SPATIOTEMPORAL VARIABILITY OF SOIL-TEMPERATURE WITHIN 3 LAND AREAS EXPOSED TO DIFFERENT TILLAGE SYSTEMS

The rational (lateral) spacing between soil temperature sensors to obt ain spatial independence of measurements in the field under various ti llage systems is not well studied. In particular, properly positioning sensors in a tillage plot requires knowledge of the spatial dependenc e of the measurements. This study was conducted to measure the horizon tal spatial variability of soil temperature and its diurnal fluctuatio n in three different tillage systems. Soil temperature was measured wi th copper-constantan thermocouples at 49 positions in each system at t he 1- and 10-cm depths. The thermocouples formed a 7 by 7 grid with a spacing of 3.0 (east-west) by 1.5 m (north-south). The three managemen t systems were fall moldboard plowing followed by spring disking and p lanter operation (MP), fall chisel plowing followed by spring dishing and planter operation (CP), and ridge tillage slot planting (RN). Each management system had been under continuous field corn (Zea mays L.) for 8 yr. A novel finding of the study is that a hysteresis within the diurnal cycle existed between the mean and the sample variance. This hysteresis was presumably due to differences in soil surface heating a nd cooling rates during the diurnal cycle. Although no definitive rela tionships could be determined, semivariograms appeared to show a time dependence and anisotropy during the 24-h observation period. Correlat ion distances were approximate to 9 m in the E-W direction along crop rows and tillage operations but <1.5 m in the N-S direction across cro p rows and tillage operations. These spatial correlation distances Hil l be useful in deciding the minimum (lateral) spacing between adjacent soil temperature sensors in the field.