The effect of container porosity on root environment and plant growth
SummaryComparison of clay and plastic pots for tomato propagation in six experiments showed plastic pots to give significantly better results than clay pots in winter and the converse to be true in summer. In each experiment the ratio of plant growth (dry weight) in clay and plastic pots was related to both the mean daily solar radiation and the mean outside night minimum temperature. In winter, plants in both types of container made significantly more growth when stood on open benches than when stood on benches with solid surfaces.These differences in growth were related to soil temperature, clay pots being cooler than plastic pots, the temperature difference between the types of pot ranging from approximately 1°C by night in winter to 4°C under high intensity solar radiation in summer. In winter, soil temperatures were often below the optimum, and in summer in excess of the optimum.By day, temperature within the pots showed highly significant correlation with solar radiation, and by night in winter pot temperature was negatively correlated with the difference between the glasshouse and the outside air temperature.Well-defined horizontal thermal gradients were found, and these were positively correlated with soil temperature. In wet soils the thermal gradients were less than in dry soils, and because of their higher thermal conductivity wet soils were warmer than dry soils.When the normal evaporative cooling from the porous walls of clay pots was prevented by a bituminous coating, the soil temperature during the night was not significantly different from that in plastic pots. By day, however, soil temperature in non-evaporating clay pots was less than that in evaporating clay pots, due to the lower thermal conductivity of the dry clay wall.
WATER FLUX IN MOIST SOIL: THERMAL VERSUS SUCTION GARDIENTS
Simultaneous flows of heat and moisture were measured across a loam soil sample under various thermal and pressure gradients. The data were collected in a manner such that the liquid and vapor flow components could be separated and such that the liquid component could further be divided into that flow due to pressure differences and that due to thermal differences. It was found that a temperature gradient of 0.5 deg C./cm at a soil moisture suction of 5 cm of Hg would move as much water through the soil as a pressure gradient of 2 cm of water per cm. At a suction of 34 cm of Hg this temperature gradient was equivalent to 250 cm of water per cm. Vapor transport through the soil was several times greater than could be predicted with Fick's law and the diffusion of water vapor in air, even though at soil moisture suctions of less than 18 cm Hg this vapor transfer accounted for less than half of the net thermally driven water flux. A linear thermodynamic moisture flux equation was developed in terms of commonly used flow coefficients and variables. This equation predicted fluxes in agreement with experimental observations. A mechanism for unsaturated flow in the liquid phase through soil is proposed. This proposal is based on current theories of the hydrogen bond distributions in water and the observed flow rates under simultaneous pressure and thermal gradients. /Author/
Handbook of Concrete Engineering
Handbook of concrete engineering , Handbook of concrete engineering , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی