Optimizing Nitrogen Fertilizer Applications for Potatoes and White Winter Wheat
(Northwestern U.S. and Western Canada)

By R.L Mikkelsen, A.M. Johnston, P.E. Patterson, and N.W Christensen


With dramatic swings in the cost of nitrogen (N) fertilizer, farmers are taking a closer look at their fertilizer application rates. Regardless of the price of fertilizer, economic principles should be carefully considered when determining how much fertilizer to apply. While economics alone may not provide “the answer” as to how much fertilizer to apply, since the optimum will vary from one situation to another, a thorough economic review will provide useful tools for production decisions.

Economists define the economic optimum as the point where the additional cost of the last unit of input (fertilizer) applied is equal to the additional revenue from the increased production (yield). However, determining how to achieve this economic optimum is possible only if the yield response function is known. In order to know the optimum amount of fertilizer to apply, you must know how much production is obtained at increasing levels of fertilizer. This type of information is usually available from your state agriculture university.

Unfortunately, reality is not always as simple as we would like. Frequently, there is uncertainty regarding the changing price of the crops being produced and the costs of inputs such as fertilizer. When the price of the input or the output changes, there is a chance that the economic optimum will also change. The yield response function is often not known with the desired precision, and variables such as weather patterns are unpredictable. In addition, the quality of many crops can be influenced by the quantity and timing of fertilizer application. But the old adage from the business world certainly holds true in agriculture: You have to spend money to make money. So, what makes sense when it comes to N fertilizer? We will answer that question for a high value crop (potatoes) and a lower value crop (wheat).

Potatoes:

Precisely managing N fertilizer for potatoes is difficult because potato roots are generally shallow and water stress is not well tolerated. Careful irrigation management is required to minimize nitrate-leaching losses and to maintain high N use efficiency. In determining crop N requirements, adjustments should be made to account for nutrients in the irrigation water and N contributions from soil organic matter. Petiole testing is a valuable tool for maintaining optimal nutritional conditions through the growing season.

Research results (provided by Dr. Jeff Stark, University of Idaho Agronomist) on N fertilizer response of Russet Burbank potatoes psprovide a good example for economic analysis (Figure 1). In this trial, the optimum economic yield occurred with an application of approximately 200 lb N/A. With a high-value crop like potatoes, the economic optimum and the maximum yield are often achieved at the same level of N. That is, it makes good economic sense to achieve the highest level of production possible.




If N fertilizer prices were to rise to unprecedented levels, or if potato prices were to drop to only $3.00 per hundredweight, the optimal fertilization rate drops only slightly- from 200 lb N/A down to 190 lb N/A (Table 1). This shows that N fertilization plays a vital part of maintaining economical stability---even with wide swings in crop and fertilizer prices! Slashing N fertilizer application rates will cause yields and profits to drop to levels far exceeding any possible savings in fertilizer costs.


Table 1. Economic optimum for N applied to Russet Burbank potatoes does not change dramatically when N fertilizer and potato price change.


Soft White Winter Wheat:

Two examples of the response of soft white winter wheat to added N fertilizer are provided from Oregon State University Agronomists; (Dr. Neil Christensen, Willamette Valley, and Dr. Mylen Bohle, Madras, Oregon). Adequate N must be available to the wheat plant at all phases of development. A lack of N during the growing season will result in fewer tillers, reduction in head size, poor grain fill, reduced yields, and low protein content. The optimum rate of N application will depend on factors such as residual soil nitrate level and the yield potential of the environment.

In the Willamette Valley trial, soft winter wheat (Stephens) was fertilized with N at rates ranging from zero to 180 lb/A (Figure 2). In Madras OR, Stephens wheat was fertilized with N at rates ranging from zero to 160 lb N/A. The response to added N fertilizer was very similar at both locations.


Figure 2. Yield increases of soft white winter wheat (Stephens) resulting from N fertilizer applications at two locations in Oregon.


At both locations, the maximum possible yield was not achieved at the highest N application rate. Sometimes, factors other than total yield influence the decision regarding fertilizer application rates. For example, very high application rates may make the plants more susceptible to lodging and influence the grain protein content. Additionally, the percentage of N fertilizer recovery generally declines as rates increase. However the economic optimal rate of N fertilization (where the price of applied fertilizer exceeds the economic return) was never achieved in these two experiments. University N fertilizer recommendations are generally in the range of 100 to 150 lb N/A, depending on site-specific conditions and the yield potential.

Within this range of application rates, N fertilization consistently enhanced profitability, even at very high fertilizer prices and very low crop prices (Figure 3). For soft white wheat and most crops, fertilizer prices typically do not have a dramatic impact on overall profit as long as you are aiming for high yields and using the optimal N rate to achieve this level of production. Reducing N application rates due to high prices commonly leads to lower yields and less money at the end of the season.



Figure 3. Within the range of recommended N fertilization for soft winter wheat (100 to 150 lb N/A), the economic return from fertilizer makes it a good investment.


When N fertilizer prices climb, remember:

Dr. Mikkelsen is PPI Western U.S. Regional Director, e-mail: rmikkelsen@ppi-far.org.
Dr. Johnston is PPI Western Canada Regional Director, e-mail: ajohnston@ppi-ppic.org.
Dr. Patterson is Extension Agricultural Economist, University of Idaho.
Dr. Christensen is Professor of Soil Fertility and Plant Nutrition, Oregon State University.












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