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Coir and applications of coir in stormwater management

Coir

This page provides information on coir. While providing extensive information on coir, there is a section focused specifically on stormwater applications for coir.

Overview and description

image of coir fiber
A close-up view of coir fibre, by Fotokannan, licensed under CC CC BY-NC-SA

Coconut (Cocus nucifera L.) pith or coir, the mesocarp of the fruit, is a waste product that has potential benefits in growth media. Coir dust is peat-like and consists of short fibres (< 2 cm). Coir has a large surface area per unit volume, is hydrophilic, and therefore has the ability to absorb water. It's primary components are lignin and cellulose, each making up about 45% of coir's dry weight. Water soluble fractions typically account for about 5% of coir, by weight (Alam).

There are three basic types of coir material.

  1. Coco pith is a rich, brown color and has a high water retention capacity.
  2. Coco fibers are stringy bundles that does not readily retain water and will break down over time.
  3. Coco chips are small chunks of coir that combine the properties of the peat and fiber. Coco chips retain water well and also allow for air pockets.

Coir production involves separating the husk from the shelled nut and soaking the husk in water. The fibers are then separated from the pith and the resulting material is screened to create a uniform particle size. A dust is created during this process and the dust may be air dried and packaged. Prematurely harvested (green) fruits are often soaked in a saline solution to facilitate the separation process, which in turn affects the chemical properties of the resulting coir dust.

Coir benefits may include but are not limited to the following.

  • Coir has a neutral pH
  • Coir improves water holding capacity of soil
  • Coir may improve drainage in fine-textured soils by creating pore spaces as it degrades
  • Coir increases the organic matter content of soil, which can improve soil structure and aggregation
  • Coir production is sustainable and therefore does not contribute to greenhouse gas emissions.

Properties of coir

This section includes a discussion of chemical and physical properties of coir, and potential contaminants in coir,

Chemical-physical properties of coir

The physical and chemical properties of coir vary with particle size. Noguera et al. (2003) varied particle size of coir dust, studying the properties of coir passing through sieves 0.125, 0.25, 0.5, 1.0, and 2.0 mm in diameter. They observed the following.

  • As particle diameter increases, air content increased and water holding capacity decreased
  • Electrical conductivity and micro-element concentrations were greatest in the smallest diameter coir
  • Bulk density decreased from 0.122 to 0.041 g/cm3 as particle size increased from <0.125 to >2 mm
  • Pore space increased from 92.3% to 97.3% as particle size increased from <0.125 to >2 mm
  • Water holding capacity (ml/l) decreased from 855 to 165, with the greatest change occurring with 0.5-1 mm particles
  • Shrinkage (volume loss on drying) decreased as particle size increased (38% to 15% as particle size increased from <0.125 to >2 mm)
  • Nutrient availability decreased with increasing particle size, but there were no significant differences between 0.125 and 2 mm. There was a large increase for the smallest particle size.

Based on generally recommended plant specifications, the researchers concluded the 0.25-0.5 mm size appears most suited for plant growth, with some addition of larger particles recommended. Abad et al. (2005) similarly concluded that a mix of particle sizes is likely to be optimum for use of coir as a plant medium.

Another factor affecting chemical properties of coir are the conditions under which it is prepared. In particular, if soaking in a saline solution is used in the preparation of coir, concentrations of potassium, sodium, chloride can be very high and may interfere with plant growth.

The following table summarizes data from the literature on physical and chemical properties of coir. Some general conclusions include the following.

  • Coir is slightly acidic but not as acidic as peat
  • Available nitrogen, calcium, magnesium, iron, copper, and zinc are low, while phosphorus, sodium, chloride, and potassium are high, particularly if the coir was prepared in a saline solution
  • Coir has a very high water holding capacity
  • Coir has a high germination index compared to compost (Lodolini et al., 2017)
  • Coir dust does not collapse when wet or shrink excessively as it dries (Cresswell)

Chemical and physical properties of coir.
Link to this table

Property Range found in literature1 Median value from literature
Total phosphorus (% dry wt) 0.036 - 0.41 0.036
Total nitrogen (% dry wt) 0.24 - 0.5 0.45
Total potassium (% dry wt) 0.4 - 2.39 0.819
Total carbon (%) 42 - 49 47.1
Total hydrogen (%) 4.4
pH 4.9 - 6.9 5.9
Cation exchange capacity (cmol/kg) 31.7 - 130 50
Electrical conductivity (ds/m) 39 - 2900 582
Total calcium (%) 0.18-0.47 0.40
Total magnesium (%) 0.11-0.47 0.36
Total copper (mg/kg) 3.1-10.3 4.2
Total zinc (mg/kg) 4.0-9.8 7.5
Total manganese (mg/kg) 12.5-92 17
Bulk density (g/cm3) 0.025 - 0.132 0.06
Water holding capacity (% by wt) 137 - 1100 566
Total pore space (%) 85.5 - 98.3 95.2

Primary references for this data:

  • Cresswell
  • Abad et al., 2002
  • Abad et al., 2005
  • Asiah et al., 2004
  • Kumar et al., 2010
  • Lodolini et al., 2017
  • Shrestha et al., 2019

Potential contaminants in coir

There are few concerns with contaminants in coir, with the possible exception of sodium and chloride in coir prepared using saline solutions. Levels of the elements may be at levels that negatively impact plant growth.

Metal concentrations are well below Tier 1 Soil Reference Values. Organic contaminants, such as polycyclic aromatic hydrocarbons, are not a concern.

Effects of coir on physical and chemical properties of soil and bioretention media

In this section we provide information on effects of coir on pollutant attenuation and the physical properties of soil and bioretention media.

Effects of coir on retention and fate of phosphorus

There are limited studies on coir retention of phosphorus at concentrations typically found in stormwater runoff (less than 0.5 mg/L). Adsorption studies show that phosphorus adsorption at higher concentrations (greater than 1 mg/L) occurs through ion exchange and chemisorption being mechanisms for adsorption, with sulfate competing with phosphate for adsorption sites ).

Shrestha et al. (2019) studied phosphorus leaching from columns containing mixtures of soil, compost, spent lime, and coir. Using tap water with no detectable phosphorus, they observed that adding coir (10% by weight) to a 70-20 soil-compost mix did not decrease phosphorus leaching compared to an 80-20 soil-compost mix. Similar results were observed for media with 40% compost. Hongpakdee and Ruamrungsri (2015) observed reduced phosphorus leaching at the flowering stage, possibly due to increased plant vigor and uptake in treatments containing coir. Herrera Environmental Consultants (2015) conducting flushing and leaching experiments for a variety of media mixtures, including mixtures containing coir. Mixtures of coir and granular activated carbon (GAC) or ash showed orthophosphorus concentrations of 0.021 and 0.052 mg/L, respectively, when flushed with solutions containing less than 0.004 mg/L. For leaching experiments, influent orthophosphate concentrations were 0.323 mg/L and effluent concentrations for coir-GAC and coir-ash mixtures were 0.025 and 0.164 mg/L, respectively. However, the researcher attributed retention of phosphorus to the GAC and ash rather than coir. The researchers also observed decreasing orthophosphorus leachate concentrations with time.

Additional research is needed to understand the phosphorus retention or leaching from media containing coir. Research to date suggests coir will not retain phosphorus in stormwater runoff but will not significantly contribute to leaching from engineered media.

Effects of coir on retention and fate of other pollutants

There is limited research on retention and leaching of pollutants from coir. Shrestha et al. (2019) observed that media containing coir performed similar to spent lime for ammonium and nitrate retention and leached significantly less of these chemicals than treatments containing compost. Herrera Environmental Consultants (2015) observed similar results and also observed that mixtures of coir and either granular activated carbon or ash reduced copper and zinc leaching compared to media mixtures consisting of just soil and compost. Because concentrations of potential pollutants are low in coir, leaching at concentrations of concern appears unlikely. An exception is coir that was soaked in salt water, which may contribute to high sodium, potassium, and chloride concentrations.

Effects of coir on soil physical and hydraulic properties

Coir has several properties that may improve soil physical and hydraulic properties (Cresswell; Noguera et al., 2003; Abad et al., 2005; Small et al., 2018; Lodolini et al., 2018; Arachchi and Somasiri, 1997).

  • Coir dust remains relatively hydrophylic (water attracting) even when it is air dry
  • Coir dust does not collapse when wet or shrink excessively as it dries
  • Increases water holding capacity
  • Increases soil porosity
  • Decreases soil bulk density

Effects of coir on soil fertility, plant growth, and microbial function

Several studies show that addition of coir to soil improves plant growth, with coir outperforming peat in several studies. In the absence of compost amendment, nitrogen and phosphorus may need to be added depending on plant requirements. Calcium and magnesium additions may also be needed. Concentrations of other nutrients and micronutrients are generally acceptable for most plant species (Cresswell; Asiah et al., 2004; Noguera et al., 2003; Abad et al., 2002; Meerow, 1997; Lodolini et al., 2017; Hongpakdee and Ruamrungsri, 2015; Small et al., 2015; Scagel, 2003; Arachchi and Somasiri, 1997). Noguera et al. (2003) showed that, based on generally recommended plant specifications, 0.25-0.5 mm diameter coir particles appear most suited for plant growth, with some addition of larger particles recommended. Abad et al. (2005) similarly recommended a mix of particle sizes.

Standards, classification, testing, and distributors

Coir standards

Distributors

Test methods

Effects of aging

Coir has a high C:N ratio, ranging from 75 to 186, with a median of 115 (Abad et al., 2002; Abad et al., 2005; Shrestha et al., 2019; Meerow, 1997; Arenas et al., 2002). It also contains a high lignin content and therefore decomposes relatively slowly unless nutrients, primarily nitrogen and phosphorus, are added to the media (Amlan and Devi, 2001). Composting is recommended to increase nutrient availability, which in turn may increase the rate of decomposition. Similarly, liming or addition of specific microorganisms can enhance decomposition (Prabhu and Thomas, 2002). Even when decomposition is facilitated, the life expectancy of coir exceeds two years (Newman 2007).

Prabhu and Thomas (2002) provide an extensive discussion of coir decomposition.

Storage, handling, and field application

Sustainability

Coir dust is a sustainable alternative to peat. Historically, very little coir has been utilized and has therefore been disposed as a waste. Prabhu and Thomas (2002), for example, estimated that in India alone, 1.5 million tonnes of coir pith could be obtained annually but only 500,000 were produced at the time of their study. Studies are underway to expand existing markets and develop technologies for manufacturing coir dust from coir fiber (Praveenkumar and Agamoorthi 2017; Varma, 2018).

References

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