Growing peat-free in Australia

John McDonald is National Biosecurity Manager at Greenlife Industry Australia (GIA). He wrote this opinion column for the May 2024 issue of FloraCulture International.

John McDonald, National Biosecurity Manager at Greenlife Industry Australia (GIA).

“Pine bark offers green life growers a sustainable and effective growing medium. Composted pine bark offers a stable, consistent base ingredient for nursery production.

It offers known properties which can be effectively managed to deliver a product fit for purpose.

In Australia, greenlife growers typically use composted pine bark (Radiata pine) for containerised plant production.

They combine the pine bark with ingredients including sand, coir, vermiculite or peat.

Peat-free options comprise either 100 per cent pine park or a combination of 85 per cent pine bark and 15 per cent coir ‘dust’ or 15 per cent sand or other ingredients.

Generally, a well-composted pine bark offers stability with a nitrogen drawdown over seven.

This provides growers with limited shrinkage over a growing season, good root growth and excellent drainage over the cropping season.

Challenges

Pine bark presents some challenges, but these are easily managed:

While pine bark is naturally acidic, the acidity is generally consistent and, therefore, easily stabilised with liming materials.

Pine bark is also hydrophobic and has poor cation exchange capacity (CEC). Growers can mitigate this by adding ingredients such as sand to reduce air-filled porosity (AFP) and increase water holding (however, this can impact root growth).

Further, adding coir has been a game changer as it not only offsets pine bark’s poor CEC but also limits its hydrophobicity effect.

One hundred per cent pine bark growing media can include various particle sizes which means a range of AFP properties. Adding commercial wetting agents can reduce the bark’s hydrophobicity.

Due to bark’s poor CEC properties, growers must manage nutrients, particularly over-irrigating and excessive leaching. Some growers are using a coir chip/dust combination (one hundred per cent coir) instead, which is proving successful as long as irrigation systems are ‘tuned’ to this growing media to avoid over-irrigation and saturation.

Opportunities

As pine bark has very little inherent nutritional value, it is a perfect platform to easily create any desired nutritional program once pH has been stabilised. Growers can provide a strong, stable foundation that minimises pH drift during cropping cycles using lime/dolomite blends (particle sizes). They can also use controlled-release fertilisers with desired N:P:K and longevity. Pine bark also lends itself to any of the other fertiliser delivery processes, such as liquid feeds and/or fertigation systems.”

This article was first published in the May 2024 issue of FloraCulture International.