Compost is the decayed remnants of plant materials and animal manure with the essential ingredient of humus. Compost is used in gardening and agriculture, mixed in with the soil to improve its structure (by increasing its organic content) and fertilizing quality. The best compost is made from roughly one third by volume of materials from the following three groups:
- Wilted green material
- Dry, straw-type material
- Animal manures
The materials are placed in alternating layers, approximately 15 cm (6 inches) thick. As the heap is built, it is wetted to the consistency of a well wrung-out sponge. The heap should be about 1 m (3 ft) wide, 1 m (3 ft) tall, and as long as is practicable – the advantage to making the heap at least 1 cu m (1 cu yd) is that it provides suitable insulating mass to allow a good heat build-up as the material decays. The heat provides a suitable environment for thermophilic (heat-loving) bacteria, which accelerate the decomposition process. The centre of the heap should get quite warm, possibly hot enough to burn a bare hand. If this fails to happen, it's nearly always because the heap is too wet, thus excluding the oxygen required by the compost bacteria, or because there is insufficient protein (animal manure). The solution is to add material, if necessary, and/or to turn the pile to aerate it.
Depending on how quickly the compost is required, the heap can be turned one or more times to bring the outer layers to the inside of the heap and vice versa, as well as to aerate the mixture. Once all the material has become barely recognisable from the original ingredients, it's ready to use. Some practitioners like to leave the compost to mature further for up to a year as this seems to make the benefits of compost last longer.
Some like to put special materials and activators into their compost. A light dusting of agricultural lime (not on the animal manure layers) can curb excessive acidity that can slow down the fermentation. Seaweed meal can provide a ready source of trace elements. Finely pulverised rock dust can also provide needed minerals, but watch out for rock dust that consists mostly of clay.
The animal manure part of compost source materials can be collected by compost toilets (in this case, human feces). However, such compost is usually not used as a fertilizer for plants that are directly edible (eg, salad crops) but should instead be used on trees, bush fruits or else on the ornametal garden.