Compost tea is a liquid extract teeming with beneficial microorganisms, created by soaking finished compost in water, often with added food sources. It acts as a microbial innoculant to improve soil health and provides a gentle, liquid feed for plants. The goal is to transfer the diverse community of bacteria, fungi, protozoa, and nematodes from the solid compost into a water solution for easy application to soil and plant foliage. Understanding how long this living brew remains effective is central to its successful use.
The Critical Usage Window
Quality compost tea, specifically the actively aerated kind, has a very short shelf life once brewing is complete. The beneficial microbes cultivated during aeration are aerobic and require dissolved oxygen to remain active. As soon as the air pump is turned off, the microbial population rapidly consumes the remaining oxygen. This swift drop in dissolved oxygen determines the tea’s viability.
Most experts recommend applying the tea within four hours of stopping the aeration to ensure the highest concentration of active organisms. At most, a well-brewed batch can be stored and used within a maximum window of 24 hours if kept cool and covered. Beyond this brief period, the microbial community shifts dramatically, and the intended benefits are significantly reduced.
Primary Factors Determining Viability
The single most important factor controlling how long compost tea remains effective is the continuous supply of oxygen. Actively Aerated Compost Tea (ACT) is defined by its high levels of dissolved oxygen, typically maintained above 6 milligrams per liter throughout the brewing process. When the constant bubbling stops, the aerobic microbes quickly deplete the oxygen, initiating the countdown to spoilage.
The distinction between ACT and simple compost extracts is important for understanding shelf life. Non-aerated extracts, often called “compost water,” involve soaking compost without mechanical aeration and are primarily used for their soluble nutrients, not for a massive boost of active microbes. Because the microbial population in these extracts is less active and less concentrated, they do not face the same rapid oxygen depletion, and their viability is not as strictly time-limited.
Temperature also plays a significant role in determining how quickly the tea must be used. Warmer temperatures accelerate the metabolic rate of the microorganisms, causing them to consume dissolved oxygen faster. If a batch is stored in warm conditions, like above 70°F, the critical usage window may shrink to just a couple of hours. Cooler temperatures slow down this microbial activity, extending the survival time of the aerobic organisms.
The initial quality of the brew, including the concentration of microbes and the type of food sources added, also impacts its stability. If the tea was poorly brewed with low oxygen levels or an imbalanced food source, it will fail faster. High-quality, mature compost is necessary to ensure the initial extraction contains a diverse and healthy population of organisms.
Identifying Failed or Anaerobic Tea
The most straightforward way to determine if compost tea has gone past its prime is by using the sense of smell. Fresh, healthy aerated compost tea should have a mild, earthy, or slightly sweet aroma, similar to fresh soil. This pleasant smell indicates a thriving, aerobic microbial community.
A foul, rotten, or putrid odor is the clearest sign that the tea has gone anaerobic. When oxygen levels drop too low, beneficial aerobic microbes die off and are replaced by anaerobic bacteria. These anaerobic organisms produce byproducts like hydrogen sulfide, which give off the characteristic rotten-egg or sewage-like smell.
Visual cues can also indicate a microbial shift. The presence of a thick, slimy film on the surface or excessive foam suggests a change in the microbial balance. Using tea that has turned anaerobic is strongly discouraged because it may contain phytotoxic compounds that can harm plants. Furthermore, low-oxygen conditions favor the proliferation of certain human pathogens.