Opened bag of growing media

What smell can you expect When you open a bag of growing medium? The aroma of a newly opened package of growing media is generally described as an earthy smell. On rare occasions during the late summer or early fall, it can be described as an ammonia or ‘litter box’ odor that can be strong and offensive. The questions then become: What is causing this odor? How do I get rid of it? Will it impact my plants?

The source of the odors are ketones, alcohols and other by-products of anaerobic microorganisms in the growing media. These microorganisms thrive in moist, warm locations with little oxygen, making the inside of growing media bags and bales an ideal growing environment. In some situations, the odor is accompanied by a very thin layer of white or yellowish mold on the moist growing media surface that was in direct contact with the plastic packaging.

Since the microorganisms causing the odor are anaerobic, they will quickly die once the package has been opened and the growing media exposed to the oxygen-rich air. Generally, the odor will dissipate within a few days or even a few hours. However, if the product is immediately used for planting, these odor-causing by-products can be leached from the growing medium when watering, so the odor dissipates faster. If the thin layer of mold is present, it will also disappear when exposed to drier air. Once these microorganisms die, the odor and mold will not return.

There are no documented instances of any negative impact on plant growth caused by these anaerobic microorganisms or their by-products. It is possible with increased microbial activity in the growing medium, that these microorganisms consume some of the starter fertilizer, but this is easily replenished with one application of a balanced water-soluble fertilizer. As a result, there is no known reason to be concerned about using growing media with this ammonia odor.

These odors are more likely to occur in the late summer and early fall after the growing media has been stored outside in the warm weather for several months, which allows time for the population of anaerobic microorganisms to build up and produce the odor. Usually, this will only occur in a few bags or bales on the pallet. It also tends to be more common in growing media that contain composted or aged bark.

While the odor may be disconcerting to some, growers may rest assured that the anaerobic microorganisms in question will have no ill effects on their crops.