• Varying the application rates of plant nutrients and other crop inputs across variable fields makes good agronomic sense. A reasonable strategy must be used to guide application.
• Grid soil sampling for P and K has improved the accuracy of fertilizer application, although greater accuracy can be attained by considering additional site characteristics.
• A "management zone" is a sub-region of a field that expresses a relatively homogeneous combination of yield-limiting factors where a single rate of a crop input is appropriate.
• Spatial information helpful in defining management zones should be quantitative, densely or continuously sampled, stable over time, and directly related to crop yield.
• The basis for accurate and profitable application of crop inputs will continue to be a clear understanding of the agronomic factors that directly affect crop growth and yield.
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Whether the goal is to determine the level of soil nitrogen (N) or the soil yield potential, management zones for N fertilizer management can be constructed using a variety of tools, including topography, aerial photographs, satellite imagery, soil electrical conductivity sensors, yield maps, and intensive soil survey data. Viewing satellite images and/or aerial photos that are relatively inexpensive and comparing them with landscape features would be a good place to start.

Zones can be constructed and managed for N using a fraction of the soil samples required to reveal the same zones through grid sampling. Zone sampling results in lower sampling costs for variable-rate fertilizer application and allows precision farming to be much more practical for producers of commodity crops.

Roger Brook