Thermal diff usivity of soil-peat mixtures: nonlinear dependence on peat content

The thermal diffusivity of the upper plow horizon of the urban soil from the territory of MSU Experimental soil station, lowland packed peat, and their mixtures was studied. The soil from Ap horizon was sieved through a 1 mm sieve and mixed with peat in various proportions; the content of peat in mixtures ranged from 1 to 80% by dry weight. Metal cylinders 10 cm high and 3.8 cm in diameter were filled with soil, peat, and soil-peat mixtures. The thermal diffusivity was measured using the unsteady-state method with a working temperature range of 20–26°C. The heating rate of the packed samples was measured after the samples were placed in a liquid thermostat with a constant water temperature. For each sample, a series of measurements was 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 vs. water content dependence was almost linear for peat, and for soil it was an S-shaped curve with a weakly expressed maximum. The lowest thermal diffusivity was obtained for peat and mixtures with high peat contents; the highest one — for soil without peat additions. The non-linear character of the thermal diffusivity vs. peat content dependence was 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. It was concluded that the patterns typical for soil-peat mixtures are similar to those earlier obtained for peat-sand mixtures, but in the case of soil-peat mixtures these patterns are less clearly expressed.