Thermal Diffusivity of Soil-Peat Mixtures: Nonlinear Dependence on Peat Content

The thermal diffusivity of the upper plow horizon (Ap) of urban soil from the territory of the Experimental soil station of Moscow State University, lowland packed peat, and their mixtures has been studied. The soil from Ap horizon was sieved through a 1-mm sieve and then mixed with peat in various proportions; the content of peat in mixtures ranged from 1 to 80% (dry mass). Metal cylinders 10 cm high and 3.8 cm in diameter have been filled with soil, peat, and soil-peat mixtures. The thermal diffusivity has been measured using the unsteady-state method with a working temperature range of 20–26°C. The packed soil samples have been placed in a liquid thermostat with a constant water temperature, and the heating rate of samples was measured. For each sample, a series of measurements has been carried out with a step-by-step change in soil water content from the maximum one after capillary saturation of the sample to the minimum one, corresponding to the air-dry state. Within the studied range of water contents, the thermal diffusivity of different samples changed by a factor of 1.3–3.5; the greatest changes with water content were typical for mineral soil. The thermal diffusivity versus water content dependence was almost linear for peat, and for soil it was characterized by an S-shaped curve with a weakly expressed maximum. The lowest thermal diffusivity has been obtained for peat and mixtures with high peat contents; the highest, for soil with no peat additions. The nonlinear character of the thermal diffusivity versus peat content dependence has been revealed. Small additions of peat to mineral soil resulted in a noticeable decrease in the thermal diffusivity of the mixture; small additions of soil to peat had practically no effect on thermal diffusivity. The thermal diffusivity of the studied substrates increased with increasing sample bulk density and decreased with increasing organic matter content. Thus, the patterns typical for soil-peat mixtures were similar to those earlier obtained for peat-sand mixtures, but in the case of soil-peat mixtures these patterns were less clearly expressed.