Topography-Based Sampling Compared with Grid Sampling in the Northern Great Plains

Abstract

A practical variable‐rate fertilizer application should be based on information gathered at low cost that represents field fertility levels. The number of soil samples gathered and analyzed may limit the effectiveness of some variable‐rate fertilizer applications. Topography‐based soil sampling is attractive because it suggests a lower number of samples needed to characterize fertility levels and patterns in a field than some current grid sampling recommendations. A 40‐acre North Dakota field consisting of Barnes loam (fine loamy, mixed, Udic Haploborolls), Swenoda loam (mixed, Pachic Udic Haploborolls), and Wyard clay loam (fine loamy, mixed, frigid Typic Haplaquolls) soil types was sampled in a 110 ft grid each fall from 1994 to 1996. Nitrate N, P, sulfate S, and chloride level patterns were similar in all 3 yr. Correlation of both area‐ (sample cores obtained from a wide area within a topography zone) and point‐based (a topography zone represented by a composite taken from a point location) topography sampling with the 110 ft grid values was compared with correlation from a 220 ft, 330 ft, and 5 acre grid. Area‐based topography sampling for nitrate N was superior in correlation to the 220 ft grid values in 2 of 3 yr. Area‐based topography sampling with P was superior to the 220 ft grid in only 1 of 3 yr, but was superior to the 5 acre grid in all years. The 220 ft grid was superior to topography sampling for sulfate S in all years, but area‐based topography sampling was better than the 330 ft grid in 2 of 3 yr and superior to the 5 acre grid in all years. Area‐based topography sampling for chloride was superior to the 220 ft grid in 1 of 3 yr, but was more highly correlated with the 110 ft grid than the 330 ft and 5 acre grid in all years. Area‐based topography sampling was more highly correlated than point‐based topography sampling in seven of 12 comparisons. Topography‐based sampling for nitrate N better represented fertility patterns than did grid sampling. Research Question In order for variable‐rate fertilizer application to be practical, soil testing information must be gathered at low cost, but at the same time represent the variation in field soil fertility levels. Since collecting and analyzing soil samples is expensive, minimizing the number of samples collected while maintaining a high level of soil fertility information is important. This study of one North Dakota field compared the use of topography‐based sampling with selected grid methods for nitrate N, P, sulfate S, and chloride to determine whether topography‐based sampling might compare favorably with information gathered from grid sampling methods, while decreasing the number of samples needed to provide similar or superior soil fertility information. Literature Summary Previous work in Wisconsin and Illinois has suggested that a one sample per acre grid might be required to gather soil fertility information needed for a variable‐rate fertilizer application. Other studies have demonstrated that some soil fertility factors may be related to l...

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