Compost FAQ
Composting Questions and Answers
When is my compost finished? - Finished compost is dark in color and has an earthy smell (like the smell of soil). Usually, it's difficult to recognize any of the original ingredients, although bits of hard-to-decompose materials (such as straw) sometimes can be seen. There is no single point at which compost is finished -- it's a bit more subjective than that. For many outdoor garden applications, for instance, it can be fine to use compost that still has a few recognizable bits of leaves or straw -- it will finish rotting in the soil. If you plan to use compost in seed-starting mixes, though, you're best off having a well-finished compost, because seedling roots may be attacked by decomposer microbes if the roots contact unfinished compost.
How can I use my finished compost? Well-finished compost looks so fine that I'm tempted to eat the stuff sometimes. However, there are several more common ways that compost can be used, on gardens, lawns, landscapes, and houseplants. See our section of Uses of Compost for additional information.
COMPOST AS SOIL AMENDMENT: Many people put compost into their garden soil by digging it in prior to spring planting. The image shows a potato harvest by apprentices at the UCSC Farm and Garden in 1990. Due to the use of copious amounts of compost, the potato beds yielded about one pound of potatoes per square foot, or about 1000 pounds total from these four beds, each 80 feet long.
Others actually - do - their composting in the soil, by burying kitchen wastes and other materials in trenches in the garden. Compost can also be used as a 'top dressing' on the soil during the growing season -- in this case it is added in around the bases of plants, where irrigation and soil animals will slowly incorporate it into the soil. On lawns, many people sprinkle/broadcast sifted compost as a top dressing in the spring -- I have been doing this on a 'problem area' of a lawn for several years, in an attempt to improve the soil there for better grass growth. It is also fine to top-dress houseplants occasionally with small handfuls of finished compost.
COMPOST AS MULCH: Compost can be left on the surface as a mulch around landscape and garden plants. This is essentially the same as a 'top dressing' application, described above, but mulches are typically meant to cover all of the soil around the plants that get mulched. Mulches protect the soil from erosion. The also save water by shielding soil from the drying effect of the wind and sun. As they decompose, mulches add nutrients to the soil, and if composed of small-enough particles, worms may slowly eat the mulch and incorporate it into the soil.
COMPOST AS TEA: Compost tea is made by combining equal parts of compost and water and letting it sit for a while. The liquid can help to provide a 'quick boost' to ailing houseplants or young seedlings and transplants (I recommend diluting it quite a bit for use on seedlings). Stu Campbell, in Let it Rot, says that the same compost - make several batches of tea. When you're finished making compost tea, use the mucky dregs as a mulch in the garden or landscape.
How does compost benefit the soil? - Compost does several things to benefit the soil that synthetic fertilizers cannot do. First, it adds organic matter, which improves the way water interacts with the soil. In sandy soils, compost acts as a sponge to help retain water in the soil that would otherwise drain down below the reach of plant roots (in this way, it protects plants against drought). In clay soils, compost helps to add porosity (tiny holes and passageways) to the soil, making it drain more quickly so that it doesn't stay waterlogged and doesn't dry out into a bricklike substance. Compost also inoculates the soil with vast numbers of beneficial microbes (bacteria, fungi, etc.) and the habitat that the microbes need to live. These microbes are able to extract nutrients from the mineral part of the soil and eventually pass the nutrients on to plants.
Fertilizer Compost 2022
Composting companies help turn food scraps into natural fertilizer Milwaukee Journal Sentinel
Preliminary design for establishing compost maturity by using the spectral characteristics of five organic fertilizers | Scientific Reports
Chicago offered its residents free fertilizer made from biosolids — but didn't say it contains PFAS Bangor Daily News
Compost part of horticulture teaching at Fox Valley Tech Spectrum News 1