The Illusive Nitrogen

The Right Form at the Right Time

by Cliff Ramsier, Technical Director

All non-legume crops benefit from nitrogen additions for optimum cropping efficiency. As a constituent of all proteins, as well as other plant tissues, nitrogen compounds represent more than three percent of green plant tissue dry weight. Since bio-availability of this nutrient is at the mercy of soil and weather conditions, management is a huge factor in both use efficiency and maximum yields.

Increases in energy prices, as well as concerns about off-site environmental effects, have heightened the value of optimizing nitrogen management. Simply put, growers cannot afford to apply nitrogen that does not ultimately add to crop yields. Since interactions between weather, soil conditions, plant requirements, and raw supply of nitrogen determine use efficiency, managing this elusive input is very challenging. Nitrogen form, timing, and placement are management components that come into play when designing an efficient system for crop production. An understanding of the basic science behind all these factors brings some order to an otherwise chaotic situation.

General plant requirements, relative to ontogeny, suggest that young tender plants benefit from the nitrate form of nitrogen since relatively low ammonium concentrations often become toxic to the small plant. The small young plant usually has an equally small root system, so placement close to the root becomes valuable. Throughout the corn belt, crops are often planted when soil temperatures are below those which enable soil microbes to convert ammonium forms to nitrates, so the form of nitrogen ideally should contain the nitrate form to facilitate plant nutrition. In fact, the microbes that are active when soil temperatures are between 55° and 62° F tend to be those that immobilize ammonium nitrogen (Martens, D. et al 2000). While nitrate is the optimum form for young plants it is also the most mobile form of fertilizer nitrogen in the soil system. This means that, depending on rainfall, nitrates may not last long in the root zone.

As soil temperatures increase above 62° F, microbes responsible for converting ammonium to nitrate proliferate, which assures ready conversion of nitrogen. Of course, by then the growing crop can tolerate higher levels of ammonium, which converts to plant proteins more easily. In the past, most growers have made nitrogen application decisions based on price and convenience. While prices were relatively low for this input, inefficiency was easy to overlook; however, with increasing prices and increasing environmental concerns, growers will benefit from understanding plant and soil system requirements to optimize efficiency.

Virtually every non-legume crop grown in the U.S. that flowers prefers nitrogen in the ammonium form after flowering. The reason for this phenomenon is twofold. First, during these growth stages, the crop has extra sugars available to detoxify the ammonium molecule. Second, the plant energy required to convert ammonium to plant protein is significantly lower than the energy required for nitrates. This means that a greater amount of plant energy is left to go directly to increasing yield, which makes nitrogen use much more efficient.

Since soils are complex and variable, managing nitrogen in them is complicated. Growers who apply nitrogen before the crop is planted are often disappointed to learn that some of the expensive nitrogen they have applied was not available to the crop during peak demand in plant development stages. This results in reduced yields and low nitrogen use efficiency. Given today’s commodity prices, neither of these results is helpful. While unpopular, multiple applications, forms, and placement, as well as using nitrification inhibitors, dramatically increases both yield and nitrogen use efficiency.

In an attempt to avoid the complexity of nitrogen management in soils, some growers opt for foliar applications of nitrogen solutions. While foliar nitrogen removes the variability of the soil from the efficiency equation, this benefit is offset by the need for multiple low dose applications. Understanding the real value of this technology is the key. First, plant tissue can tolerate only very low concentrations of reduced nitrogen (the form in most foliar nitrogen materials). This limitation would require many applications throughout the season, which reduces the cost effectiveness of foliar nitrogen. While foliar nitrogen applications cannot effectively replace soil-applied nitrogen management, they can and do dramatically improve the response of the plant to soil-applied nitrogen, especially during reproductive plant growth phases. Fruit fill demands on photosynthate usually preclude the crop from significant root growth and development (Nielsen, R. L. et al 2007). Limited root growth (in fact most often roots shrink during fruit fill) reduces the volume of soil that roots can exploit for soil nitrogen. Foliar feeding can reignite photosynthate production, which stimulates root growth and increases the soil volume for nitrogen exploitation. If done properly, foliar nitrogen management increases yield, as well as enhances overall nitrogen and water use efficiency.

The Maximum Farming System emphasizes nutrient form, placement, and timing as one of its primary components. Every Ag Spectrum associate understands that efficiency and uniformity are the keys to crop success. Nitrogen management is an integral consideration of Maximum Farming from planting through harvest and, depending upon the crop, soil type, and moisture level, can include varied and multiple strategies, such as banding, in-furrow placement, side-dress applications, and foliar feeding.

Optimum nitrogen management is not simple or convenient, but it is doable. With an understanding of the basic sciences, a desire to replace convenience with efficiency, and good leadership, a very effective, high-yielding nitrogen management plan can deliver profits to a field in virtually any weather condition for any crop. Are you ready to take the next step to Maximum Farming?