Finished compost is the stable, dark, earthy-smelling organic matter resulting from the decomposition of raw materials. Gardeners often store this resource until planting time or when soil amending is required. Proper storage preserves the quality and biological activity achieved during the curing phase. This article explores the factors that determine how long finished compost can be stored before its benefits diminish.
Shelf Life Variables
The longevity of finished compost depends heavily on whether it is stored in open bulk piles or sealed in containers. Compost stored in large, uncovered outdoor piles often maintains peak quality for only about four to six months. Exposure to the elements, particularly direct sunlight and heavy rain, accelerates the loss of beneficial properties by encouraging continuous microbial activity.
Rainwater moving through an open pile causes soluble nutrients, such as nitrates and potassium, to leach out. Nitrogen loss is particularly noticeable because active bacteria and fungi convert nitrogen compounds into gaseous forms or consume them. This process results in a measurable decline in the compost’s immediate fertilizer value over time.
Compost that has been properly cured and then sealed in bags or covered bins can maintain quality for a much longer period, typically 12 to 18 months. This extended shelf life results from minimizing environmental stresses, such as moisture fluctuation and temperature extremes. When sealed, the microbial ecosystem stabilizes, slowing the metabolic rate of the organisms.
Successful bagged storage relies on the compost having optimal moisture content, similar to a wrung-out sponge, before sealing. This moisture level keeps microbial populations in a state of stable activity. If the material is too dry, microorganisms cease activity, and the compost becomes dusty and inert. Conversely, if the moisture is too high when sealed, the environment quickly turns anaerobic, leading to undesirable chemical changes and the production of phytotoxic compounds.
Optimizing Storage Conditions
Maximizing shelf life starts with protecting compost from harsh outdoor fluctuations. Storing the material under a roof, such as in a shed, garage, or under a tarp, prevents nutrient leaching caused by heavy rain. Protection from direct sunlight is also important because UV radiation can degrade the complex organic polymers that give compost its structure. Covering the compost also helps deter pests and insects that might introduce pathogens or disrupt stability.
Maintaining the correct moisture level is the most important factor in preserving beneficial microbial life. Compost should be stored at a moisture content of around 40 to 60 percent; it should feel damp but no water can be squeezed out when compressed. Storing the material too dry causes microorganisms to enter a dormant state, while saturation encourages the growth of undesirable anaerobic bacteria.
Extreme temperatures are detrimental to the microbial community responsible for compost quality. Storage locations should be cool and shaded, avoiding excessive heat buildup that can denature enzymes and kill heat-sensitive organisms. Storing compost in larger volumes helps, as the thermal mass buffers the internal temperature against rapid external changes. Conversely, prolonged deep freezing halts biological activity, though the compost often recovers once the temperature rises above freezing.
The choice of storage container influences the internal environment and degradation rate. Breathable containers, like woven polypropylene bags or wooden bins, are preferred because they allow for passive gas exchange. If plastic bins or bags are used, they should not be airtight, especially if the moisture content is slightly higher than ideal, to prevent noxious gases and anaerobic conditions.
When storing bulk quantities, prevent excessive compaction of the material. Overly dense compost restricts airflow and can lead to localized anaerobic pockets within the pile. Turning or fluffing the stored compost every few months, if possible, helps maintain aeration and prevents particle settling.
Recognizing Degraded Compost
Compost that has exceeded its useful shelf life or was stored poorly often exhibits recognizable physical changes. Properly stored material remains crumbly, light, and maintains a consistent texture. Degraded compost, especially if stored uncovered in heat, may become overly dusty and dry, indicating moisture loss and microbial dormancy. Conversely, if stored too wet and compacted, it may become dense and difficult to break apart, signaling structural collapse. This change in physical texture directly impacts its ease of spreading and ability to aerate the soil.
A significant indicator of improper storage is an unpleasant odor, which should not be present in finished, stable compost. A sour, rotten egg, or strong ammonia smell suggests anaerobic decomposition has occurred, likely due to excessive moisture and lack of oxygen. This odor indicates a loss of beneficial microbial diversity and the production of potentially phytotoxic organic acids.
Even when degraded, old compost is rarely considered “bad” in the sense of being harmful, but rather less potent. The primary loss is rapid-release nutrients, especially soluble nitrogen, meaning its value as a fertilizer has diminished. However, the material retains its high organic matter content and still functions effectively as a long-term soil conditioner, contributing stable humus to improve soil structure and water retention.
Compost showing signs of degradation can often be revitalized before use. This process involves spreading the material out, re-moistening it to the optimal wrung-out sponge consistency, and mixing it with a small amount of fresh, nitrogen-rich organic matter. Allowing it to sit for a few weeks ensures aerobic microbial populations become active again, restoring some lost biological function.