Air and Soil Temperatures during Spring Burning of Standing Wheat Stubble 1

Winter wheat ( Triticum aestivum L.) residues in the Pacific Northwest are often burned even though long‐term detrimental effects on soil properties may ensue. Better weed and disease control are cited as reasons for burning even though results have been highly inconsistent. Inadequate duration of lethal temperature or a nonuniform burn pattern may allow substantial weed, disease, and insect survival. We investigated temperature patterns during burning of 5.0, 6.3, and 6.9 Mg ha −1 of wheat residue on March 21 and 6.5 Mg ha −1 of residue on May 3. Wheat residue consisted of about 60% standing stubble (350 mm tall) and 40% harvested litter deposited on the surface of a coarse‐silty, mixed, mesic Typic Haploxeroll. Temperature maximum, duration, and variability below, at, and above the soil surface were recorded with copper‐constantan thermocouples. Air temperatures reached a maximum within 30 s after ignition and returned to near‐ambient level within 180 s. Maximum temperatures averaged 301, 416, and 315°C at 25, 75, and 250 mm, respectively, above the soil surface. Burning had no effect on the temperature 25 mm below the soil surface. Neither residue level nor date of burning substantially influenced temperature maximum or duration. Soil surface temperatures during burning were highly variable, seldom exceeding 120°C when the litter‐burn was incomplete, but ranging between 170 and 330°C where litter was completely burned. Substantial occurrence of surface temperatures below 120°C suggests that uniform residue distribution and complete combustion are critical for achieving temperatures capable of destroying weed seeds and disease organisms located on or near the ground.