Nitrogen Management Practices
for Potato Production

This article by Don Smucker, Michigan State University Extension, appeared in “The Vegetable Growers News,” July 2001.

The following is a list of commonly recommended practices for nitrogen (N) management for many crops to protect water quality.

Use Recommended Rates
Apply N at recommended rates for crop being produced. The one most important practice affecting N contribution to groundwater is the rate of N applied. We know that crop recovery of N decreases with increasing amounts applied, and N that plants do not take up can leach into groundwater.

Nitrogen from Other Sources
Appropriate credits from N contributions of legumes, other crop residues, or manure must be properly credited to accurately determine crop fertilizer needs. Irrigation water can also supply nitrogen, and can be easily measured. Multiply the ppm nitrate N in the irrigation water by 3 to estimate the pounds of N supplied.

Use Testing Where Appropriate
Wise use of soil and plant N testing can improve the accuracy of N recommendations. New techniques, such as the pre-side-dress nitrogen test can help us monitor soil-profile nitrate content before planting or side-dress applications. Petiole testing, using dry assay, sap nitrate testing, Cardy meter, or SPAD meter can help monitor the plant nitrate content during the growing season.

Set Realistic Yield Goals
Optimum N application rates can only be accurately determined if you use realistic yield goals based on recent yield experience. Generally, yield goals should not be more than 10 to 20 percent above the recent average yield experience for a particular field. Accurate records for crop yields are very important, to enable realistic yield goals to be set.

Apply According to Crop Demand
An important factor that affects the potential for nitrate loss is the timing of nitrogen applications. The longer the timing before crop uptake of N and application, the more potential there is for leaching.

Incorporating Applications
Urea or N solutions should be incorporated, to minimize the potential for volatilization losses.

Minimize Leaching
Excess water from over-irrigating or precipitation can cause nitrates to move below the root zone. The use of irrigation scheduling can help reduce the likelihood of over-watering and N leaching loss.

The following are practices that we still are learning more about that we need to develop into more precise recommendations.

Timing Issues
Keith Kelling, University of Wisconsin, feels that evidence points to a critical N nutrition availability status during early-to-mid tuberization. The general suggestion is that about one-third of supplemental N (50 to 70 lbs N/acre) be applied at emergence, and the remainder (100 to 150 lbs N/acre for Russett Burbanks) be applied early-to-mid tuberization.

Leaching Versus Non-Leaching Years
Kelling has found that on non-leaching years in sandy soils, at least 50 to 60 lbs of N/acre needs to be applied at emergence or quality is lost. When mid-season leaching is low, multiple splitting of the remaining N tended to reduce yield or quality. Kelling recommends monitoring petiole nitrate levels, and if leaching occurs prior to 65 days after emergence, an additional 30 to 50 lbs N/acre be applied. What is the impact of applying N 65 days after emergence, or within 4 to 6 weeks of vine killing? This is a tough question to answer, but experience shows that often we prolong the growth of the plant, delay maturity, and do not enhance potato tuber quality. Green vines are difficult to kill, and immature tubers are easily damaged by harvest operations.

The ideal situation is to have the crop start to slow tuber growth and vines start to sinesce by the time you would like to vine kill or dig. Excessive N impacts quality, perhaps to an extent that we may not always be aware of, by reducing tuber-specific gravity and raising the reducing sugar content, which can cause storage and marketing problems.

Timing Application Precisely
Some feel that we need to be able to factor in something that gives us a more precise measure of development, such as growing degree days or P days after emergence. We know that varieties do behave differently regarding N use, and research and experience has determined more precise recommendations for some varieties. Measuring maturity by sugar analysis is being done, and gives valuable information to help make storage management plans and decisions. Perhaps we need to be able to relate the sugar status back to N and irrigation management practices, to help us become more precise in potato N management and maximize tuber quality.

Monitoring Petiole N
Established recommendations specify that you must sample consistently each time you take petiole samples. You should sample the most recent fully-expanded leaf on the main stem, which is usually the fourth or fifth petiole from the top. You need to sample the same area of the field also when you repeat sampling, to accurately monitor changes. But one factor that needs more investigation is due to the fact that our sandy fields are often quite variable in soil types and productivity. What is the best sampling pattern for a given field, as we consider the more productive versus the less productive areas of the field? If we cannot vary N rate by soil type, do we apply N, considering the more productive or less productive areas, or somewhere in between? Precise N management is a key to optimizing yield, quality, and environmental stewardship.