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Not All Wood Fibre Is Equal – Here’s Why the Process Matters

Woodfibre in horticulture is predominately made by using two methods, most common is Disc Refined and the patented BVIS Twin Screw method. Proptec's Hortifibre, using the BVIS method has been successfully used in our TopMix for various horticultural crops. This technical paper explains the difference.

Technical Note: Comparative Processing Routes for Wood Fibre in Horticultural Substrates 

Scope: disc refining, twin‑screw defibration. Key metrics: fibre morphology, pore structure, air‑filled porosity (AFP), wet‑cycle stability, and nitrogen (N) immobilisation risk. 

Executive summary 

This note compares two thermo‑mechanical processing routes used to convert wood into fibrous components for growing media. The processes differ primarily in applied specific mechanical energy, pressure/temperature profiles, and the degree of geometric constraint (die/no die), which in turn drives fibre length distribution, surface morphology, elastic recovery, and pore‑space stability under repeated wetting and load. 

  • Disc refining typically produces a broad particle size distribution with higher fines content and lower elastic recovery, increasing the likelihood of AFP loss and compaction after wet cycles. 

  • Twin‑screw defibration (non‑die) generally yields short‑to‑medium fibres with higher surface roughness and moderate stiffness, providing improved initial structure versus disc refining but with some susceptibility to packing over time. 

  • N dynamics: Higher surface area/roughness and fines can increase microbial colonisation and short‑term N immobilisation risk; extrusion‑type fibres often present lower biologically accessible surface area. 

Definitions and abbreviations 

  • AFP (air‑filled porosity): Volumetric fraction of air space in a substrate at a defined suction/measurement method. 

  • Wet‑cycle stability: Retention of pore volume/structure after repeated wetting and drying cycles under representative handling or pot load. 

  • N immobilisation: Temporary reduction in plant‑available nitrogen due to microbial uptake during decomposition of carbon‑rich materials. 

This document is intended as a process‑to‑performance mapping for wood‑fibre components used in horticultural substrates. It describes expected directional effects on morphology and substrate function; site‑specific outcomes depend on feedstock species, moisture content, pre‑treatment, operating set‑points, and downstream screening/blending. Brand names are used only to identify representative equipment configurations. 

Twin‑screw defibration (non‑die; e.g., BIVIS‑type systems) 

Machine architecture 

  • Two parallel, intermeshing screws (e.g. BIVIS machines) 

  • Screws include drive zones + braking zones 

  • No die forming a shaped extrudate 

  • Continuous process 

Energy & physics 

  • Thermo‑mechanical: heat is generated by shear + compression 

  • Pressure ≥ ~90 bar 

  • Temperature ~120–150 °C 

  • Residence time: seconds to tens of seconds 

What happens to the wood 

  • Chips are mechanically compressed, braked, and torn apart 

  • Lignin softens; fibres split along grain 

  • Fibres are short‑to‑medium length, rough surface, irregular cross‑section 

Resulting fibre characteristics 

  • Good initial AFP 

  • Moderate fibre stiffness 

  • High surface roughness → water retention 

  • Some collapse/packing after multiple wet cycles 

Implications for substrate performance 

Expected behaviour (directional): Maintains aeration at moderate inclusion rates with generally reliable rewetting. Under repeated over‑wetting and/or compaction, fibre packing can reduce AFP over time. Elevated surface roughness and accessible carbon may increase the probability of short‑term N immobilisation relative to more densified/low‑surface fibres. 

Summary 

Non‑die twin‑screw defibration applies high compressive and shear stresses (thermo‑mechanical softening of lignin) to separate fibres without imposing die‑controlled geometry. The resulting morphology is typically more structured than disc‑refined material, but less geometrically constrained than die‑based extrusion products. 

 

 

Disc refining (thermo‑mechanical wood fibre) 

Machine architecture 

  • One or two rotating refiner discs 

  • Chips fed between discs 

  • Mostly batch or semi‑continuous 

Energy & physics 

  • Mechanical shear + frictional heat 

  • Lower pressure than twin‑screw systems 

  • Temperature rise is less controlled 

What happens to the wood 

  • Fibres are ripped apart radially 

  • Significant fines generated 

  • Fibre length distribution is wide 

Resulting fibre characteristics 

  • Short fibres + fines 

  • Low elastic recovery 

  • Irregular pore size distribution 

Implications for substrate performance 

Expected behaviour (directional): Typically lower cost with high throughput, but higher fines content and lower elastic recovery increase sensitivity to compaction and wet‑cycle AFP loss. Greater biologically accessible surface area may increase the probability of N immobilisation (especially where labile carbon fractions are present), and process variability can contribute to batch‑to‑batch performance variation. 

Summary 

Disc‑refined fibres can provide economical bulk and water‑holding contribution, but are generally less suitable where repeatable, wet‑cycle‑stable macroporosity is required without additional stabilising components. 

 

Side‑by‑side comparison (what actually matters) 

Feature 

Disc refining 

Twin‑screw defibration (non‑die) 

 

Machine complexity 

Low 

Medium 

 

Uses twin screws 

No 

Yes 

 

Uses die extrusion 

No 

No 

 

Pressure level 

Low 

High 

 

Fibre length control 

Poor 

Moderate 

 

AFP stability 

Poor 

Moderate–good 

 

Wet‑cycle collapse 

High 

Medium 

 

N immobilisation risk 

High 

Medium 

 

Indicative cost per m³ 

Low 

Medium 

 

 

Terminology guidance (defensible descriptions) 

The statements below are appropriate for technical documentation and should remain accurate provided they reflect the actual equipment configuration and operating window. 

  • Florentaise uses a patented twin‑screw defibration process. 

  • This process is not disc refining. 

  • This process is not classical die‑based extrusion. 

  • The resulting performance is often designed as a cost–stability balance for commercial growing media. 

A defensible phrasing: 

Florentaise fibres are produced using a patented thermo‑mechanical twin‑screw defibration process. This creates a more structured and stable fibre than disc‑refined materials, but without the cost and rigidity of full extrusion. 

 

Application implications 

In applications where target AFP and hydraulic conductivity must be maintained across handling, transport, and multiple irrigation cycles, the processing route materially

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