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Which is more cost-effective: ammonium or nitrate nitrogen fertilization of tobacco?

Bill Easterwood

For more than 40 years, researchers from universities in tobacco-growing regions have tested the agronomic benefits of nitrate or ammonium forms of nitrogen in terms of leaf yield and quality. They’ve determined that nitrogen has a more pronounced effect on tobacco than any other nutrient. But will the form of nitrogen used influence grower profitability at the receiving station under the new contract production system?

Agronomically, research findings indicate that tobacco leaf yield and quality is superior with nitrate nitrogen. Tobacco plants prefer nitrate since excessive ammonium-nitrogen assimilation is toxic and stunts transplant growth.

For ammonium nitrogen to be a viable tobacco fertilizer, it must be converted to nitrate by soil microbes via the nitrification process. But beneficial microbes involved in the nitrification process cannot survive in large-enough populations to be effective in soils that have been fumigated, are acidic, flooded, dry or have high soil temperatures. Hence, ammonium nitrogen remains the dominant form of nitrogen in these soils and stunts growth. In addition, when optimal soil and environmental conditions for nitrification occur, a large surge of nitrate can be released for uptake and may be detrimental to leaf yield and quality depending on the stage of plant growth. Growers applying ammonium nitrogen are gambling that environmental conditions will be favorable for nitrification early in the growing season. Ammonium nitrogen also possesses negative attributes of increased soil acidification and as a potential source of potassium and magnesium deficiency in tobacco leaf.

Conversely, when nitrate is applied, the grower controls the nitrogen uptake timing of his tobacco crop and is not as subjugated to environmental conditions. Also, nitrate nitrogen maintains soil pH and aids in potassium and magnesium uptake by the crop. For these agronomic reasons, national and international extension publications during the past 40 years state the following: “At least 50 percent of the N in the transplanting fertilizer should be in nitrate form and the remainder in the ammonium form; in some cases a favorable response can be expected with up to 100 percent of nitrate N. The N applied after the transplant period should be entirely in nitrate form.”

Why then are tobacco growers applying urea (0 percent nitrate), ammonium sulphate (0 percent nitrate), 13-0-14 (20 percent nitrate), UAN solution (25 percent nitrate), and ammonium nitrate (50 percent nitrate) alone or in blends and ignoring valid long-term university research and fertilizer management techniques taught by their fathers?

The answer, of course, is fertilizer cost. As a general rule of thumb, ammonium fertilizers are less costly per pound of nitrogen than nitrate. But economically, is this a wise decision? Are growers leaving money in the field because of their fertilizer choice?

Flue-cured production
The first economic data comparing nitrate and ammonium nitrogen fertilization of flue-cured on fumigated soil, published in 1963 by McCants and Woltz of North Carolina State University, shows the value of nitrate nitrogen. Crop value (based on largest yield and highest quality) of all nitrate nitrogen-treated tobacco was valued at 100 percent, indicating the all-nitrate treatment had the highest yield and quality. When the nitrate content of the applied fertilizer was decreased to 33 percent (67 percent ammonium nitrogen), a crop value of 92 percent compared to nitrate was observed. All-ammonium nitrogen fertilizer generated just 88 percent of the value of the all nitrate-treated tobacco (See Figure 1).

Using the 1963 data, what then would be the value of 2,500 pounds of leaf per acre priced at $1.80 per pound from the above treatments? The all-nitrate treatment had a value of $4,500 per acre; the 33 percent nitrate treatment $4,140 per acre (4,500 x 0.92); and the all-ammonium treatment $3,960 per acre (4,500 x 0.88). So, if the grower chose an all-ammonium fertilizer management plan, he could lose $540 per acre. If he chose 33 percent nitrate, he could lose $360 per acre. From the 1963 data, we observe that investment in nitrate nitrogen was far more important than the “savings” derived from applying ammonium nitrogen fertilizer on tobacco. University extension recommendations to use nitrate nitrogen and our fathers’ choice of nitrate nitrogen sources were economically valid.

But this is “old” 1963 data with different technology and flue-cured varieties. Is the research and science developed 43 years ago antiquated or still applicable today? To find out the answer, David Smith, Ph.D., of North Carolina State University compared UAN-30 (25 percent nitrate) to CN-9 (liquid calcium nitrate) as the sole nitrogen sources applied in six tests during 2003-2005. Average CN-9 yield was 2,952 pounds of leaf per acre, valued at $4,543. Average UAN-30 yield was 2,841 pounds of leaf per acre, valued at $4,251 (Figure 2). Dr. Smith’s flue-cured research findings indicate that the all-nitrate nitrogen fertilizer application was superior to the UAN-30 fer-
tilizer program by $292 per acre and compares very favorably with results from McCants and Woltz.

But what about fertilizer costs? Typically, UAN-30 and CN-9 each retail for $200 per ton in North Carolina. Therefore, 80 pounds of nitrogen per acre from UAN-30 would cost approximately $27, and 80 pounds of nitrogen per acre from CN-9 would cost approximately $89—a substantial investment of $62 more per acre for nitrate.

What then is the flue-cured bottom line? If you did not invest the $62 for nitrate, you lost $292 per acre—a tremendous loss for today’s grower. If you invested the $62 for nitrate, your net return was an additional $230 per acre.

Burley production
Do the same nitrate nitrogen advantages apply for burley tobacco production? During the 2000-2004 seasons, Gary Palmer, Ph.D., of the University of Kentucky, conducted 19 research trials to answer the question. His treatments included 150 pounds of nitrogen applied as ammonium nitrate (50 percent nitrate/50 percent ammonium) followed by sidedressing 100 pounds of nitrogen as ammonium nitrate or an all-nitrate source like sodium nitrate, calcium nitrate or 15-0-14. The research objective was to determine how sidedressing with ammonium nitrate compared to all-nitrate nitrogen sources.

What was the result? Ammonium nitrate produced 2,070 pounds of burley leaf per acre, valued at $4,140, and all-nitrate sources produced 2,186 pounds of leaf per acre, valued at $4,372—an increase of $232 per acre with all-nitrate sidedressing (Figure 3).

How do fertilizer costs affect the increased profitability? Retail fertilizer costs for ammonium nitrate or calcium nitrate are each approximately $325 per ton. Ammonium nitrate sidedressing cost for 100 pounds of nitrogen per acre is $48. Calcium nitrate cost is $105 per acre—an investment of $57 per acre for nitrate. Therefore, nitrate sidedressing increases grower net profitability by $175 per acre.

In addition, nitrate does not acidify the soil. Manganese toxicity and molybdenum deficiency problems have developed from excess soil acidity produced by ammonium nitrate fertilization. As a result, burley leaf yield and quality have suffered when grown in acidic soils.

The bottom line
In the words of the wise man Solomon, “There is nothing new under the sun” in terms of nitrogen fertilization of tobacco. During the past 40-plus years, tobacco research-
ers, university extension personnel and our fathers knew that nitrate nitrogen is superior to ammonium nitrogen. It produces higher yield, increases quality and gives greater profitability per acre to those who use it. That is the bottom line. In our current market environment, nitrate nitrogen fertilization of tobacco would indeed be a wise choice.