Effects of Particle Size, Pore Structure, Water-Holding Capacity, Air-Filled Porosity, and Reuse of Cocopeat as a Growing Medium: A Literature Review, 2017-2026

Abstract

Cocopeat, or coir pith, is a soilless growing medium widely used in greenhouse, hydroponic, nursery, and container cultivation systems. Although the commercial quality of cocopeat is commonly evaluated using pH, electrical conductivity (EC), moisture content, and expansion volume, its agronomic performance is primarily governed by the relationships among particle-size distribution, pore structure, water-holding capacity, air-filled porosity (AFP), wetting behavior, and changes that occur during reuse. This narrative review synthesizes scientific publications issued between 2017 and 2026 that examined these characteristics. A targeted literature search was conducted through official journal and publisher websites, DOI resolution services, PubMed/PMC, AGRIS, institutional repositories, and academic search engines. Twenty-two publications were mapped; 19 articles were used as primary evidence and three as supporting context. The synthesis indicates that fine fractions generally increase water retention, whereas coarse fractions improve infiltration pathways, drainage, and aeration. However, particle-size effects are not always linear because fiber morphology, compressibility, pore distribution, container height, initial moisture content, and amendments jointly determine the air-water balance. Total porosity alone is insufficient to predict plant performance; dynamic parameters, including water-retention curves, hydraulic conductivity, rehydration capacity, and wetting-front patterns, should also be considered. Cocopeat can be reused for several cycles while maintaining relatively stable crop yields when structure, salinity, nutrient status, and disease risks are controlled. Nevertheless, microbiome shifts and increases in potentially pathogenic fungi demonstrate that physical stability does not automatically guarantee biological safety. This review proposes a more comprehensive cocopeat quality framework and a research agenda linking laboratory specifications with crop requirements and fertigation systems.

Keywords-air-filled porosity, cocopeat, hydraulic properties, particle size, porosity, reuse, water-holding capacity.

1. Introduction

The expansion of soilless cultivation has increased demand for growing media that are lightweight, uniform, capable of supplying water and oxygen in a balanced manner, and compatible with precision fertigation. Cocopeat is the fine fraction produced during coconut husk processing and is also referred to in the literature as coir pith or coir dust. Commercial products may contain different proportions of pith, fiber, and chips. These terminological distinctions are not merely semantic because each fraction differs in morphology, size, surface area, and pore arrangement, thereby producing different air-water behaviors [1], [6], [7].

In commercial practice, cocopeat is commonly classified according to low or high EC, pH, moisture content, expansion volume, block weight, and washing level. These parameters are important, but they are insufficient to explain why two products with similar EC and pH values may require different irrigation regimes, exhibit different drainage rates, or produce different root responses. Research conducted during the past decade has shown that particle-size distribution, particle density, total porosity, water-holding capacity (WHC), container capacity, AFP, wettability, saturated hydraulic conductivity, and the shape of the water-retention curve are interrelated components [2]-[5], [8], [9].

Conceptually, particle size governs how particles are packed and how pores are formed. Fine fractions tend to increase surface area and the volume of water-retaining micropores, whereas coarse fractions form macropores that accelerate infiltration, drainage, and gas exchange. However, cocopeat is fibrous, elastic, and susceptible to compression or structural rearrangement upon wetting. Consequently, the relationship between particle size and hydraulic properties is not always linear. The addition of perlite, vermiculite, pine bark, compost, or biochar may also produce mixture properties that cannot be predicted solely from the weighted averages of the components [3], [4], [13].

Growing-medium reuse is another issue of increasing importance. From economic and environmental perspectives, extending the service life of cocopeat can reduce production costs and substrate waste. Studies on lettuce and strawberry have shown that coir-based media can be reused without substantial yield losses under certain conditions [12], [14]. However, substrate age may alter microbiome composition and has been associated with increases in potentially pathogenic organisms [15]. Reuse assessments must therefore include physical, chemical, agronomic, and biological dimensions.

This review aims to: (1) analyze the effects of particle size on pore structure and water behavior in cocopeat; (2) compare the roles of total porosity, WHC, and AFP in determining the root-zone environment; (3) evaluate the implications of hydraulic properties for irrigation and substrate formulation; (4) examine the evidence concerning cocopeat reuse; and (5) identify research gaps and quality parameters relevant to producers, exporters, greenhouse operators, and researchers.

2. Literature Search Method

This review used a structured narrative approach supported by thematic mapping. The publication period was defined as January 1, 2016, to August 2, 2026, to capture research developments over the previous ten years. Searches were conducted through official journal and publisher websites, DOI links, PubMed and PubMed Central, AGRIS, institutional repositories, and academic search engines. Keywords used in various combinations included “cocopeat,” “coconut coir,” “coir pith,” “coir dust,” “particle size,” “particle size distribution,” “porosity,” “pore structure,” “water-holding capacity,” “container capacity,” “air-filled porosity,” “air space,” “hydraulic conductivity,” “water retention,” “rewetting,” “reuse,” “reused coir,” “strawberry,” and “soilless substrate.”

Articles were included when they met the following criteria: publication in a scientific journal within the specified period; direct examination of cocopeat/coir or inclusion of it as an important comparator; assessment of at least one principal variable, namely particle size, porosity, WHC, AFP, hydraulic properties, or reuse; and availability of a verifiable DOI or publisher page. Articles that merely mentioned cocopeat without presenting physical, hydraulic, agronomic, or reuse data were excluded from the main synthesis. Publications issued before 2016 were considered only when cited as conceptual foundations in the reviewed articles and were not treated as units of analysis.

A total of 22 relevant publications were mapped in the initial database. Nineteen articles were treated as primary evidence because of their direct relevance to the review themes, whereas three were used as supporting context. Extracted data included publication year, cultivation system, coir type or mixture, measured variables, principal findings, novelty, and limitations. The synthesis followed a mechanistic sequence: particle size -> pore structure -> water retention and flow -> air-water balance -> plant response -> changes during reuse.

This study was not a PRISMA-based systematic review and did not conduct a meta-analysis because methods used to measure WHC, AFP, porosity, matric pressure, container dimensions, and reporting units differed substantially among the articles. Accordingly, numerical values from different studies were not treated as fully equivalent; the discussion emphasizes directions of effect, underlying mechanisms, and consistency of findings.

3. Discussion

3.1. Particle Size as an Initial Determinant of Substrate Structure

The study by Ilahi and Ahmad was among the early investigations to measure particle-size distribution, bulk density, total porosity, WHC, wettability, and saturated hydraulic conductivity simultaneously in a cocopeat-perlite mixture [1]. Approximately 82.93% of the mixture particles were within the 0.425-4 mm range, with total porosity of approximately 79% and very high gravimetric WHC. The study demonstrated that sieve data can provide an initial description of substrate arrangement, but their interpretation must be linked to density and hydraulic behavior.

Kiran et al. separated several agricultural residues, including coconut coir, into fine fractions below 2 mm, medium fractions of approximately 3 mm, and coarse fractions of approximately 5 mm [6]. In general, reducing particle size increased water retention and decreased AFP because smaller particles filled interparticle spaces and increased the proportion of small pores. However, the response of coconut coir was not always as pronounced as that of other materials, suggesting that fiber morphology and elasticity may preserve some air space even at smaller nominal sizes. This finding emphasizes that sieve diameter does not fully represent the geometry of fibrous particles.

The comparability of particle-size data is also influenced by testing procedures. Bartley, Fonteno, and Jackson identified substantial variation in agitation duration, sample mass, sieve arrangement, and reporting practices across horticultural substrate studies [9]. For coir, the sieving time required for the distribution to stabilize may reach approximately 11 min. If two laboratories use different agitation periods, the reported percentages of fine and coarse fractions may differ even for the same sample. Commercial descriptions such as “fine,” “medium,” and “coarse” should therefore be accompanied by clearly defined sieve ranges and analytical procedures.

An important methodological advance was reported by Yao et al., who used micro-computed tomography and pore-scale flow simulation [18]. More than 90% of the pore diameters in the investigated samples were within the 0-400 micrometer range. Changes in particle size and the addition of perlite affected three-dimensional porosity, pore diameter, tortuosity, permeability, and flow-path connectivity. This study demonstrated that two substrates with similar total porosity can nevertheless differ in pore connectivity and resistance to flow.

Feng et al. extended these findings by examining infiltration patterns and predicting moisture content using RGB images and machine learning [19]. Coarse-particle media produced faster vertical wetting, whereas fine fractions retained water more strongly and restricted vertical penetration. A mixture of coarse and fine fractions at an initial moisture content of approximately 20-30% provided a better balance between infiltration and retention under the experimental conditions. Overall, recent evidence supports using particle-size distribution as a design parameter, while also accounting for particle morphology, compression level, and pore connectivity.

3.2. Total Porosity, Water-Holding Capacity, and Air-Filled Porosity

Total porosity represents the entire pore volume within a substrate, whereas WHC or container capacity represents the water retained after specified wetting and drainage conditions. AFP is the proportion of pore space that remains air-filled under those conditions. These three parameters are related but are not interchangeable. A substrate with high total porosity may still have poor aeration if most pores are water-filled micropores, or it may dry excessively rapidly if its pore system is dominated by macropores.

The accuracy of total-porosity calculations depends on bulk density and particle density. Bartley et al. measured the particle density of several substrate components and reported a value of approximately 1.40 g cm-3 for coir [8]. The use of a generic particle-density value may bias porosity estimates, particularly when mixtures contain inorganic materials or wood fibers with different densities. Standardized particle-density measurements are therefore an important prerequisite for comparing total porosity among products.

Londra et al. compared peat, coir, perlite-containing mixtures, and commercial media using particle-size distribution, water-retention curves, pore-size distribution, and saturated and unsaturated hydraulic conductivity [2]. All substrates could exhibit high total porosity, but the addition of perlite shifted the distribution of water and air. This does not imply that increasing perlite content is invariably beneficial because improved drainage may be accompanied by reduced water retention. Gohardoust et al. likewise emphasized that mixtures of coir with perlite, volcanic tuff, or other inorganic materials do not always display properties equal to the linear average of their components [3].

In pine-bark mixtures, coir amendment increased water storage and plant-available water [4]. A coir proportion of approximately 25% was sufficient to improve available water, whereas formulations with higher coir contents produced the greatest water content under the study conditions. Total porosity could remain relatively stable even as air space decreased and container capacity increased. Changes in pore composition are therefore more informative than changes in total pore volume alone.

The results of Prabhadharshini et al. provide a quantitative example of differences in air-water balance [16]. A 1:1 coir-vermiculite mixture had a WHC of approximately 69.4% and an AFP of 17.1%, whereas coir without vermiculite had a WHC of approximately 57.7% and an AFP of 12.28% under the methods used in that study. The same mixture also supported strong spinach performance when combined with an appropriate nutrient formulation. These values should not, however, be adopted directly as universal standards because WHC and AFP are highly dependent on wetting procedures, container height, compaction, and measurement pressure.

Mogale and Maluleke compared two coir products with AFP values of approximately 28% and 33% with loam soil having an AFP of approximately 17% [17]. The coir with moderate AFP provided advantages in fruit number in some observations, whereas the higher-AFP coir was associated with increases in certain quality attributes, including lycopene and selected minerals. These findings indicate a trade-off: greater aeration does not necessarily maximize every response variable, and optimum AFP must be matched to crop species, climate, fertigation frequency, and yield or quality objectives.

3.3. Dynamic Hydraulic Properties and Their Implications for Irrigation

Static characterization at saturation and after drainage captures only part of cocopeat behavior. During cultivation, the substrate undergoes wetting-drying cycles, changes in root distribution, compaction, and salt accumulation. Water-retention curves, saturated hydraulic conductivity, unsaturated conductivity, rehydration capacity, and wetting-front velocity are therefore important parameters for irrigation design.

Schulker et al. tested peat, coir, and pine bark under different initial moisture contents, irrigation depths, and pulsed application patterns [5]. Coir exhibited rapid initial water capture and retained approximately 81% of the applied water across the experimental sequence. The response of coir to pulse intervals was smaller than that of peat or bark because coir rewetted more readily. These results suggest that, in cocopeat, initial moisture content and application volume may be more influential than dividing irrigation into numerous short pulses, although the actual response remains dependent on substrate formulation and container configuration.

Bartley et al. examined substrate sorption after initial moisture conditioning and wet-dry cycling [10]. Coir and bark were relatively stable across initial conditions compared with peat. Even a small coir addition improved hydration in peat-based media, whereas peat showed a greater reduction in wetting capacity after drying cycles. These findings reinforce the value of coir for supporting rewetting, but they do not imply that all cocopeat products are invariably easy to wet. Excessive fine dust, high compression, or extreme drying may still create nonuniform wetting.

Papadimitriou et al. demonstrated that container geometry interacts with the hydraulic properties of coir dust-perlite mixtures [13]. A substrate with the same composition may exhibit a different air-water distribution when container height and shape are altered. In addition, mixture properties did not change proportionally with the volume fractions of coir and perlite. Consequently, cocopeat intended for long grow bags, short slabs, tall pots, or Dutch buckets cannot be evaluated using a single WHC or AFP value without specifying the container configuration.

In a Dutch bucket system, Yang et al. found that coir generated approximately 24% less leachate than perlite while supporting comparable cucumber yield [11]. This efficiency indicates that coir can reduce water and nutrient losses, but its greater retention also requires adjustments to fertigation volume and frequency to avoid prolonged saturation. Irrigation programs should therefore be derived from the actual characteristics of the substrate rather than copied from schedules developed for rockwool or perlite.

3.4. Cocopeat Formulation and Amendment Materials

Commercial cocopeat is rarely homogeneous. Machado et al. compared coir pith, coir chips, and a pith-fiber mixture for spinach production [7]. Coir pith and the pith-fiber mixture supported growth more comparable to peat, whereas chips alone tended to produce poorer growth. These differences are consistent with the physical roles of the fractions: pith increases water retention, fiber maintains structure and connectivity, and chips create macropores but may reduce water-root contact when present at excessively high proportions.

Perlite is commonly added to improve aeration and reduce density, but its effects depend on perlite size and cocopeat structure. Vermiculite increases water retention and nutrient-holding capacity, whereas pine bark can act as a porous and renewable amendment. Compost and biochar may add nutrients, exchange capacity, or microbial habitat, but they can also modify EC, pH, porosity, and irrigation requirements. Because mixture properties are often nonlinear [3], [13], each formulation should be evaluated as a new system rather than estimated solely from the average values of its components.

The literature indicates that substrate formulations should be differentiated at least according to crop species, container dimensions and shape, climate, emitter number, drainage target, and fertigation frequency. For greenhouse tomato and cucumber, mixtures that maintain stable macroporosity are important because of long production cycles and large root biomass. For nursery crops or leafy vegetables, water retention and uniform wetting may be more important. The concept of a single cocopeat formulation suitable for all crops is not supported by the available hydraulic evidence.

3.5. Cocopeat Reuse: Physical Stability and Plant Response

Reuse is a potential advantage of cocopeat, but its benefits depend on the substrate’s ability to maintain structure, porosity, salinity, and sanitary status. Machado et al. reused coir-based mixtures that had previously supported spinach and subsequently grew lettuce in them [12]. In general, the reused coir blends continued to support lettuce growth, although amendment composition affected nutrient uptake and phytonutrient accumulation. These results show that reuse cannot be evaluated on biomass alone; changes in pH, drainage EC, nutrient status, and product quality must also be assessed.

Woznicki et al. compared new coir with substrates that had been used for one and two years in strawberry production [14]. Fruit yield in coir remained relatively stable across substrate ages, and coir preserved its structure and organic matter more effectively than some alternative media. The study supports reuse for up to three seasons or production cycles under the tested conditions, particularly in the absence of disease problems. However, the number of seasons is not the only criterion. Fertigation intensity, water quality, dead-root accumulation, salt precipitation, and compaction may accelerate functional decline.

In practice, a reuse assessment should begin with evaluations of volume and physical stability, drainage distribution, pH, EC, specific ion concentrations, and substrate cleanliness. Old roots and plant residues must be managed without excessively disrupting substrate structure. If the medium shows shrinkage, an excessive fine fraction, nonuniform drainage, or anaerobic zones, the addition of coarse material or partial replacement may be necessary. A medium that still meets physical criteria nevertheless requires biological assessment before reuse.

3.6. Microbiome, Pathogens, and Biosecurity in Reused Cocopeat

The biological dimension is an important distinction between new and previously used media. Xu et al. used amplicon-based metagenomics to assess changes in bacterial and fungal communities in strawberry coir of different ages [15]. Substrate age was associated with increases and decreases in several amplicon sequence variants, indicating that microbial communities were progressively selected over multiple seasons.

Yield decline was associated with an increase in the potentially pathogenic fungus Ilyonectria destructans. This association does not establish a single causal pathway, but it is sufficient to show that stable porosity and yield at one site cannot guarantee the general safety of reuse. Localized biochar application in planting holes also failed to restore yield consistently. Physical amendments therefore do not automatically resolve established biological problems.

Reuse protocols should incorporate crop disease history, root inspection, facility sanitation, water quality, and-for high-value production-pathogen testing. Heat treatment, steam, solarization, or chemical disinfection must be validated to suppress pathogens without damaging substrate structure or leaving harmful residues. Future research should determine microbiological risk thresholds that support practical decisions: whether a substrate can be reused directly, requires sanitation, should be blended with new material, or must be removed from the production system.

3.7. Implications for Cocopeat Quality Standards

The literature synthesis shows that cocopeat specifications for professional use should extend beyond block weight, expansion volume, moisture content, pH, and EC. Additional relevant parameters include: particle-size distribution measured using a fully specified sieve method; proportions of pith, fiber, and chips; bulk density and particle density; total porosity; WHC or container capacity under clearly defined testing conditions; AFP at a specified container height or pressure; saturated hydraulic conductivity; rehydration capacity after drying; and batch-to-batch variation.

Method reporting is as important as the numerical result. Gravimetric WHC expressed as a percentage cannot be compared directly with volumetric water content. AFP measured in a 10-cm-high container is not equivalent to AFP in a taller slab because gravitational gradients alter water distribution. Initial substrate EC also differs from nutrient-solution EC or drainage EC during cultivation. Without clearly defined methods, specification values may be misinterpreted by buyers and users.

For producers and exporters, products may be grouped into function-based grades, such as high water-retention grade, balanced air-water grade, high-drainage grade, nursery grade, tomato/cucumber grow slab, or strawberry reuse-ready grade. Such claims must, however, be supported by laboratory and plant tests. Batch consistency is particularly important because changes in raw materials, sieving, washing, buffering, compression, and storage can modify physical properties even when block dimensions and EC comply with specifications.

3.8. Synthesis of Novelty and Research Gaps

The principal novelty of research developments during the past decade has been the shift from bulk measurements toward mechanistic explanations. Sieve analysis has been standardized [9], particle density has been measured specifically [8], hydraulic behavior has been evaluated using retention curves and flow models [2], [3], pore structure has been visualized using Micro-CT [18], and moisture content has been predicted using RGB imaging and machine learning [19]. At the same time, reuse studies have progressed from yield measurements to microbiome analysis [14], [15].

Several gaps nevertheless remain. First, no fully uniform international protocol is available for determining the particle-size distribution of fibrous cocopeat. Second, quantitative relationships among size fractions, three-dimensional pore structure, retention curves, and root responses have been investigated for only a limited range of products. Third, studies of reuse beyond three seasons, under tropical conditions, and in crops other than strawberry remain scarce. Fourth, interactions among particle size, initial salinity, ion accumulation, and fertigation frequency have not been comprehensively mapped. Fifth, biosecurity thresholds for reused cocopeat have not yet been established.

Future research should combine multi-producer and multi-batch sampling, standardized laboratory protocols, Micro-CT or image analysis, real-time moisture sensors, water-nutrient transport models, and crop experiments. Predictive models linking initial specifications with irrigation requirements and crop performance would have greater commercial value than classifications based only on EC and block dimensions. Life-cycle assessment is also required to ensure that the benefits of reuse are not offset by excessive energy, water, or chemical consumption during sanitation.

4. Conclusion

Particle size is an initial determinant of cocopeat structure, but its effects on plants are mediated by fiber morphology, compression, pore connectivity, amendment materials, container geometry, and irrigation management. Fine fractions generally increase water retention, whereas coarse fractions improve infiltration, drainage, and aeration. Total porosity cannot be used alone to assess quality because substrates with similar pore volumes may differ in WHC, AFP, tortuosity, permeability, and rehydration capacity. A more meaningful assessment should integrate particle-size distribution, particle density, WHC/container capacity, AFP, water-retention curves, hydraulic conductivity, and wetting patterns. Cocopeat can be reused for several cycles while maintaining crop yield when structure, salinity, nutrient status, and drainage remain controlled. However, microbiome changes and potential pathogen accumulation indicate that physical stability is not the sole basis for decision-making. Reuse must be supported by sanitation and biological-risk assessment. Based on this synthesis, cocopeat quality standards for professional and export markets should shift from general specifications toward function-based specifications supported by transparent test methods. Future studies should develop cross-product models linking particle distribution and pore structure with air-water dynamics, fertigation requirements, crop performance, and reuse safety.

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