IS SOIL-TEMPERATURE BETTER THAN AIR-TEMPERATURE FOR PREDICTING WINTER-WHEAT PHENOLOGY

In predicting wheat (Triticum aestivum L.) phenology, logic suggests t hat basing thermal unit accumulation on near-surface soil temperature should give a better representation of shout apex thermal unit accumul ation than air temperature until internode elongation raises the apex above the soil surface. A study was undertaken to determine if predict ions of winter wheat phenology are improved when based on measured nea r-surface soil temperature rather than air temperature. Air temperatur e 1.5 m above the soil surface and soil temperature at crown depth (th e position of the shoot apex before stem elongation) were collected Fo r 23 site-years across the U.S. Central Great Plains representing a ra nge of cultivars, soils, management practices, and climates. Seven sit e-years from different sites were randomly selected to calculate the m ean thermal units from both seeding and I January to specific growth s teps based on both air and soil temperature. These means were used to predict occurrence of growth stages for the remaining 16 site-years. I n no instance did soil temperature significantly improve prediction of winter wheat phenology, From these results, we conclude that the addi tional effort and expense of using soil temperature in predicting wint er wheat phenology are not justified.