The Nitrogen Cycle

The nitrogen cycle is a complex chain of microbial and chemical processes that play a major role in crop production. Nitrogen takes up about 78% of our atmosphere and is one of the 17 essential nutrients necessary for plants to complete their life cycle. It plays important roles in the plant such as assisting with protein and chlorophyll production (Sustainable Corn, 2015). The nitrogen cycle is a natural occurrence, but farmers can influence this process based on the 4Rs of nutrient management: right source, right time, right rate and right place (“The 4Rs”, n.d.).

It might be helpful to explain the nitrogen cycle by giving a real-life example:

Farmer Joe is on a corn-soybean crop rotation and applies 180 pounds of nitrogen in the fall with anhydrous ammonia to his soybean stubble. His corn yield goal for the next crop year is 250 bushels per acre. The term “anhydrous” in anhydrous ammonia means “free from water”.  Once this product is injected into the soil, it searches aggressively for water – and if there is proper soil moisture at the time of application, anhydrous ammonia (NH3-) will quickly react with water and convert to ammonium (NH4+). At this point, positively charged ammonium ions attract and bind with negatively charged clay particles and organic matter. If there is not an adequate amount of soil moisture at the time of application, the anhydrous ammonia will likely volatilize and be lost to the atmosphere. Ammonium is a very stable source of nitrogen that the plant can use. When soils are 50 degrees or less, the microbial activity slows down, and the nitrogen from the fall anhydrous ammonia application will likely remain in the ammonium form until spring (“Why Do”, 2019).

Once soils begin to warm up in the spring months, the microbes in the soil will become more active and begin converting ammonium (NH4+) to nitrite (NO2) and then to nitrate (NO3-) through a process called nitrification. Nitrate is another form of nitrogen that the plant can use, but it is less stable than ammonium because its negative charge does not attract and bind with the negatively charged clay particles and organic matter. At this time, nitrate can be lost due to excessive rainfall by leaching through soil profile or by denitrification due to over saturation and ponding (Sustainable Corn, 2015).

Could Farmer Joe have decreased his chances of nitrogen loss? Looking back at the 4Rs of nutrient management will help.

Right Source: Anhydrous ammonia is one of the most stable forms of nitrogen, but the addition of N-Serve (nitrogen stabilizer) would have helped slow down the process of nitrification – resulting in less nitrogen loss. If a farmer insisted on only applying one pass of nitrogen on their corn acres, then anhydrous ammonia would be the best product to use. Urea and UAN are more likely to be lost quicker than NH3.

Right Time: Executing at least one extra nitrogen application to spread out risk will decrease the likelihood of loss. A split application can be made on the planter with UAN, side-dress or y-drop application with UAN, or by topdressing urea. Of course, these methods do not stop the loss of nitrogen and come with their own risks, but they do help time the fertilizer application closer to when the crop will use it. A top-dress urea application, for example, requires rainfall shortly after application or will risk the chance of volatilization.

Right Rate: 180 pounds of nitrogen is a respectable rate for rotated corn acres. If you figure 30 units of nitrogen as a “soybean credit” from the following year and assume there’s natural mineralization of organic matter throughout the growing season, there will be enough nitrogen to cover a corn yield of 250+ bushels per acre (assuming 0.9 bushels per unit of nitrogen use efficiency and limited nitrogen loss). If someone had a yield goal of 180 bushels per acre, this rate would most likely be excessive.

Right Place: Anhydrous ammonia is placed in a concentrated band accessible for the roots to capture and utilize. Planting closer to the fertilizer band will likely increase efficiency. When looking at a split application of nitrogen, side-dressing with coulters places the product in the middle of the row compared to a y-drop application which places the product closer to the root zone.

It would take a much longer article to cover an in-depth look at the nitrogen cycle, but this should demonstrate the importance of the process and how it affects growers. Nitrogen is one of the most difficult nutrients to manage, but using the 4R approach will likely decrease nitrogen loss and increase the ROI.

 

Sources

Why do they do that? Anhydrous. (2019, March 21). Iowa Agriculture Literacy Foundation.

https://iowaagliteracy.wordpress.com/2019/03/20/why-do-they-do-that-anhydrous/.

Sustainable Corn. (2015, April 27). Speed science: The soil nitrogen cycle [Video]. YouTube.

https://www.youtube.com/watch?v=SVFywwreP0I&t=229s.

The 4Rs of nutrient stewardship. (n.d.). 4R Plus. https://4rplus.org/nutrient-management/.