10 Must-know Growing Media Facts

  • Thread starter MIMedGrower
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I was looking for expiration info on potting soil. Many common problems I have had and see online are caused simply from using out of date or improperly stored potting soil.

From PT Horticulture (Pro-Mix)


1. Use growing media shortly after blending or delivery.

Growing media does not go bad, but changes do occur with aged product. Changes include draw-down of certain nutrients (if the blend has a starter nutrient charge), activation of limestone and decreased effectiveness of the wetting agent. For example, wetting agents can be consumed by native microorganisms found in the growing media components and can chemically degrade.

Microbes in the growing media can potentially consume the fertilizer charge, especially nitrogen and iron. This may explain why crops planted in aged growing media get off to a slower start. These processes occur more rapidly with hot storage temperatures than with cold.

Product aging begins the day the growing media is manufactured. Most manufacturers print a manufacturing date on the package. If you use packaged growing media, check with the manufacturer for this date. If you mix your own, keep records for the day you mixed it. The best use of a typical packaged peat-based growing media is eight to nine months during the summer months and 10 to 12 months through colder, winter months. Growing media components that have been composted contain a high population of microorganisms that can also consume the wetting agent. Therefore, packaged growing media containing bark and compost have a shorter best-use date of four to six months.

Some additives incorporated into growing media are more critical for the best-use date. Growing media that contain controlled-release fertilizers should be used within one month of the manufacturing date due to the release of fertilizer within the package. Some fungal-based biofungicides have a shelf life of a few months when incorporated into the growing media; however, most bacteria-based biofungicides and mycorrhizae can last more than a year.

The take-home message for aged product is that you may need to modify your cultural practices when planting in an older growing media. The best way of knowing how to use aged product is to send unused samples to a laboratory for testing and discuss recommendations with the media manufacturer.

2. When switching to a new growing media, expect to water differently.

The most difficult change to overcome is the water requirements. Select a growing media that has similar watering characteristics to what is currently being used. Having the same appearance when it dries out is also ideal, as many growers use the color of the growing media to determine when to water. The adjustment is often difficult for the employees who are most familiar with the current growing media.

Other considerations when switching include:

  • Will there be changes required in my fertility program?
  • Will the growing media influence the application rate of plant growth regulator drenches?
  • How will the limestone react and change the pH of the growing media?
  • How will the buffering capacity of the growing media be affected by the limestone?
  • Keep in mind that switching growing media may not always be a solution to a growing problem.
3. Most growing media contains a starter fertilizer charge that is designed to help with initial plant growth.

Once in use, it generally lasts about one to two weeks, depending on the rate the manufacturer incorporates, how much water is applied to plants watered (volume divided by frequency) and plant uptake. If you are unsure of the fertilizer content, check with the growing media manufacturer for fertility levels. Some growers prefer to water in with a water-soluble fertilizer immediately after planting with about 100 ppm of nitrogen, while others prefer to wait a few weeks. This is a personal preference. However, if you water in immediately, you should adjust the fertilizer level of the growing media. This will account for any imbalances or loss of nutrients in the growing media as a result of product aging.

4. Peat moss, the major component in most potting soils, is acidic with a pH from 3.5 to 4.5.

Limestone is added to neutralize the acidity of peat to raise the pH from 5.5 to 6.2. Limestone is a rock and needs time to break down from the moisture in the growing media. The drier the growing media in storage, the longer this will take. It is not uncommon for a dry, unused growing media less than three months old to have a pH close to 5.0. To get the true starting pH of fresh, unused growing media, take a sample of unused mix, saturate it with deionized water to make a paste and let it sit inside a plastic bag. Take pH readings at day one, day three and day seven to verify the pH rise to the normal range.


One word about pH meters: Not all pH meters work well with porous, soilless growing media. Testing the pH of a growing media by sticking the probe directly in the root ball generally does not produce accurate readings. This is especially true of inexpensive metal probe pH readers. Often they read a pH near 7.0 because there is not enough moisture contact to get an accurate reading. It is best to test the pH in the mud of a saturated media extract or in leachate collected by the pour through method.

Avoid pH meters that do not require or cannot be calibrated. Inaccurate pH readings are problematic because any corrective measures that are implemented as a result of the inaccurate pH may cause nutrition problems and crop damage. The best meters are those with glass probes that are stored wet and have at least a two-point calibration.

5. Occasionally, when opening the growing media packaging, mold can be seen growing on the medium surface.

No need to panic. These molds are saprophytic, meaning they live off of dead plant material such as bark, compost, peat moss or coir. They occur more frequently on composted components such as bark. They do not harm plants. These molds often require high humidity and stagnant air to survive. Typical greenhouse environments and nurseries are generally too dry and have high airflow, so these molds rarely come back.

6. In rare cases, offensive odors can come from the growing media.

This is especially true for bulk products that have been sitting for a while and products sealed in plastic packaging. It generally does not occur in all packages within a shipment. The odors are from by-products of anaerobic microorganisms naturally found in the organic component of the growing media, such as bark or peat moss.

When the package is opened and oxygen is introduced, odors tend to dissipate within a few days. The presence of oxygen shuts down anaerobic microorganisms, so they no longer produce odor-causing by-products and the odors do not come back. Any by-products causing the odor evaporate easily and leach when the growing media is watered.

7. Using the wrong media can cause uneven growth.

Crops sometimes grow unevenly in areas where taller plants are next to short plants. When this can be seen throughout the entire crop, it creates a wavy appearance. The cause is often a lack of fertilizer application. Nutritional stress affects each plant differently within a crop. Some plants are a little stronger and continue to grow, even with low nutrient levels, while others are weaker and stop growing. The easy solution is to apply the appropriate fertilizer at the recommended rate. Verify that the correct amount of fertilizer is added to the stock tank and that the injector is working properly.

Uneven growth could also be caused by variation in the size of growing media components from one container to the next. Either the unused growing media was manufactured this way or it was caused by excessive fluffing with a bale breaker or soil mixer.

8. Change your mix if your growing media does not dry out.

The growing media slowly dries out early in the crop cycle when the weather is cool and the plants are small. If this is a recurring problem, consider a high-porosity or bark-based growing media that dries out more rapidly.

Increased airflow through the plant canopy can also help. Achieve this result by installing horizontal airflow fans. This will move stagnant, humid air out of the plant canopy and replace it with drier air found above the canopy. Avoid growing plants on the ground and place them on benches where the air is warmer and airflow is better. If possible, reduce the number of hanging baskets above the crop. These baskets shade the crop and produce more humidity.

If plants are watered with clear water and followed up with another clear watering, crop nutrition can be a contributing factor. If the growing media is fresh, the starter fertilizer charge will last about one to two weeks. If it is aged, the starter fertilizer charge is a reduced rate. As a result, the plants do not have access to the nutrients required to maintain active growth. The plant just sits there, does not use the water and is susceptible to overwatering and attack by root-rot pathogens. To avoid this, water plants with a weak solution of water-soluble fertilizer.

9. Algae may form on the top of the growing media.

Algae can come from a number of sources, but the most common source is the irrigation water. Surface water sources, such as ponds, lakes or rivers, have the highest concentration of algae, but even sources from wells and municipalities contain algae.

Algae needs light, nitrogen and water to grow. You cannot take away light and nitrogen, as this compromises crop quality, but you can limit the time that the growing media surface stays wet. Algae growth can be discouraged by allowing the growing media surface to dry out between waterings. Other controls include selecting a growing media that dries out more rapidly or completing surface drenches with peroxide-based pesticides.

10. Every greenhouse operation has a certain population of weeds that end up in a crop.

But sometimes there are a lot of them. Growers often blame the growing media, but sphagnum peat moss has few, if any, possible weed seeds.

Consider the environment in which it grows. Peat bogs are aquatic, very acidic and have low plant-nutrient content. Weeds do not grow in these conditions. Generally weeds originate inside or around the greenhouse. Weeds produce seed in the summer and fall when houses are open or weeds are allowed to grow under benches. The weed seeds can settle on potting benches, fan jet tubes, vents, exhaust fans, benches, etc. If supplies are stored outside, weed seeds can adhere or blow into containers and growing media packaging.

To keep weeds from getting into future crops, make sure to remove all weeds growing around the greenhouse property. This keeps them from going to seed. Remove any weeds that sprout inside the greenhouse, whether they are under the bench or in the crop. Keep the greenhouses clean, and weed issues in the crop will be reduced or eliminated.

Once in use, be sure to periodically test growing media, plant tissue, water and nutrient solution. You can test pH and EC of growing media, water and nutrient solution with your own testing equipment or send it to an independent laboratory for a more comprehensive analysis. If you have questions about growing media, consult the manufacturer to confirm you are well informed about the product’s specifications, intended use and expected results. Manufacturers can assist and provide technical support to ensure the best growing experience.

PRO-MIX® is a registered trademark of PREMIER TECH Ltd


I learned a lot from their education center. It is not about our plant but general container crop production but the info in straightforward and carries over perfectly. Even the nutritional and deficiency stuff is super helpful. Plenty of articles and videos.

One great tip I got when learning about water alkalinity was to water in new plants and seedlings with un-adjusted tap water for the first week. This keeps the ph up in the beginning while the lime in the mix is activated and starts buffering the medium.

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