1.2.1 Requirements to compost as growing media
When compost is used as a growing medium in organic greenhouse production a number of requirements should be fulfilled. First, the compost structure must be suitable for plant growth. Water retention, air filled porosity and volume weights are important parameters in a growing medium (Gruda and Schnitzler, 2004) and are dependent on particle size and geometry. Hence, the particle size distribution should allow pores of different sizes to be formed with the largest amount in the range 30-300^m. These will retain water, but will not bind it so tightly, that it is unavailable to plants at water pressures relevant for containerised plants (Payne, 1988). Secondly, nutritional quality of an organic substrate is an important parameter. As greenhouse crops have high nutrient demands, and addition of organic fertilisers to plants grown in closed containers can be difficult and time consuming, initial nutrient contents must be high. Organic liquid fertilisers are to be preferred over solid fertilisers, as application is more easily conducted, although the liquid fertilisers can cause a variety of practical problems such as microbial growth in the irrigation system. Another disadvantage of fluid organic fertilisers is that they often are based on wastes from animals, leading to repellent odours. Thus, compost containing an initially large amount of the nutrients is to be preferred. However, a high nutrient content increases the risk of continuous decomposition of the media in the container, leading to anaerobic conditions or unstable compost, loosing a large amount of the media during the plant production time (Jensen et al., 2001; Gruda and Schnitzler, 2004). Additionally, high initial nutrient levels can damage the roots and inhibit plant growth, and will increase the risk of loosing nutrients.
In greenhouse vegetable production composted manure is often used as fertiliser. As organic farms and greenhouse production are not always closely located, transport of the manure can be an expensive and laborious process. As growing media for ornamentals, different types of composted waste have also been proposed, but these are not suitable as peat substitutes as they are too compact and often have a high content of nutrient salts and heavy metals (Weinhold and Scharpf, 1997). Alternatively, replacement of the animal manures in favour of easily available composted plant residues could be a more sustainable solution. Nutrient rich plant material such as clover could be grown in the field close to the greenhouse productions, and residues from other field productions such as different straw materials could be used. Compost based on plant residues must contain at least two different plant material types nutrient poor materials such as straw as structural component, and nutrient rich materials such as legumes to supply nutrients for microbial metabolism and subsequent plant nutrition.
Plant growth is dependent on a wide range of nutrients at different amounts. Nutrient release during decomposition is dependent on the nutrient source and varies between nutrients. However, nitrogen is one of the most important nutrients for plant growth and the main focus in this review.
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