Apparent Thermal Conductivity of Mulch Materials Exposed to Forced Convection

Soil temperature controls plant growth and many related processes in the soil. A mulch or crop residue covering the soil may alter soil temperatures significantly. Available simulation models often lack experimental data for the mulch thermal conductivity and its dependence on air velocity. The apparent thermal conductivity (k) of wheat straw, pine straw, tire chips, dry sandy soil, and the thermal resistance of Bermudagrass sods were measured using a guarded hot plate at air velocities between 0 and 5 m/s. For all mulch materials, k ranged between 0.1 and 0.6 W m–1 °C–1, and increased with increasing air velocity, except for the more compact materials such as soil and, to a lesser extent, small tire chips. We found a minimum in k around 1 m/s for the thicker (> 0.1 m) layers of wheat straw and pine straw, which was tentatively attributed to interactions between the straw and the convection (free versus forced mechanism at the 1 m/s velocity). A model was created for predicting apparent thermal conductivity through mulches in thermally unstable environments. Using estimated mulch opacity parameters and fitting convection parameters, r2 values ranging from 0.72 to 0.99 were obtained. The model may be used in field situations where the soil under a mulch is warmer than the air above the mulch, which is a typical nighttime condition. The model should be tested using independently measured data.