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Myth Series: Does Growing Media pH Change in the Package?

Often, the assumption is made that the pH of an unused, professionally manufactured growing medium in a compressed bale, loose-fill bag or tote will not change over time. Due to the chemistry and moisture inside the package and environmental pressures outside the package, the pH of unused growing medium can increase slowly overtime.

Most professionally produced growing media are composed of any combination of the following physical components:  Sphagnum peat moss, aged/composted soft or hardwood bark, wood substrates, coir, compost, perlite, vermiculite, pumice and/or rice hulls. The component percentages vary based upon the type of growing media and its intended use.

What is the pH of Growing Media Ingredients?

The pH of these physical components can range from 3.5 (acidic) to 8.0 (basic) depending on the type and its origin.  Sphagnum peat moss, the major component in most growing media, is acidic with a pH from 3.5 to 4.5. Limestone is added to neutralize the acidity of peat and help balance the pH of the media during the growing cycle.

In addition, most professional growing media contain other chemical components, which include but are not limited to: fertilizer (starter charge or controlled release fertilizer), wetting agent and moisture. There are also natural microorganisms found in growing media components, which can also have an effect on the chemical characteristics of the media.


Depending on the growing medium manufacturer, the type of packaging and the moisture content of the physical components, the moisture content within the packaging will vary. Over time, all of these factors can have an impact on growing media pH during storage.

What Happens When a Growing Medium is Stored?

Once a growing medium is produced, packaged and shipped, it may be used in plant production shortly after delivery or it may be stored for a given period of time before use. As packaged growing media sits in storage, the physical and chemical components within the package begin to react and affect the pH of the growing medium. The rate of the reaction and change in pH is dependent upon the environmental conditions such as the moisture inside the package, storage temperature and exposure to sunlight.

Sunlight Effects on Growing Medium

If the growing medium is stored in an area that is exposed to excessive sunlight and/or high temperatures, the chemicals, moisture and natural microorganisms react with the limestone at an increased rate which gradually causes the pH of the packaged growing medium to rise. Conversely, during winter months when growing media is stored outside in cold temperatures, there is very little microbial activity and minimal pH change from manufacturing.


The pH of unused growing medium can slowly increase overtime depending on the moisture, temperature and biological activity. Test the pH of the unused growing medium prior to planting to ensure that it is within normal ranges. Source: Premier Tech

Aging begins the day the growing medium is manufactured

Understanding that the potential is there for the pH of packaged growing media to change is an important part of the handling process.  It is important to know that aging begins the day the growing media is manufactured. That is why most growing media producers print a manufacturing date on the package, so you can track the time that has passed and allow for possible pH changes.

When to use peat-based growing media?

The best use of a typical packaged peat-based growing media is 9 months during the summer months and 10 -12 months through the colder, winter months. For bark based growing media, the best use is within 6 months of manufacturing date. It is always a good idea to test your growing media for pH and EC at time of initial use and throughout the growing cycle.

And last, but not least, it is also recommended that you manage inventory rotation with ‘First-In - First-Out’ Best Use Practices.

PRO-MIX® is a registered trademark of PREMIER HORTICULTURE Ltd.

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