Tobacco plants were grown on the following three kinds of ridges. Soil temperature, soil moisture and growth of tobacco of these ridges were studied. The first was the high ridge thoroughly covered with polyethylene film (thoroughly mulched ridge), the second was the conventionally mulched ridge of normal height (mulched ridge), and the third was bare ridge of normal height (non-mulched ridge) .
Maximum and average soil temperatures were the highest at the thoroughly mulched ridge, and the lowest at non-mulched one.
At thoroughly mulched ridge water vapor evaporated at the soil surface was forced to go downward into the soil and the latent heat in the water vapor was changed into the sensible heat in the soil and raised soil temperature. At mulched ridge the downward movement of water vapor was not so large as at thoroughly mulched ridge, and the temperature raising effect of mulched ridge was relatively small. At non-mulched ridge water vapor ran into air and gave small influence on soil temperature.
The influence of soil moisture increase on the thermal diffusivity was conspicuous at thoroughly mulched ridge.
Average soil moisture during the tobacco cultivating period was the lowest at thoroughly mulched ridge and the change of soil moisture was the least at thoroughly mulched ridge.
The quality of cured tobacco leaves was the best at thoroughly mulched ridge, and the worst at nonmulched ridge.