Mid‐Continent Fall Temperatures at the 10‐cm Soil Depth

Abstract

Recommendations for applying nitrogen (N) fertilizer in autumn involve delaying applications until daily soil temperature at 10‐cm depth is ≤10°C. Daily soil temperature data during autumn were examined from 26 sites along a transect from 36° to 49° N latitude in mid‐continent USA. After soils first cooled to ≤10°C (First10), temperatures usually rebounded for varying amounts of time before the final date, at which they decreased to ≤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 in mid‐continent USA accumulated more degree‐days (base 10°C) between 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 with soils in Missouri or North Dakota and northern Minnesota. Progressively more conservative guidelines for applying N fertilizer in autumn may be to wait until the dates of (i) average Last10, (ii) Last10 plus one SD, or (iii) Last10 plus two SD units. All of these alternative dates can be estimated reliably through simple polynomial equations for latitudes between 35° and 50°.