Fertilizing Corn with Expensive Nitrogen

Emerson Nafziger

Emerson Nafziger

Professor of Agromic Extension Deartment of Crop Sciences

Phone: (217) 333-4424

E-mail: ednaf@illinois.edu

A bushel of corn contains about 0.8 lb of nitrogen (N), so a 150-bushel corn crop removes about 120 lb of N from the field. About two-thirds of this N is in the grain, so a 150-bushel crop would have about 180 lb of N in the plants before harvest. These figures result in 1.2 lb N per bushel, which has been the factor we have used to convert expected yield into N rate recommendations—"1.2 is the most (we) should do." Why should we need a new approach to N rate recommendations?

Why New Guidelines, and Why Now?

The main reason we are re-examining N rates is that the use of the amount of fertilizer N suggested by the yield goal-based N recommendation is not supported by recent research results as N prices increase. In most studies, especially studies with corn planted following soybean, there is little or no relationship between yield and the N rate required to achieve those yields (Figure 1). The main reason for this discrepancy seems to be the amount of N that the soil can provide to the crop. This N is released from the organic matter in the soil in a process called mineralization. Unfortunately, the rate of mineralization depends on the temperature and soil moisture content and is variable over years. Additionally, mineralized N may not all be available to the plant, depending on how the plants grow.

Even as we look for new ways to develop N rate guidelines, it should be noted that the proven-yield (PY) guideline has been misinterpreted: "1.2 is the MOST you should do" has become an inflexible "rule." The PY method was based on a lot of N response data, much of it from trials in which corn followed corn. Data were averaged across trials, and it was found that the optimum N rate divided by the yield at that rate produced the factor of 1.2 lb N per bushel. There was an economic adjustment suggested based on the ratio of corn price to N price. Corn following soybean was found to need less N per bushel of yield than corn following corn, giving rise to the "soybean N credit" of 40 lb, which was subtracted from the PY-based N rate.

The question about whether the YG-based N recommendation system is accurate is the wrong question to ask. In an era of low N cost, the YG-based N recommendation provided a reasonable guideline for N use. So why change? Recent corn yields of 200+ bu/ acre have raised some doubt about the data from trials conducted more than 20 years ago, using very different hybrids. But as noted previously, the main reason for reexamining N rate recommendations is because a great deal of recent N response data show that we can provide better-backed recommendations than the YG basis provides.

The New Approach

One way to use data from a large number of trials is to average the data over trials, producing single curves that describe average N responses (Figure 2). This approach is straightforward, and we can apply economics to such a response curve. However, it can be difficult to average data over different trials conducted differently, and there usually is little sense of, or adjustment for, variability among response curves.

Most N response data show a curvilinear (decelerating) response, usually (depending on the highest rate) leveling off at some point, with a flat curve after that. Yield decreases at high N rates occur rarely now compared with results from trials conducted a few decades ago, probably due to better standability in more recently conducted trials. In Figure 3, we show such a response from one trial. After finding a line to fit the data, we can subtract the yield at zero N fertilizer and multiply the yield from N at each N rate times the price of corn to produce the gross return from N. Subtracting the cost of N gives a "return to N" (RTN) line, which gives the profit from N at each N rate (Figure 3). The high point of this line is the "maximum return to N" (MRTN) point, and is where the yield increase from adding N just paid for the N added.

Figure 3
Figure 3 –Gross return to N, N cost, and net return to N (RTN)
at different N rates. Data are from a single N rate trial
conducted on corn following soybean at Urbana in 2002.

We calculate the RTN curve for each trial in the dataset, then average these lines to produce an RTN line for the whole dataset. The MRTN is the high point on this average line over all trials, and is the point where the data indicate that we can expect the maximum return to fertilizer N. We refer to the N rate at the MRTN as the MRTN-NR. Because this curve tends to be rather flat on top (Figure 4), we think it makes sense to use a range of N rates instead of a single rate. We arbitrarily chose this range to be the N rates over which the RTN is within $1.00 per acre of the MRTN. In the database we have, this range of N rates usually is about 20 lb on either side on the MRTN-NR, so the range is about 40 lb of N wide. Ranges allow some individual choice based on personal approach to risk, environmental fragility, and other factors.

What Changes with the New Guidelines?

Table 1 provides a comparison of recommended N rates using the Proven-Yield method and the rates suggested by the new approach. For this example, we use a corn price of $2.10 per bushel and an N price of $0.30 per lb.

Table 1 – Recommended N rates using the proven-yield
method and the new N guidelines, University of Illinois
Area Rotation "Proven" yield,
PY N rec
lb N/acre
MRTN-NR New N guideline
range, lb N/acre
Southern IL SC
125 to 165
135 to 175
Central IL SC
142 to 182
176 to 213
Northern IL SC
96 to 130
131 to 169

Although recommended N rates do not change drastically using the new approach compared with the PY method in central and southern Illinois, recommended N rates from the new approach decline considerably in northern Illinois. The new rates in northern counties are in line with the rates calculated using this approach in Iowa. It is possible that higher soil organic matter, more manure application in the past on many fields, and weather increase the supply of N from the soil in northern Illinois. We need more data to consolidate these guidelines.

One of the features of these new guidelines is the absence of a soybean N credit. Because the guidelines are based on trials where corn followed soybean, the suggested ranges already include this "credit," so nothing should be added or subtracted from the rates provided by the guidelines. The new approach will take some getting used to, but to ease the transition, it might be helpful to think of the 40 lb N credit as "available" to the crop in addition to the applied N rate. In this way, an applied N rate of 150 lb N/acre becomes 190 lb N/acre.

Dealing with N Fertilizer Cost

One of the more useful features of the new N rate guidelines we describe in this paper is that they incorporate corn and N prices into the recommendations. An increase in the price of N fertilizer while the corn price stays low means less economic return to N fertilizer. But the amount of N that we apply should also decrease whenever the ratio between N cost and corn price increases. Using an N cost of $0.30 per lb of N (about $500 per ton of ammonia) and a corn price of $2.00 per bushel, this ratio is 0.3/2.00 = 0.15. Figure 5 shows how MRTN decreases as N price (that is, the ratio between N and corn prices) increases, and also how the N rate at MRTN changes, using all of the Illinois data for corn following soybean. Within today's general range of prices, each 1-cent increase in the price of a pound of N calls for a reduction in N rate of about 1½ lb of N per acre.

Dr. John Sawyer at Iowa State University has developed a Website at which N rate guidelines can be calculated using the new approach. For Illinois, calculations can be made only statewide for now, although we will divide the state into three regions (north, central, south) when we have defined databases to support the calculations for each region. Calculations can be made for single N and corn price combinations, or different price combinations can be compared on the same graph. The url address for the Website is http://extension.agron.iastate.edu/soilfertility/nrate.aspx.

Remaining Issues

Other issues associated with the new approach include the need for more response data to fill in those areas of Illinois where N responses have not been determined recently. Although the question about how many N response trials are necessary to produce full confidence is not easy to answer, for now the answer clearly is "more." We will undertake some efforts to produce such data. Another question is whether we can ever use factors such as soil type, cropping history, tillage, drainage, and N form and timing to fine-tune N recommendations. It seems logical that such approaches would be useful, but we don't have nearly enough data to even guess at such possibilities. And even though it might be logical, the year-to-year variability in N response we have observed in some of our longer-term studies will make it difficult to see consistent effects of soil and management on N response.

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