Complete Guide to Peat Alternatives: How New Substrates Behave

The search for alternatives to peat in nursery and ornamental plant production has increased significantly in recent years. This trend is driven not only by the need for new cultivation solutions, but also by growing environmental responsibility. When correctly selected and managed, peat alternatives can deliver several agronomic and operational benefits—an approach also supported by professional bodies such as RHP and EPAGMA.

The Advantages of Peat-free and Peat-reduced Substrates

When properly used, alternative substrates can offer multiple benefits:

• strong root development, supported by an optimal air–substrate ratio
• effective water drainage
• improved rewetting capacity
• lower bulk density (with the exception of compost)

That said, it is essential to fully understand the characteristics of each peat substitute, as none are completely free from constraints or management considerations, when it comes to plant nutrition. Let’s start with an overview of their characteristics and then let’s dig into the possible challenges.

The Characteristics of Main Peat Substitutes Used in Ornamental Plant Production

The most common peat substitutes currently used in professional ornamental horticulture include bark, wood fiber, green compost, and coconut fiber.
The table below summarizes the main characteristics relevant for ornamental plant production.

Specific Characteristics of Peat alternatives Related to Plant Nutrition

Why Reduce Peat Content in Growing Media?

Peatland protection is increasingly included in environmental sustainability strategies adopted by growers, garden centers, landscapers, and large-scale retailers. In many countries, the elimination of peat from hobby substrates has become a strategic objective.

Peatlands play a crucial environmental role: they act as carbon sinks, helping to absorb CO₂ from the atmosphere, and serve as important freshwater reservoirs. However, peat regeneration is a very slow and fragile process that does not align with the speed and extraction volumes required by the market.

In professional ornamental horticulture, peat can be partially or fully replaced with alternative materials, each offering specific physical and chemical properties.

How Plant Nutrition Changes When Substrate Composition Changes

Using alternative substrates requires recalibrating fertilization strategies, with particular attention to nitrogen management. Nitrogen availability may vary considerably due to microbial nitrogen immobilization processes that occur during substrate maturation. To dig deeper into this subject, we have created an ad-hoc article that explores the C:N Ratio in Peat-free and Peat-reduced Substrates

Factors Influencing Growing Media’s pH

Key factors affecting substrate pH include:

• Substrate composition: pH is influenced by the substrate’s ability to retain hydrogen ions (H⁺), which depends on the quantity and quality of total organic matter
• Calcium addition (liming): calcium (Ca²⁺) is generally added to peat-based mixes; this is often more challenging when using alternative substrates
• Fertilization strategy: cation exchange processes (including peat complexes); ammonium-based nitrogen tends to acidify the substrate, while nitrate-based nitrogen has an alkalizing effect
• Irrigation: water quality, volume applied, and especially bicarbonate (HCO₃⁻) content play a decisive role

Buffering Capacity of Peat and Peat Substitutes

Compaction and Shrinkage in Substrates Containing Peat Substitutes

Over time, substrates can change volume inside containers and this needs careful management. A 100% peat substrate tends to be heavier, making it essential to ensure adequate air-filled porosity (AFP) and to avoid excessive drying.

When adding low-bulk-density alternative materials (high AFP), growers should:

• avoid excessive pressure during pot filling
• use well-composted and fully matured materials

Attention to the dilution effect: excessive shrinkage can result in reduced substrate volume in the pot, leading to lower fertilizer availability than initially planned.

Irrigation Management When Using Peat-Reduced Substrates

Key points to consider:

• different substrate compositions require different irrigation strategies
• crops grown in different substrates should ideally be managed in separate irrigation zones
• lower water-holding capacity leads to faster substrate drying
• thorough wetting of the root zone is essential for optimal root and plant development
• the use of H2Gro wetting agents is recommended

Peat-based growing media tend to dry out and, once completely dry, can become almost water-repellent. Without a wetting agent, rehydration can be difficult and uneven. There are specific trials run at Weihenstephan-Triesdorf University in Germany which show the effect of H2Go and Peat Substitues.

Challenges When Changing Substrate Type

Switching to alternative substrates may introduce several practical challenges:

• changing the substrate means changing the cultivation approach
• differences in water-holding capacity
• variations in container temperature compared with peat-only media
• nitrogen management to compensate for microbial immobilization
• reduced peat content decreases lime buffering (where required)
• reduced peat content lowers overall pH buffering capacity
• fertilizer rates must be carefully recalibrated, especially when using wood fiber

ICL Fertilizers Suitable for Low-peat Substrates

Two key nutritional needs must be addressed when reducing or replacing peat:

1. compensating nitrogen availability
2. supplying calcium and magnesium

Below are ICL Growing Solutions designed to support consistent plant nutrition, regardless of substrate composition.

1. Compensating Nitrogen in Substrates Containing Wood Fiber and Organic Materials

Cal-BOOST 20-5-5+7CaO+2MgO, a controlled release fertilizer, offers the following benefits:

• Coated N for optimal nutrient use efficiency
• Greener leaf color thanks to N, Ca, Mg and S
• Higher Calcium level in substrate, contributing to plant quality

 Osmoform High N 38-0-0, a slow release fertilizer, offers the following benefits:

• provides additional nitrogen during the first 10 weeks of growth
• slow-release nitrogen
• suitable for all cultivation systems

* Osmoform slow release fertilizers contain methylene urea chains of varying length, which gradually decompose. The release rate is influenced by temperature, microbial activity and moisture.

2. Supplying Calcium and Magnesium (+ Nitrogen) with Controlled Release Fertilizers

Lower peat content results in reduced buffering capacity for calcium and magnesium, making targeted fertilization strategies even more important.

Cal-BOOST 20-5-5+7CaO+2MgO is an extremely efficient product in relation to this specific nutrition need.

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