A commercial winter wheat farm near St. John, Washington U.S.A. was intensively sampled along four 655 m long transects separated by 122 m. Samples were taken at intervals of 15.2 m to a depth of 1.8 m to obtain profile available water content, organic matter content, pH, nitrate nitrogen, ammonium nitrogen, and available phosphorus. Criteria used to evaluate spatial patterns in soil properties and wheat grain yield along the transects included fertility status, organic matter content, available profile water content and mapped extent of erosion. Grain yield was also used as a criterion to divide the field into zones having different soil properties. Extent of erosion was not a satisfactory criterion, since rates of recommended nitrogen fertilizer did not differ significantly between the resulting management zones. The other four criteria were determined to be acceptable since, on average, they allowed the field to be divided into three large zones that each had significantly different grain yields (averaging 3.29, 4.05 and 4.75 Mg ha−1, respectively) as well as significantly different recommended rates of nitrogen fertilizer (ranging from 0 to 90 kg N ha−1). In zones delineated using the four acceptable criteria, the field averaged rates of recommended nitrogen were about 35 kg ha−1, which represents a significant reduction compared to the grower's typical uniform application of 73 kg N ha−1.
Criteria, eroded wheat fields, management zones
