GETTING THE MOST FROM FERTILIZER

Nitrogen Plays the Starring Role in High Yields

KIM POLIZOTTO, Ph.D.
PotashCorp Senior Agronomist

Did you know that about 41% of corn yield in the United States is directly attributable to the use of nitrogen (N) fertilizer?¹ For cotton, rice, sorghum and wheat, the contributions of N to yields are 37%, 27%, 19% and 16%, respectively. Regardless of the advances that crop breeding and modern bioengineering have brought to increased yield potential, fertilizer is still a major component of yield.

And it's not just N fertilizers that are critical to yield! Crops require 16 other essential nutrients to maximize their yield potential. The three that are required in the greatest amounts are nitrogen, phosphorus (P) and potassium (K). There is no disputing the fact that for most crops, nitrogen is the nutrient that has the largest effect on yield. For crops that don't require nitrogen fertilizer, like soybeans and alfalfa, not applying appropriate amounts of phosphorus and potassium usually limit yield the most.

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The figure above shows that at nearly the same nitrogen rate, the maximum economic yield (MEY) level with adequate P nutrition yields was 41 bushels higher than when P levels were low. In the case of the low P treatment, the effective N response was 0.89 bu of corn per pound of N applied, whereas in the high P treatment, the effective N response was 1.17 bu of corn per pound of N applied. This is a tremendous improvement in N-use efficiency.

Source: Schlegel, Dhuyvetter and Havlin, 1996 JPA 9:1

Even in crops that require nitrogen fertilizer, phosphorus, potassium and the other essential nutrients are critical to the nitrogen response. Balanced nutrition may be one of the most important components of high yields. No nutrient stands alone in importance to crops. Adequate amounts of soil phosphorus promote the uptake of ammonium nitrogen, and in the crop itself, phosphorus is critical for nitrogen metabolism and for all of the energy transfer reactions.

Potassium is critical for efficient nitrogen utilization because it is necessary to drive the enzymes that assimilate nitrogen into biochemical compounds. Also, potassium is critical to maintaining water relations in crops.

It appears that the most profitable fertilizer rates are based on crop response, fertilizer costs and commodity prices. The optimum rate is where the yield increase just pays for the last pound of nutrient. This is usually just below the maximum yield level, but this relationship changes with the price of fertilizer and commodity prices. Given today's fertilizer and commodity prices, maximum economic yield (MEY) is still very near the maximum yield potential.

To get the most return from applied fertilizer, we need to put it where we have the best chance of getting a return on our investment. Obviously, this is going to occur on our higher yielding fields and/or on soils with low soil test for P and K.

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The figure above shows that with adequate potassium the optimum nitrogen rate for MEY is 180 lb/A, whereas at low potassium levels it took 280 pound of N to reach maximum yield potential, which is considerably lower than what is achieved with the high-K treatment. Remember, maintaining good P and K fertility maximizes crop response to nitrogen.

Source: Johnson et al., 1997, Ohio

In 2008, farmers will be shooting for yields higher than ever before. Many factors contribute to crop quality. Good hybrids and varieties, timely planting, proper plant populations, good pest management, and good weather are all necessary, but a good fertility program will contribute to a significant portion of the yield.

High yields require high fertility. Your goal should be to build your soils to a high enough fertility level so that you can reach your maximum yield potential for any given crop rotation and weather condition. High soil fertility supports yields in good years; it also provides yield stability in less than ideal years. All in all, maintaining a good fertility program will help get the most from your fields in any given year.

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The table above shows a Purdue University estimate of the probability and magnitude of responses of corn and soybeans to P and K fertilizer at different soil test levels. You can see that lower testing soils are most likely to respond to added fertilizer.