Evaluation of soil survey scale for zone development of site-specific nitrogen management

Zone sampling for site-specific N application has been shown to be effective in North Dakota and other areas of the Great Plains. Printed and sometimes digitized soil surveys are presently available for most agricultural counties in the USA. Order 2 soil surveys generally have scales that range from 1:12 000 to 1:31680. These surveys were developed for general planning purposes. There is interest in using Order 2 soil surveys as a basis for delineating N management zone patterns, especially where the soil-mapping units have been digitized. This study was conducted to evaluate soil survey scales at the Order 1 (scale >1:15 840) and Order 2 level against grid- and topography-based zone sampling to determine whether soil surveys at these scales could be used to delineate N management zones for site-specific fertilizer application. Fields mapped at a finer scale (Order 1 survey) showed some similarity between mapping units and N management zones defined by topography. Order 2 soil-mapping units, which are the present mapping scale of most agricultural soil surveys, were often not similar to N management zones. Published Order 2 soil surveys should not be used to develop N management zones for site-specific agriculture unless the soil patterns are verified with other zone development tools of site-specific management. Alternatively, a major benefit of Order 1 soil surveys would be to reinforce or redefine apparent N management zones.