Effect of solar radiation on soil temperature in pots housed in cold frames

Abstract

Heat gain in pot soil within cold frames covered with glass and without an artificial supply of heat and ventilation may take place during the day: (a) by advection from the cold frame air, as the air is at a higher temperature than the pot soil; (b) by advection from the inner glass surface, as this surface is at a higher temperature than the cold frame air; (c) by radiation from the inner glass surface; (d) by solar radiation. A theoretical consideration of the heat gain by advection from the cold frame air resulted in an equation which expresses the relationship between pot soil temperature and cold frame air. An application of this equation to experimental data showed that the heat gain by advection from the air is only 10–20% of the total heat gain. The heat gain by advection and radiation from the inner glass surface is very small compared with the solar radiation transmitted through glass and can be omitted. Therefore the main source of heat in cold frames without an artificial supply of heat is solar radiation since (a), (b) and (c) above have been proved negligible. The pot, acting as a black body, accumulates the solar energy falling on it. Using a theoretical approach the effect of solar radiation on soil temperature was found Θ sj − Θ so = A ss r G C (R sun t j) The parameters involved in this equation are; an initial temperature of pot soil ( Θ so), the pot soil temperature ( Θ s j ) at any given time ( t j ) before daytime maximum, the pot soil surface area ( A ss), the transmission coefficient of glass ( r G), the thermal capacity of pot soil ( C) and finally the net energy from sun ( R sun t j ) integrated from the initial time up to time t j . From the above equation a prediction of pot soil temperature from solar radiation can be made with a good degree of accuracy, since the experimental data fit this equation satisfactorily.