Organic fertilizers in greenhouse production systems - a review

From a sustainability point of view, there are strong arguments of nutrient recycling within the society, which means more use of organic nutrient sources within agriculture and horticulture. At the same time, there is an increasing consumer demand for certified organic products, and incentives from governments to increase the conversion of production areas to organic production. This also applies to greenhouse horticulture. Many different raw materials for organic fertilizers are used as of today, such as animal manures, slaughterhouse byproducts, vegetable byproducts, green manure, algae, composts, anaerobic digestates etc. In common for all these fertilizer types is that they are limited in availability, not always consistent with respect to nutrient content, and that they require microbial degradation in order to mineralize its content of nutrients, and are thereby more or less to be characterized as slow release fertilizers. Greenhouse horticulture is different from open field agri-culture in several ways with respect to nutrient supply. Firstly, the use of fallow crops and crop rotation is not practical due to the high investment costs bound in the greenhouse structure. Secondly, growth per unit area is significantly higher than in outdoor production, with subsequently higher nutrient demand, often concentrated to a relatively short period of time. On the other hand, climatic factors such as soil temperature and moisture can be controlled which is beneficial for the control of nutrient release. Traditionally, animal by-products such as manure and slaughterhouse wastes have been widely used as organic fertilizers. However, limited availability and ethical concerns is currently driving forces in the search for alternative nutrient sources. The use of solid and liquid anaerobic digestates as fertilizers is a promising practice for greenhouse horticulture. Energy is a "by product" from the production and the nutrient content of the digestates can be modified by feeding the anaerobic reactor with different stock. Furthermore, it is suggested that techniques for fine-tuning the nutrient supply in organic greenhouse horticulture is further developed and adopted, such as the use of microbial biofertilizers and foliar sprays.