Measuring compost structure

Compost structural properties were quantified by measurements of particle size distribution and water retention capacity (Dresboll and Thorup-Kristensen, submitted B). As these were the initial tentative comparisons between composts based on different structural material, these parameters were expected to be sufficient. Particle size is generally determined by sieve fractionation (Agnew and Leonard, 2003), but this has been found to underestimate the fractions of small particles. Thus, in order to determine small fractions as well, particle size was determined by software analysis of a scanned picture of 1 g compost (Clemmensen, pers.comm.). This method can be criticised by the small sample size which to some extent can be overcome by a high number of replicates. Nonetheless, it has proved to be an illustrative method (Dresboll and Thorup-Kristensen, submitted B). Beside the quantitative measurement it also provided a qualitative knowledge of the surface appearance and the geometry of the particles visualised at the scanned pictures (Fig.9) (Dresboll and Thorup-Kristensen, submitted B).

Figure 9. Particle geometry of A) Mischanthus compost based on wheat straw, Mischanthus straw and clover-grass. B) Wheat compost based on wheat straw and clover-grass. C) Hemp compost based on wheat straw, hemp straw and clover-grass (Dresboll and Thorup-Kristensen, submitted B).

Water retention is generally measured on a sand box keeping compost samples in metal rings and varying the degree of suction (PrEN 13041, 1999; EN 13040, 2000). Suctions can be varied in the range potted plants can experience, although when using compost as a growing

Figure 9. Particle geometry of A) Mischanthus compost based on wheat straw, Mischanthus straw and clover-grass. B) Wheat compost based on wheat straw and clover-grass. C) Hemp compost based on wheat straw, hemp straw and clover-grass (Dresboll and Thorup-Kristensen, submitted B).

medium only part of the compost will probably experience the zero suction where the compost is water filled. Plants will typically be watered by the "ebb and flood"-method and the capillary forces in compost will probably not be sufficient to pull water to the upper layer of the container.

Scanning electron microscopy has been used in some studies following the decomposition during composting (Atkey and Wood, 1983; Davis et al., 1992; Lyons et al., 2000). Examining decomposition of plant material by the use of SEM makes it difficult to quantify the degradation. However, information from the qualitative results should not be underestimated. The visualisation of the actual decay provide valuable insight into how the tissues are degraded, the extent and rate of decomposition, and the spatial distribution of microorganisms (Dresboll and Magid, submitted). Most important, the SEM studies show the post-composting appearance of the plant material, implying the capability as structural element in compost based growing media. However, when interpreting the scanning electron micrographs it is important to distinguish between signs of decomposition and artefacts caused by the cutting and drying procedures when preparing the specimens for examination.

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