Crop Management

Predicting N Requirements for Corn: Past & Future

by Eric G. Beauchamp, Dept. of Land Resource Science, University of Guelph

There has always been and continues to be considerable uncertainty in predicting N requirements for crops. In this article, Eric Beauchamp presents an historical overview of this field of study and outlines best steps for the future.

Improving the economics of N recommendations for corn through more accurate predictions of crop requirements has consumed enormous financial and human research resources in Ontario and throughout the world. This is not surprising, as the dynamics of soil N are very complex. Although laboratory studies can improve our knowledge, the much more difficult and costly field studies are what matter most as we try to understand soil N dynamics in the field.

The availability of soil N in the field is affected by no fewer than 10 processes. Half of these are microbial processes, whereas the others are of a physiochemical nature. The following do not take into account additions of crop residue N or manure N.

All of these processes act together in field soil, making it somewhat amazing that we are able to predict N requirements with any degree of accuracy at all.

The annual reports of the Ontario Agricultural College since 1874 reveal studies of crop response to chemical fertilizers. Prior to the 1950s, the major focus was on crop response to mixed chemical fertilizers containing P and K as well as N. Hundreds of demonstration trials were conducted on Ontario farms: a typical fertilizer treatment would be 100 lb 2-10-6 per acre, which meant that only 2 lb N per acre was applied. Although some form of soil testing was promoted for P and K as early as the 1920s, a rigorous soil test research program was carried out in the 1950s. This program focused on crop responses to rates of individual fertilizer nutrients along with economic assessments. Rates of fertilizer N recommended for corn increased during this decade partly because corn had become a key cash crop in Ontario. For example, in 1953, the Fertilizer Advisory Board for Ontario recommended 45 to 116 lb N per acre be applied partly preplant as a mixed fertilizer and partly sidedressed as ammonium nitrate providing there was no previous clover crop. These were regarded as general recommendations based on historical corn crop response information. There is no precise information available on how such recommendations were determined from an economic standpoint.

A novel soil N test involving an incubation laboratory procedure was developed for use in Ontario in 1963. But after several years, it was abandoned because of primarily logistical shortcomings. It was not until 1991 that another soil N test for corn was implemented. It is based on the quantity of nitrate accumulated up to shortly before sidedressing time. In principle, these two tests are similar. The old test involved mineral N production during a 2-week incubation period of a field soil sample in the laboratory, whereas the newer one relies on nitrate accumulation under field soil conditions. The new test (like the old one) determines the ability of the soil to produce mineral (available N). With proper calibration in the field, the test is used to determine the quantity of N needed to grow a corn crop with maximum profitability.

In the mid 1980s, corn yield response to N data from more than 200 field trials conducted since 1960 across southern Ontario were evaluated to improve general recommendations. The data sets from individual trials were fitted to a quadratic model and the most economical rate (MER) of N was determined. It was noted that these rates generally differed for corn crops in three regions in Ontario: southwestern, central and eastern. No clear reason for these differences has been established, although short and long season cultivars likely have different N requirements. There are probably other factors involved as well.

A further evaluation in 1995 of corn yield response to N from an additional 100 field trials basically confirmed the general N recommendations first proposed in 1986 and is still used. These general N recommendations are based on historical data and cannot therefore take into account future weather conditions, agronomic practices, cultivars, etc. The N from manures and leguminous crops can be taken into account in a general way.

The management of manure N (and P or K) has been much improved recently with the NPAC Program developed by OMAF.

There has always been and continues to be considerable uncertainty in predicting N requirements for crops. Recent attempts to use spectral analysis (e.g., chlorophyll concentrations) has met with moderate success at best. Success has been marginal because so many plant-related factors other than N supply affect chlorophyll content.

Improving N Recommendations for Corn: What Needs to be Done?

1. General N recommendations should be reviewed at least every 10 years. Cultivar yields continue to increase and cultivars are recycled rapidly.

2. The current soil N test, in particular its interpretation, needs to be improved. Suggestions for improvement include taking into account antecedent soil temperature and moisture in the period before sampling, and considering the exponential increase in soil nitrate during the late May/early June period before sampling for the presidedress N test.

3. Better soil N tests need to be identified – recent suggestions include the amino sugar N test, measurement of both concentration and depth of soil organic matter in the soil profile, and ‘hot potassium chloride’ extraction of ammonium for short term predictability of N availability.

4. Continue to investigate spectral analyses. It will probably be possible to ‘track’ changes in plant N content by satellites in conjunction with changing weather patterns and growing conditions. Breakthrough in interpretations of spectral data could be possible.

5. We need a better understanding of the dynamics of decomposition of crop residues and manures coupled with the release of available N over the growing season. Most research on soil N dynamics has been confined to the laboratory: field studies that also include the roles of soil structure and changing soil water content are essential for a better understanding of the soil/crop N system and interpreting soil N tests.

6. A biological N fixation system specifically for corn needs to be developed. There is little, if any research being conducted in this area.
Efforts to improve the economic and efficient use of N will also result in minimizing losses. Although it is unlikely that a ‘closed’ crop/soil N system can be achieved, much can be done to improve the efficiency of N use.

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