Top 9 growing media questions

Heavy percentages of fresh or “green” bark should not be used in growing mixes because micro-organisms compete with the plants for nitrogen. This photo demonstrates the impact fresh bark with limited fertility has on a four-week-old perennial salvia plug.Starting with and maintaining a good growing medium is critical for producing a high quality crop. New and inexperienced growers often have numerous questions about what growing media they should be using and how to get maximum performance. Even experienced growers can encounter problems with media, whether it’s a substrate they’ve used for years or new product.

1. Does a growing mix have a shelf life?
Although there are a lot of “what ifs,” growing mixes are best used within six months of manufacture.

Since certain chemical and physical properties can change during storage, use a mix that’s fresh as possible. The degree of change depends on both storage time and conditions.

Whether bagged, baled or bulk, in storage a mix can dry out and become difficult to wet. A mix stored outdoors in humid, rainy conditions can absorb moisture and become wet and moldy. During handling and potting, wet mix is easily compacted, resulting in decreased aeration porosity.

Many mixes contain a starter fertilizer. As mixes age during storage, naturally occurring micro-organisms can consume some of this fertilizer, reducing the electrical conductivity (EC) and nitrogen content.

If a grower were to use an older mix side by side with a fresh mix, some initial growth differences might be seen, especially if the first fertilizer application is delayed.

Sunlight can degrade and eventually disintegrate plastic bags or bales, allowing for contamination by weed seeds or disease organisms.

2. The water pH is very high. Will this affect the growing mix or my crops?
Water pH alone has little impact on the mix. Instead, the water alkalinity level affects the growing mix pH. Irrigation water high in alkalinity can induce rising growing mix pH.

High water pH is a bigger factor when preparing pesticide solutions than its effect on growing mixes.

Low pH water can also cause problems, as it can also lower the pH of the growing mix.

3. Is there an advantage to using some coir in the mix?
Coir, derived from coconut husks, is somewhat peatlike in physical nature, but there are differences. Because coir is denser than peat during watering in, mixes with a high percentage of coir are less prone to settling down from the top of the pot than peat mixes.

Coir is naturally higher in pH than peat. Compared to peat, coir contains higher potassium and sodium, resulting in higher soluble salt levels. There is also some evidence that mixes containing coir are less subject to fungus gnat infestations.

4. How long can I store a mix that contains a controlled-release fertilizer?
Use it as soon as possible. The release rate is dependent on the temperature of the moist growing mix. The higher the temperature, the quicker the release rate.

Because growing mixes contain some moisture, the release process for a controlled-release fertilizer begins soon after the mix is made, resulting in increasing soluble salt levels.

During storage, release rate is affected by temperature, time, a growing mix’s degree of moisture and the fertilizer’s longevity factor. While any mix containing a controlled-release fertilizer can be safely stored for a week or two before use, soluble salt levels should be checked after longer storage periods.

If necessary, the salt level can be reduced by leaching after potting. Packaged mixes should be stored cool and dry.

5. Does bark tie up nitrogen?
All organic mix components undergo natural decomposition. Bacteria and fungi are responsible for this process.
The micro-organisms consume nitrogen along with the organic material, and the nitrogen becomes “tied-up” within the cellular structure. In the growing mix, these micro-organisms compete with the plant for nitrogen.

Mix made with readily decomposable organic material can support a large population of micro-organisms. Fresh bark taken directly from trees is subject to rapid decomposition. To compensate for this, growing media manufacturers age or compost the bark. The resulting composted bark is resistant to further rapid decomposition.

Incorporation of aged or composted bark in a mix supports a much less active population of micro-organisms than fresh bark. Properly aged or composted bark ties up only slightly more nitrogen than peat moss.

6. Are bark mixes disease suppressive?
Aged or composted bark contains many species of naturally occurring micro-organisms. Some of these micro-organisms are relatives of the bacteria and fungi used in commercially available biocontrol products.

When used in a growing mix, properly processed bark suppresses some disease pathogens for a short time.

Mix components that are processed properly, including bark, becomes populated with micro-organisms that suppress (but do not prevent) certain fungal diseases.

Disease problems can occur in suppressive mixes under conditions of poor sanitation or heavy disease pressure. Suppressive mixes should be used as a disease-control tool along with fungicide and sanitation programs.

7. Why is the growing mix pH so low when tested fresh from the bag?
Pine bark and peat are naturally low in pH – ranging from 3.5-4.5. Add limestone to increase the pH.

Once containers are filled with the mix and it’s moistened, limestone reacts, but not instantly.

It takes a period of three to five days for the reaction and the subsequent pH increase to occur.

8. There is green scum growing on the surface of the mix. When the mix dries, it turns brown and then water doesn’t penetrate the surface. What causes this?
That’s algae growing on the mix surface, and it’s not a good thing. When dry, the scum can become an impermeable barrier that makes watering difficult.

Excessive algae growth occurs when mix conditions are too wet. Over watering, poor air circulation and/or compacted soil result in conditions that inhibit drying and encourage algae growth.

Proper water and environmental management are needed to minimize algae problems.

9. Isn’t there a shortage of peat land in Canada? Isn’t peat harvesting depleting the wetlands?
The North American peat industry takes environmental concerns seriously. For example, when a peat bog is at the end of its productive life, it is mandatory that the area be restored to a functioning wetland. These restored areas can become ecologically balanced systems within five to 20 years.

There is a misconception that peat is in short supply, and that the harvest rate is not sustainable. Peat is a renewable resource that is accumulating 70 times faster than it is being harvested.

For more information, go to the Canadian Sphagnum Peat Moss Association website (www.peatmoss.com).

Bob Steinkamp and Michael Tilley are technical services managers, Jamie Gibson is director of research and development, and Hugh Poole is director of technical services, Conrad Fafard Inc., Fafard Technical Services, www.fafard.com.