[Spatial and temporal variation of soil temperature extremum under plastic mulch in Xinjiang]

The upper and lower limit values of soil temperature affect crop growth and development greatly. Observations on the soil maximal and minimal temperatures under different mulching and cropping conditions in Xinjiang showed that during crop growth period in 1998 and 1999, soil temperature extremums were both at 0 cm, and varied with different observation time. The soil minimal temperature under plastic mulch was higher than that without mulch, indicating that plastic mulch could remarkably increase soil temperature. The diurnal variation of soil minimal temperature could be expressed by quadratic function, while the maximal temperature at 14:00 and 20:00 could be expressed by ellipse function and linear function of soil depth, respectively. Soil temperature extremums correlated linearly with air temperature under different conditions. The correlation between soil minimal temperature and air temperature was higher for bare soil than for mulched soil, and higher for maize field than for cotton field, while the correlation between soil maximal temperature and air temperature was lower than that between soil minimal temperature and air temperature.