This article originally appeared in the AGVISE Laboratories Fall 2022 Newsletter
Good crop prices encouraged late planting beyond crop insurance deadlines, but additional June rainfall kept some producers from planting all their acres, leaving some unplanted fields or unplanted parts of fields. There are many questions about soil testing on these unplanted fields: When should you start soil sampling? What kind of residual soil nitrate-nitrogen amounts can you expect? The extremely wet soil conditions may have caused considerable soil nitrogen losses to leaching or denitrification. Through summer, warmer and drier weather added nitrogen through mineralization of soil organic matter. In addition, cover crops and any weedy growth will acquire nitrogen from soil. The amount of soil nitrate-nitrogen remaining for next year will depend on soil type, environment, and management factors, which vary from field to field and zone to zone.
• What was the crop grown in the previous year?
• What was the nitrogen fertilizer rate and application timing? Was it applied last fall?
• Did you do any summer tillage? More tillage promotes nitrogen mineralization.
• How was your weed control? Did the weeds get large and acquire substantial nitrogen?
• Did you plant a cover crop? Did the cover crop get incorporated later?
• Did excessive rainfall cause nitrate leaching on well-drained soils?
• Did excessive rainfall cause denitrification on poorly drained soils?
• Were summer temperatures warm? Warm temperatures promote N mineralization.
Soil testing on these unplanted fields can begin as soon as good quality soil samples can be collected after mid-August. There is no reliable way to guess how much residual soil nitrate may be present in these unplanted fields or unplanted parts of fields. Soil testing is the only accurate way to learn how much residual soil nitrate remains in the soil profile. To obtain the best information for nitrogen management, we recommend splitting fields into management zones for soil testing. The unplanted field areas can vary considerably from the rest of the field, which will skew the field-average soil test result and resulting nitrogen fertilizer rate.