Organic production in Denmark has evolved during the last decades. The number of organic farms has increased from 400 in 1989 to 3500 in 2003 (Danish Plant Directorate, 2003). However, the organic greenhouse production has not followed the same steep increase. The greenhouse vegetable production covers only 6.2 ha (Danish Plant Directorate, 2003) corresponding to less than 5% of total greenhouse vegetable production, and production of ornamentals and herbs are almost non-existing. One of the major obstacles towards organic greenhouse production is controlling the fertilisation in order to obtain sufficient nutrition without risking high leaching losses. When growing plants in closed containers the nutrition is even more important since suitable supplementary organic fertilisers are scarce. In addition, structure and stability of the substrate influence plant growth. Thus, in order to develop organic greenhouse production more focus on nutrition and structure of growing systems is needed.
Most organically grown greenhouse vegetables are grown directly in the soil. In contrast to field grown vegetables, crop rotation is not straightforward in greenhouses. Every crop requires specific greenhouse equipment and rotation would be very costly. Thus, the crops are grown for years in the same soil with the risk of concentrating pathogens in the soil. As chemicals cannot be used in organic production and sterilisation by steaming is questionable due to the effect on the microflora and the high energy consumption, alternative ways of avoiding pathogens in the soil must be considered. Greenhouse vegetables are very nutrient demanding, needing up to 20 times the amount of N of field grown vegetables. Thus, large amounts of organic fertilisers are added to the soil. In addition, the vegetables can be irrigated in excess, presenting the risk of loosing nutrients by leaching. One way of avoiding these potential problems could be by growing the plants in compost in confined beds. After the season the used compost could be added as soil conditioner in the field while fresh compost is added to the beds, in this way the greenhouse production can become an integrated part of an organic cropping system. In addition, leachate could be collected from the beds thus reducing leaching losses. However, many growers see this system as being in conflict with the principles of organic farming, and see the many advantages of growing plants in soil. In addition, establishment of confined beds will be time and labour consuming. Soil can be a more stable and reliable growing medium than compost as the rooting volume is higher and soil functions as a good buffer for water and nutrients. Hence, an intermediate system where plants are grown in confined beds in compost, but where the roots also have the possibility to develop outside the beds, could potentially combine the advantages of the two systems (Sorensen, 2003).
When organic plants are grown in small containers there are many requirements to the structure, stability and nutrient content of the growing medium. Peat is approved as an organic substrate but increased concern has risen due to the exploitation of these slowly renewable natural resources. Environmentalists throughout Europe have warned against the intensive utilisation of the peat bog reserves, as resources are being depleted and valuable biological habitats destroyed (Carlile, 2004). As large peat reserves exist throughout the world (Lappalainen, 1996) these worries might be overestimated. However, in organic production an increased focus on natural resources and environmental concerns is desirable. Additionally, peat is a very inert substrate and requires large amounts of organic fertilisers during the production. This can cause anaerobic conditions in the containers due to microbial activity and be harmful to plant growth. Thus, alternatives to peat are desirable in organic greenhouse production.
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