the other hand, fix atmospheric nitrogen via rhizobia bacteria living on their root surfaces. Massive amounts actually. More than the plant needs, and they need plenty! As a result, plowing down a well populated field of alfalfa that’s been living in good status for 3-4 years can contribute almost enough to feed a field of corn for the entire first season. Some starter nitrogen to help the crop get started, and in many cases, that’s all you need following a good alfalfa sod. If your legume that is being plowed down is productive and has good population, you’ll receive from 70-120 lbs of available Nitrogen per acre or 175-300 lbs per hectare. A very important step when growing legumes, especially if the field hasn’t been grown in the legume you’re planting, is to INOCULATE the seed before planting with the proper strain of rhizobia bacteria. If the crop hasn’t been grown in the field in the past several years, there is not likely enough native rhizobia in the soil to grow on the roots of the crop you’re planting.

Animal Manures and Composts

Using animal manures is a valuable contributor to your nitrogen requirements of the coming crop. Incorporating fresh manure immediately after application can double the nitrogen contribution when applying manures that are high in ammonia. Incorporating the ammonia fraction of the manure keeps it from volatilizing into the atmosphere and stabilizing it for use by the crop to be grown. Using composted manures or compost from other sources depends on the source. They will also have a lower amount of total N than fresh forms of the waste from which they are made because composting removes the ammonia fraction. Composts will elevate all your nutrients through their value as organic matter. They can help you balance the general ‘health’ of your soil, but their value in providing immediate nitrogen to the crop is dependent on their C:N ratio, which is driven by their original source. Testing your compost will assist you with calculating how much available N can be assumed. Chances are, however, you will want to plan for supplementing N in addition to composts at the proper timing for the crop by using sources that are higher in available N. Chemical fertilizer, poultry manures, etc.

Chemical Fertilizers

Chemical based fertilizers are meant to be used at rates you determine after calculating:

(Predicted uptake needs – Organic Contributions) x Nitrogen Efficiency of the soil = additional N needed

The primary sources of N will be found as:

Ammonium Nitrate (32-0- 0): highly stable, but degrades in sunlight rapidly
Ammonium Sulfate (21-0- 0): highly stable, contains sulfur, but can be expensive
Urea (44-0- 0): not as stable as nitrate or sulfate, but typically less expensive. Should be incorporated with rainfall or tillage as it mineralizes a significant portion to NH 3 and will volatilize to the atmosphere rapidly.
UAN (34-0- 0) Liquid: a liquid blend of Urea and Ammonium Nitrate offering ease of application through a sprayer or squeeze pump on the planter (ground driven).
Anhydrous Ammonia (80-0- 0): applied as a gas, must be injected, hazardous to handle, but very low in cost in some areas. Toxic to organisms in the soil in the short run. Not widely used due to restrictions on application and handling equipment.

Other than Anhydrous, your fertilizer blends will contain one or more of the first three sources…nitrate, sulfate, or urea. You can ask your source to know which ones and in what percentages.

Nitrogen Extenders

A number of extenders to keep nitrogen more stable are used for various applications. Examples include sulfur-coated urea, nitrapyrin (N-Serve TM ), dicyandiamide (DCD or cyanoguanidine), Urease Inhibitors, Polymer Coated Urea, and others… each has its own fit, and drawbacks associated with their use. Read more here: Nitrogen Extenders and Additives for Field Crops – North Dakota State University Publication SF1581 (Revised)



The most important thing to consider when balancing for Nitrogen is to understand how it is lost. The more healthy your soil is, the better it will mineralize Nitrogen by providing an optimum environment for the desirable microbes that work to make it available to the plant. This is affected by many cultural practices like rotation, tillage, use of manures, and other factors.

Ultimately, know your soils, know what you’re receiving to meet the needs of your crop, and FINALLY, calculate how much additional N you’ll need. Too many farmers do it the other way around and your pocketbook (and crops) will thank you for doing it in this manner.

Here are some more sources to reference on Nitrogen Cycling and Management: – The Nitrogen Cycle – Processes, Players, and Human Impact
Nitrogen Extenders and Additives for Field Crops – NDSU Crop Science
Beginning Farmers : The Nitrogen Cycle: Agricultural Science Basics
Environmental Impacts of Nitrogen Use in Agriculture – Ontario Ministry for Agriculture