What Gap?
The gap between what's possible and what we accept...the gap between our potential and our actual...the gap between what our combine harvests and what some combines have harvested.

Think about it. Narrowing that yield gap is:

	The greatest profit opportunity available today to crop producers.
	The most substantial potential source of food for the additional 2.5 billion people expected on this planet by the middle of the 21st century.
	A significant source of environmental relief through enhanced carbon sequestration, increased nutrient use efficiency, and through freeing more land for buffer strips, wetlands, rain forests, and recreation.

But, is there a real yield gap to be explored and exploited?
You bet, and some data, such as the Iowa corn data in Figures 1 and 2, suggest it 's getting larger with each passing season. The yield gap between average corn growers and Master growers has averaged over 100 bu/A since 1938 and appears to be increasing at a rate of 0.56 bu/A/yr. Average Iowa corn yields have increased at a rate of 1.64 bu/A/yr, but Master grower yields have increased at a rate of 2.20 bu/A/yr.

Figure 1. Highest annual corn grain yields entered in the Iowa Master Corn Grower Contest and Iowa state average corn grain yields.(Data: Iowa Crop Improvement Association and National Agricultural Statistics Service).	Figure 2. Annual difference between the yields attained in the Iowa Master Corn Grower Contest and Iowa state average yields. (Data: Iowa Crop Improvement Association and National Agricultural Statistics Service).

The yield gap is not unique to Iowa or to corn.
Table 1 summarizes verified crop yields for North America. They clearly illustrate the remarkable attainable yields of today's genetic material and at the same time, in striking fashion, reveal the huge gap between attainable yields and the yields normally harvested.

Some argue that these record-setters are just lucky and got all the needed rains that their neighbors missed, or that they have some "weird " soil that does not occur anywhere else. However, the facts show that frequently it 's the same growers who set and break their own records and that the soil types they do it on are often the dominant soil types in the area. A good example is Mr. Francis Childs of Manchester, Iowa. In 1999, he harvested a verified corn yield of 393.7 bu/A, a new world record. He has won the non-irrigated class of the Iowa Master Corn Grower Contest for several years and has done so on the Kenyon-Clyde-Floyd soil association that comprises 51 percent of the soils in his county. What is unique about Mr. Childs is his management, not his weather or his soil type.

So what does it take for an individual to exploit the yield gap?
In one word, management …in a phrase, management and long-term dedication. Consider the following characteristics relative to your own management program and its potential for producing and sustaining high yields.

	A fundamental planning process that estimates what the actual attainable yield levels might be within each field, recognizing existing controllable limiting factors and their interactions for the cropping system. That 's a challenge. Producing high yields often requires on-farm experimentation in which a yield goal is set that you believe is slightly out of reach. Then all input levels from seeding rate to variety selection to fertilizer rates are set assuming you can attain that yield. As you learn, practices are applied to whole fields and farms.
	A focus on timeliness of all operations and a record keeping system that allows quantification of what works and what doesn't work.
	Use of technologies such as genetically enhanced varieties and site-specific management to control risk.
	Long-term dedication to soil improvement, including physical, chemical, and biological properties. Individuals who produce top yields seldom do it overnight. That's because properties such as soil tilth, water holding capacity, and subsoil characteristics can be improved, but only over a period of several years.
	A constant watch for yield limiting factors and dedication to removal of those that can be controlled. Insufficient soil fertility is an example of a controllable limiting factor that can be profitably removed, given sufficient time.

Soil fertility programs designed to narrow the yield gap have a goal of removing the soil supply of immobile nutrients such as phosphorus (P) and potassium (K) as potential yield limiting factors.

Nearly all record yields have been produced with high or very high soil test levels. In most soils you only need to build soil levels once, so build-up should be viewed as a capital investment or a land improvement that can be amortized over many years. What soil test levels are necessary? The answer starts with relevant soil test calibration research …relevant to the soils, tillage practices, rotation, varieties, and yield levels for your farm. If your management system differs significantly from that of the calibration research, required levels could also differ.

Points to consider relevant to the impact of yield potential on soil test level requirements:
Factors common to high-yield environments tend to increase crop shoot growth more than root growth, increasing the amount of nutrient that must be absorbed per unit of root length present. This would potentially increase soil test level requirements. Factors known to increase shoot to root ratios include increasing nitrogen (N) or P levels, increasing water available to the plant, decreasing light intensity (as in high populations), and soil temperatures in the 75 to 85 degrees F range.

Good root development in the subsoil will often increase the supply of water to the plant, but this also means that a higher percentage of roots might be growing in less fertile soil, increasing the need for nutrients in the surface soil.

If hybrids or varieties with the highest yield potential direct more of their photosynthate to producing shoot growth rather than root growth, their soil test level requirements could be higher.

The fact is that we don 't know what the minimum soil test levels are for yield levels that extend beyond the range of our calibration research. However, one thing is certain. If they do differ from levels required for typical yields, they won 't be lower.