Bark: Technical Guide to Its Properties, Stabilization, and Agronomic Use

As explained in our article on how bark contributes to substrate characteristics, this organic raw material is a by-product of the timber industry. Following appropriate processing, bark can be used as an organic component in growing media. Its primary value lies in its ability to improve aeration and structural stability, while also influencing water-holding capacity depending on particle-size distribution. 

The overall characteristics of bark depend on several factors, including

• tree species and maturity,
• bark age,
• debarking methods,
• wood content,
• particle-size distribution
• degree of stabilization.

Pine bark is among the most widely used materials because of its good biological stability and its versatility in producing products with different physical properties through crushing and screening. 

Bark: Physical and Chemical Properties 

Particle-size distribution

This physical property has a major influence on substrate physical properties: 

• coarse particles generally increase aeration and drainage; 
• fine particles generally enhance water retention. 

For this reason, bark should not be considered solely as an aerating component. Its effect depends on both particle-size distribution and its interaction with other substrate components. Excessively coarse bark can reduce plant-available water, whereas an overly fine fraction may impair aeration and increase the risk of compaction. 

Bark composting

Composting typically reduces the hydrophobicity of bark. However, depending on the origin and characteristics of the material, the use of wetting agents (surfactants) may be beneficial to promote uniform substrate wetting and rewetting. At this link, you can access a full article about moisture monitoring on peat-based substrates vs peat alternatives  

pH, EC and CED of Bark

• The pH of bark is typically acidic to moderately acidic (approximately 4.5 to 6.5)
• Electrical conductivity (EC) is generally low, although it can vary according to origin and post-harvest treatments
• Bark also contributes to the cation exchange capacity (CEC) of the substrate; however, these values should always be evaluated alongside physical properties and biological stability. 

For a deeper understanding of pH management, read our article on the pH stability in peat-free and peat-reduced substrates. To learn more about monitoring EC and soluble salts in alternative substrates, read our dedicated article to the importance of EC and salt measurements.  

Biological Stability and Composting 

Fresh bark may contain readily degradable organic matter. The resulting microbial activity can lead to oxygen depletion, heat generation, and temporary nitrogen immobilization (N-drawdown), potentially negatively affecting plant growth. 

Composting progressively breaks down the most easily degradable organic fractions and increases biological stability. Insufficient composting can leave high residual microbial activity and induce phytotoxicity, whereas an excessively prolonged process can alter physical properties by reducing the coarse particle fraction. 

Consequently, simply classifying bark as fresh or composted is not sufficient to determine its suitability for horticultural use. Its degree of biological stability should always be evaluated according to the intended application. 

This aspect is particularly important in substrates used for seed germination and young plant production (plugs and transplants), where limited container volume can amplify the effects of any physical or biological imbalance. 

Processing and Quality Control 

1. Processing typically involves
   • size reduction,
   • removal of physical impurities,
   • creening (sieving),
   • (when necessary) composting or other stabilization processes aimed at reducing or transforming potentially phytotoxic compounds such as terpenes, tannins, phenols, and manganese. 
2. Screening produces a more homogeneous particle-size distribution and thus more predictable physical behavior.
3. Attention should also be paid to tree species and wood content, as commercial bark products may contain varying proportions of wood fragments that can significantly alter both physical and biological properties. 
4. The origin of the raw material and its phytosanitary status should also be monitored. Bark sourced from infected timber or contaminated during handling and storage may act as a vector for plant pathogens. Properly processed and stabilized bark significantly reduces this risk and may also support the development of beneficial micro-flora that are antagonistic to several root pathogens. 

How to Use of Bark in Growing Media 

As discussed in our article on bark’s contribution to substrate characteristics, bark can improve both aeration and structural stability, although very coarse grades may reduce water-holding capacity. The net effect depends on its interaction with the other components of the mix. 

Therefore, bark should not be regarded simply as a volumetric replacement for peat. Rather, it should be considered a raw material capable of modifying the balance between air, water, and the solid phase within the growing medium. 

There is no universal inclusion rate suitable for all situations. In substrates intended for young plants, it is generally advisable to begin with conservative inclusion rates and increase the proportion only after evaluating the characteristics of the final mix. Higher inclusion levels may be appropriate when bark particle size, biological stability, and water-holding capacity are aligned with crop requirements and container volume. 

Bark: Evaluation Criteria 

For professional applications, the following parameters should be systematically monitored: 

• Physical: particle-size distribution, bulk density, water-holding capacity, and air-filled porosity. 
• Chemical: pH, EC, organic matter content, ash content, manganese concentration, and, where appropriate, nitrogen and other major nutrients. 
• Biological: biological stability, respiration rate, and potential phytotoxicity. 

For composted materials, pH and EC alone are not sufficient indicators of biological maturity. Evaluation should ideally be carried out on the final substrate blend, as interactions between components can significantly influence overall performance. 

Advantages and Limitations of Bark as Substrates’ Ingredient  

Bark Key Advantages:

• Valorization of a timber industry by-product. 
• Opportunity to reduce peat use. 
• Improved substrate aeration and structural stability. 
• Availability in a range of particle-size grades suited to different applications. 

Bark Key Limitations:

• Variability in raw material quality. 
• Potential nitrogen immobilization. 
• Risk of phytotoxicity when insufficiently stabilized material is used. 
• Risk of selecting particle-size grades that are unsuitable for the crop or container volume. 

Conclusions

Bark is a valuable raw material for growing media formulation when sourced through controlled supply chains and subjected to appropriate selection and processing. Its value lies not only in its ability to replace part of the peat fraction, but also in its capacity to improve substrate structure, aeration, and physical stability. 

Pine bark benefits from extensive commercial use and practical experience; however, every batch should be evaluated according to its origin, particle-size distribution, wood content, biological stability, and chemical characteristics. The optimal inclusion rate should always be defined according to the properties of the raw material and the agronomic objectives of the growing medium. 

Given the number of variables involved, growers may benefit from tailored technical guidance. Our regional technical support teams are available to help. Visit your country’s website to find your nearest ICL expert, or complete the contact form accessible through the banner at the bottom of this page.