If you’ve listened to the radio in rural Nebraska over the past few months or follow many folks in the agriculture community on Twitter, you’re probably aware that nitrogen management is a top-of-mind issue for farmers in the 2022 growing season. Not only are there concerns about the price of nitrogen fertilizer, but there are also concerns about acquiring enough nitrogen fertilizer to supply the upcoming season’s crop. Both concerns are pushing growers and their trusted agronomic advisors to seek reliable methods for scaling back nitrogen fertilizer use without reducing yield. Achieving this objective is challenging due to the complexity of the nitrogen cycle within agricultural production systems. In this article I will discuss the complexity of nitrogen management and some of the concerns that need to be eased for farmers and their advisors to comfortably scale back nitrogen fertilizer applications. I will follow up on this topic in next week’s article by exploring two primary classes of data-driven nitrogen management technologies available to improve nitrogen fertilizer efficiency – models and sensors – and discuss their strengths and weaknesses.
A Contemptible Management Challenge
What makes nitrogen management challenging is that the amount of plant available nitrogen (in the form of ammonium and nitrate) in the soil varies spatially and temporally based on numerous factors including weather, topography, soil characteristics, and management practices (e.g. fertilizer application, crop rotation). The agricultural nitrogen cycle graphic below from the Nitrogen Notes article series by the International Plant Nutrition Institute shows the many processes and pathways by which nitrogen enters and exits the soil. Nitrogen enters the soil and becomes available to crops by three basic pathways: fixation, mineralization of organic material, and application of nitrogen fertilizer. Mineralization of organic material has significant potential to supply crop available nitrogen during the growing season. The best part about it? It’s the least expensive source of crop available nitrogen (manure and cover crops come at a bit of a cost so it’s not always free). The total amount of mineralization that occurs and when it occurs during a growing season, however, is highly variable and hard to rely on. It also likely isn’t enough to achieve peak yields. Therefore, farmers rely on nitrogen fertilizer to supply the balance of the crop’s nitrogen needs and smooth out low contributions of crop available nitrogen from mineralization and fixation.
Figure 1 - The agricultural nitrogen cycle as communicated in Nitrogen Notes (No. 1) from the International Plant Nutrition Institute linked to above.
There are also several paths by which nitrogen may exit the system. Nitrogen may be lost from the system before entering the soil via volatilization and runoff. This may occur with any synthetic or organic (e.g. manure) fertilizer application. Both harm the environment: runoff poses risks to surface water resources and volatilization increases greenhouse gas concentrations in the atmosphere. Once nitrogen enters the soil and becomes plant available, it may also be lost via nitrification or, more predominantly, leaching when in the form of nitrate. Leaching of nitrogen fertilizer is a major point of concern for groundwater quality, particularly in heavily irrigated agricultural regions. This is one of the primary reasons why many regions in Nebraska and throughout the world have groundwater nitrate levels that exceed the safe drinking water standard put forth by the EPA. Leaching is most significant in coarser soil textures (i.e. sandier soils) but can occur in any soil. Risks for leaching are highest when there is a significant amount of nitrogen in the soil and soils are exposed to significant water infiltration, such as from excessive irrigation or large rainfall events. The bottom line? The soil is a terrible place to store nitrogen.
Misalignment between crop nitrogen uptake and crop available nitrogen in the soil poses a leaching risk when crop nitrogen need is low and crop available nitrogen is high. It also poses a yield loss risk when crop nitrogen need is high and crop available nitrogen is low. Maximum uptake of nitrogen fertilizer by corn and several other nitrogen intensive crops (e.g. cotton) does not occur in the early parts of the growing season. Instead, it occurs when the crops have reached a critical level of maturity (i.e. the V8-V9 growth stage in corn) often reached in late June in Nebraska and other parts of the corn belt. Many current nitrogen fertilizer application practices – including all up-front fall or spring applied anhydrous – store all applied nitrogen for the growing season in the soil for extended periods of time when crop nitrogen uptake is lowest and the potential for large rain events is highest. Split application of nitrogen fertilizer involving a partial up-front application followed by a sidedress application when the crop has matured somewhat is an improvement, but still carries some leaching risk. If fertigation is used as an application technique, nitrogen applications may be spread across the growing season minimizing the amount of excess crop available N from fertilizer in the soil at any given time and thereby minimizing nitrogen loss to the environment. However, minimizing the amount of excess crop available N from fertilizer in the soil increases the risk of not having enough nitrogen available in the soil for the crop when it needs it, depending on nitrogen availability from non-fertilizer sources.
In an ideal world, the amount of nitrogen fertilizer applied at any given time would be the exact difference between the crop’s nitrogen need and what is supplied by non-fertilizer sources. But how can we be sure there is enough nitrogen supplied from non-fertilizer sources to sustain the crop’s needs and make a fertilizer application if there isn’t? How can we make an application before a nitrogen shortage affects the crop? If we are splitting nitrogen fertilizer across many applications during a growing season, like in fertigation, how can we determine when those applications should be made so we don’t stress the crop? If I’m using a biological, inhibitor, cover crops, manure, or other product, how can I determine how that product is affecting my crop’s nitrogen uptake during the season? Considering the complexity of nitrogen management, these are tough questions to answer reliably without the right information. Fortunately, there are data-driven technologies that seek to quantify crop nitrogen needs helping farmers and their trusted agronomic advisors make smarter nitrogen management decisions. I’ll get into these technologies in next week’s article as we continue in our Winter 2022 blog series.
Jackson "Jack" Stansell
CEO & Founder
Sentinel Fertigation