堆肥是利用含有肥料成分的动植物遗体和排泄物，加上泥土和矿物质混合堆积，在高温、多湿的条件下，经过发酵腐熟、微生物分解而制成的有机肥料。

堆肥制作历史攸久，制造堆肥必须先收集适当材料，例如稻草、茎蔓、野草、树木落叶或是禽畜粪便等，然后将其适当混合，并添加适量的氰氨化钙，促其发酵，然后覆盖上破席、破布、稻草或塑胶布，以避免肥份丧失。但因为氰氨化钙有毒性，因此目前已很少使用。

然后每隔大约三、四个星期翻积一次，大约经过三个月左右，堆肥就能使用了。

堆肥最好放置在堆肥舍。若无堆肥舍也可使用露天堆肥，但必须选择适当地点，以免因日晒、雨淋及风吹，导致矿物质溶失。

Compost (/ˈkɒmpɒst/ or /ˈkɒmpoʊst/) is organic matter that has been decomposed and recycled as a fertilizer and soil amendment. Compost is a key ingredient in organic farming.

At the simplest level, the process of composting simply requires making a heap of wetted organic matter known as green waste (leaves, food waste) and waiting for the materials to break down into humus after a period of weeks or months. Modern, methodical composting is a multi-step, closely monitored process with measured inputs of water, air, and carbon- and nitrogen-rich materials. The decomposition process is aided by shredding the plant matter, adding water and ensuring proper aeration by regularly turning the mixture. Worms and fungi further break up the material. Bacteria requiring oxygen to function (aerobic bacteria) and fungi manage the chemical process by converting the inputs into heat, carbon dioxide and ammonium. The ammonium (NH₄) is the form of nitrogen used by plants. When available ammonium is not used by plants it is further converted by bacteria into nitrates (NO₃) through the process of nitrification.