Fall vs. Spring Soil Sampling for Calibrating Nutrient Applications on Individual Fields

Historic soil test results have been proposed as a method for adjusting the nutrient applications on specific fields but the method has not been tested. The method requires consistency of sampling and analysis. An assumption that fall or spring soil sampling could be used for this purpose was tested in a field monitoring study. Soils of six production fields at Agassiz Research Station were sampled in the fall (3–8 November) and the subsequent spring (3–10 March) following three successive cropping years (1984–1986) to examine changes in standard soil test analyses over the winter and from year to year. Four of the fields were cropped to a grass-legume mixture and, in most cases, removed as silage or hay, and two were used for silage corn. Management practices were relatively uniform within each crop type and nutrient inputs were recorded. The fields were sampled in triplicate to determine variability and sampled at 0 to 6,6 to 12, and 12 to 24 in. depth intervals to monitor possible leaching. All samples were immediately air dried and stored until analyses could be done as a batch to ensure uniform analytical conditions. Some soil tests (P, Mg, Cu, Fe, and Zn) did not change over the winter but others (NO₃-N, K, Ca, pH, S, Mn, and B) did. Only B changed from fall to spring independent of field or year of sampling, with a consistent decrease. All the other analyses that changed over the winter were influenced by the field, year, or both. The change in Ca over the winter was consistent over all fields, but was variable over the three years. Sulphur, Mn, and NO₃-N tended to decrease, and K and pH tended to increase over the winter, but the changes were not consistent over all fields and all years of the study. The decreases observed over the winter were frequently not accounted for by leaching when subsurface samples were examined. Hence, surface samples were adequate to assess residual levels of most elements. Subsurface sampling was preferred for residual NO₃- assessment. Several of the analyses (Mg, P, Mn, K, and NO₃-N) were consistently higher in the corn (Zea mays L.) fields than in the grass fields and frequently increased from year to year, probably reflecting the influence of the crop and application of inorganic fertilizer and manure during the period of study. The other analyses varied from year to year and field to field, but not consistently with respect to differences in general field management during the study period. The results of this study show that the use of soil test records to adjust on-farm field-specific fertilizer practices must be approached with caution and that fall rather than spring soil sampling would be preferable unless local data on the effect of winter on soil test analyses are available to recommend otherwise.