Why can vegetable pits in the north keep vegetables fresh?

The vegetable cellar is warm in winter and cool in summer, which fully meets the refrigeration conditions. In the courtyard of the northern countryside, many families will build a vegetable cellar at home. In the past autumn and winter, after the garden was closed, there were no supermarkets and other ways to buy food. In order to prevent vegetables from freezing, the northern rural areas usually dig an underground basement with a depth of 2 to 3 meters to store vegetables, and put several cans of water in the vegetable cellar to keep the temperature at 0-5℃. Potatoes, radishes, cabbages and other vegetables are suitable for storage in vegetable cellars, and can generally be preserved until the next spring.

The vegetable cellar is warm in winter and cool in summer, which fully meets the refrigeration conditions. Vegetables will not continue to grow and rot, so they should be replenished in time and kept fresh. What is the principle behind this? In winter, the land heat transfer is also very slow, which leads to the underground temperature being generally higher than the ground. The surface temperature of the earth is very low, but the underground temperature is about 1 m, which is the second lowest, and the temperature is getting higher and higher. After reaching a certain depth, the underground temperature will remain at a high level, so northerners can store vegetables in the cellar. In winter, buckets of water are usually put in the vegetable cellar, so that vegetables are not easy to rot. This includes changes in the state of matter. In winter, the temperature in the vegetable cellar is very low, and the water is easy to freeze. Liquid water freezes when it is cold. At the same time, it releases heat to prevent vegetables from freezing because the temperature of the vegetable cellar is too low.

Besides the higher temperature, the humidity of the vegetable cellar is higher than the ground, which can reduce the water loss of vegetables. In addition, vegetables will breathe, absorb oxygen and release carbon dioxide in the vegetable cellar. When the concentration of carbon dioxide gas is high, the breathing of vegetables will be inhibited, which is beneficial to the preservation of vegetables. In addition, Chinese cabbage and radish will also produce methane and other toxic gases during long-term storage. At the same time, the cellar is tightly sealed and lack of oxygen, so it is easy to inhale carbon dioxide and methane when the cellar is opened rashly. Once poisoned, it will usually die in a short time.

Before entering the vegetable cellar, people usually light candles and put them in. If they can keep burning, they can enter. If the fire goes out, it needs ventilation to prevent the "vegetable cellar effect". After all, the air in the vegetable cellar is isolated from the outside world, with poor ventilation and little oxygen. Coupled with the respiration of vegetables in the cellar, its carbon dioxide content is very high. Once people enter, they are in danger of suffocation due to lack of oxygen. For the safety of life, it is necessary to detect and exhaust before entering the cellar.