华北、东北和西北广大地区,农村90%以上居民都睡火炕。火炕结构几乎全是土坯和泥巴涂抹而成,表里粗糙,极易形成烟垢,天长日久,炕洞里便挂满了黑褐色胶状物。这种物质,人们称之为炕洞焦油,是多种有机物的混合体——作物的优质肥料。农民用这种肥料栽培作物已有悠久历史,所谓“新土 North China, northeast and northwestern vast areas, more than 90% of rural residents sleeping sleeping kang. The structure of the kang is almost entirely adobe and mud smear, rough in appearance, easy to form soot, over time, kong hole covered with dark brown jelly. This material, what people call kang-dong tar, is a mixture of many organic compounds - a high-quality crop fertilizer. Farmers have long history of cultivating crops with this fertilizer, the so-called ”new soil."