EVALUATING THE RELATIONSHIP BETWEEN THERMAL CONDUCTIVITY AND VINEYARD SOILTEMPERATURE IN THE NORTHERN WILLAMETTE VALLEY AMERICAN VITICULTURAL AREA

Soil temperature exerts important controls on wine grape development, including the onset of the phenological stages of ripening. Many variables affect soil temperature including thermal conductivity, air temperature, sunlight intensity, and ground cover. In order to evaluate the relative importance of thermal conductivity, laboratory conductivity values were obtained from samples obtained from 24 sites within 13 vineyards located in the northern Willamette Valley AVA. The samples were dried, crushed, and sieved to particles <2 mm and compacted to a uniform bulk density. Thermal conductivity values were compared with average soil temperatures recorded by data loggers buried at each sample site. Soil temperature readings were recorded hourly for four months. The measured conductivity values did not correlate with average soil temperature, average diurnal temperature variation, average minimum temperature, or average maximum temperature. The lack of correlation between soil temperature measurements and the thermal conductivity measurements obtained in the lab was presumed to be related variable soil properties such as moisture content and density that affected thermal conductivity at the field sites. To investigate the impact of these variables, thermal conductivity was measured in selected samples using volumetric water contents of 0, 14.1, and 28.1 percent and bulk densities of 1.1, 1.3, and 1.5. Thermal conductivity was strongly affected by changes to both water content and bulk density. Further experiments revealed that different ground covers place a significant role in soil temperatures. The effect on vineyard soil temperature of variations in thermal conductivity as measured in standardized laboratory samples was overwhelmed by variations in density, water content, and ground cover in the natural setting. Standardized laboratory measurements of thermal conductivity are therefore of little value as a predictor of vineyard soil temperatures.