Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/20/2006
Publication Date: 5/11/2007
Citation: Forcella, F., Lachnicht Weyers, S.L. 2007. Mid-continent Fall Temperatures at the 10-cm Soil Depth. Agronomy Journal. 99:862-866.
Interpretive Summary: The rule of thumb for applying N fertilizer in the fall is to wait for soil temperature at 4" depth to fall below 50° F. Unfortunately, after soils first cool to less than 50° (First50), they usually rebound for varying periods of time before they decrease to less than 50° (Last50) for the remainder of the winter. Because N may be lost during these warm periods between First50 and Last50, understanding their extent, duration, and timing is important for N management. Therefore, daily soil temperature data during the fall were examined from 26 sites along a transect from 36° to 49° N latitude in the mid-continent USA. Soils at sites between 41° and 44° N latitude accumulated more degree-days between the First50 and Last50 than soils north or south of this region. Thus, if N-fertilizer is applied too early, soils in Iowa, South Dakota, and southern Minnesota are more susceptible to N losses compared to soils in Missouri or North Dakota and northern Minnesota. A more conservative guideline for applying N-fertilizer in the fall would be to wait until average Last50. Even safer guidelines would be to wait until the date of Last50 plus the number of days equivalent to one standard deviation (a statistical measure of variation in time of Last50), or Last50 plus two standard deviations. These alternative and conservative dates for fall N applications can be estimated reliably for latitudes between 35° and 50° and determined from simple tables provided in the article. The results should be valuable for producers, fertilizer applicators, and the fertilizer industry for determining when to apply N in the fall with the least amount of risk of N loss.
Technical Abstract: Recommendations for applying N-fertilizer in autumn involve delaying applications until daily soil temperature at 10 cm depth is = or < 10° C. Daily soil temperature data during autumn were examined from 26 sites along a transect from 36° to 49° N latitude in the mid-continent USA. After soils first cooled to = or < 10° C (First10), temperatures usually rebounded for varying periods of time before the final date at which they decreased to = or < 10° C (Last10) for the remainder of the winter. Because N may be lost during warm episodes between First10 and Last10, understanding the extent, duration, and timing of these warm periods is important. Soils at sites between 41° and 44° N latitude accumulated more degree-days (base 10° C) between the First10 and Last10 than soils north or south of this region. Thus, if N-fertilizer is applied too early, soils in Iowa, South Dakota, and southern Minnesota are more susceptible to N losses compared to soils in Missouri or North Dakota and northern Minnesota. More conservative guidelines for applying N-fertilizer in autumn may be to wait until average Last10, Last10 plus one standard deviation (SD) unit, or Last10 plus two SD units. All of these alternative dates can be estimated reliably through simple polynomial equations for latitudes between 35° and 50°.