The microstructure of coconut coir dusts for use as alternatives to peat in soilless growing media

Abstract

The microstructure of the mesocarp pithy tissue and short-length fibres of 2 differently processed coconut coir dusts from Mexico and Sri Lanka was studied using scanning electron microscopy and compared with that of a Sphagnum moss peat. Selected physical properties of these materials were also determined for their use in potting media. The 2 coir dusts did not significantly differ in the microstructural parameters studied, whereas they differed remarkably in their particle size and therefore in their physical properties. Pithy tissue was the major constituent of coir dust. Coarse pithy tissue particles (larger than 1 mm) were highly porous, with an internal porosity of 41.0% by volume. These particles exhibited round-shaped external pores which ranged in diameter from 30 to 80 μm, with an average of 44.3 μm, and which accounted for a relative surface porosity of about 40.9%. By contrast, the cells of Sphagnum peat moss leaves showed oval-shaped pores of 20.1 × 11.5 μm, which accounted for a relative surface porosity of only 12.0%. The internal porosity of peat moss leaves was 51.1% by volume. The retting process (soaking in water) applied to the coconut husks in Sri Lanka partially degraded the pithy tissue and coir fibres. Coir dust from Mexico showed lower water-holding capacity and higher aeration than peat, whereas air–water relationships in coir dust from Sri Lanka were similar to those in peat. Although differences observed in microstructure and porosity characteristics could explain the differential physical properties of the three materials studied, the major reason was the different size distribution of their constituent particles.