Choice of material anatomical and economical considerations
Plant material to be used as growing medium must be chosen carefully in order to obtain sufficient physical properties in the end product. Based on results from the SEM studies as well as water retention capacities and particle size distribution, hemp was expected to be a suitable structural component in plant based compost (Dresboll and Magid, submitted; Dresboll and Thorup-Kristensen, submitted B). Although a large part of the hemp stem is readily degraded, the remaining parts seem to be suitable as they consist of a large amount of helical secondary walls, which probably will enhance the capillarity of the compost by connecting larger particles. In addition, external bast fibres will also increase the continuity of the media. Wheat and Mischanthus straw are both degraded to much more rigid parts, containing a large amount of small particles and long almost intact parts of straw in contrast to the hemp particles having a more uneven surface and size of particles (Dresboll and Thorup-Kristensen, submitted B). Thus, plant species having high fibre content might be good choices when producing compost as a growing medium. Clover-grass was chosen as nutrient rich source in the studies by Dresboll and Thorup-Kristensen (submitted A) since it is an easily available nutrient supply, readily mixable and has really good nutritional properties. Despite the structural qualities of hemp there are certain obstacles towards using hemp. When the material is harvested fresh and cut immediately the nitrogen content can be high, which will increase decomposition and make the material unstable (Dresboll and Thorup-Kristensen, submitted B). However, if hemp is not used until after the retting process the shredding of the material becomes complicated as the bast fibres are very strong and it takes special thresh mills to cut the fibres. After this processing the fibres have a 'wool-like' structure which makes it difficult to mix with other materials. This however, might be overcome by harvesting the hemp late in the season and shredding it without going through the primary retting process.
A less scientific approach to the choice of material for plant-based substrate is an economical approach. In order to make compost applicable in practice, it has to be able to compete with the prices on peat. Clover-grass is grown in large quantities in Denmark and could be used without large expenses. This is more uncertain when it comes to the structural elements. Wheat straw could be used at low expenses but results showed that wheat straw alone was not suitable as growing medium as stability and water retention capacities were not satisfactory (Dresboll et al., in prep; Dresboll and Thorup-Kristensen, submitted A). Growing hemp and Mischanthus straw would be somewhat more costly; however, hemp is becoming a more common crop for different purposes, the leaves can be used for tea, the seeds for oil production for cosmetics and fibres for robe, clothes and insulation. Both fibres and stem could though be produced without being much more expensive than peat. Mischanthus straw is grown as an energy crop and to some degree as material for thatched roofs, and it is the same part of the plant that should be used for composting. As Mischanthus in addition is very costly to establish, this might bring too high expenses on the growers. Thus, hemp-based compost, maybe combined with wheat straw to keep expenses down, is believed to be an economically and structural alternative to peat.
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