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 Haplo-borolls), 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.
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