Optimisation of growing media for organic greenhouse production

Summary

Organic production of greenhouse crops has developed during the last decade. Greenhouse crops in general, have higher nutrient demands than field grown crops and therefore, in order to optimise production it is essential to focus on the growing media and fertilisation. The studies comprised in this dissertation describe the suitability of compost as a growing medium for organic greenhouse crops. The choice and composition of plant material as well as shredding of material and management of composting processes can alter the nutritional and structural quality of compost based on plant residues. This dissertation is based on several studies on the nutritional quality of compost as well as studies of the structural properties of different straw materials and how these are transformed during composting.
Composting is an aerobic process driven by microbial activity. The process is highly dependent on a wide range of different parameters such as temperature, pH, and moisture content. In addition, the nutrient content and structural properties of the material to be composted influence the end product.
The availability of nutrients is important when plant-based compost is used as growing medium. However, mineralised nitrogen is generally only present in small amounts in compost due to immobilisation processes and nitrogen losses during the process. Thus, management of composting processes in order to obtain more mineralised nitrogen could improve the compost as a growing medium. In the first study, nitrogen mineralisation was managed by postponing the addition of part of the nutrient rich material by 3 weeks. One fourth of the nutrient rich material was added at the start, as this was expected to be sufficient to decompose the readily available carbohydrates of the structural straw material, without a large amount being immobilised. The remaining nutrient rich material was added after 3 weeks, when decomposition was expected to be dominated by microorganisms with a less N demanding metabolism degrading more recalcitrant carbohydrates, thus leading to more mineralised and less immobilised N. The results showed that the mineralisation pattern was altered, resulting in almost twice as much of mineralised nitrogen after 8 weeks of composting as compared to the control where all material was added initially. This has proved to be a simple way of influencing the composting processes without altering the type or amount of material used. In the second study, the mineralisation pattern was followed in a long-term experiment revealing that even in nutrient poor compost, postponed addition had a significant effect on the mineralisation rate, although the effect was much delayed compared to the nutrient rich compost. However, the experiments showed that the stability of the compost was not satisfactory.
Structural quality of the compost is also a key factor in the successful use of compost as a growing medium. Decomposition of plant material is not only dependent on chemical composition but on anatomical arrangement of tissues as well. In the third study, the decomposition of three different straw materials; wheat (Triticum aestivum), hemp (Cannabis sativa), and Mischanthus (Mischanthus ogiformis) were examined in order to get a basic understanding of the extent and rate of decomposition, and most importantly to evaluate the remains of the decomposition process. These changes were examined by the use of Scanning Electron Microscopy, and revealed great differences in the appearance of the straw materials after composting. Due to the post-composting appearance, hemp was suggested to be a suitable structural element to enhance the physical properties of the compost.
The physical properties of the growing medium are important parameters for successful plant growth, as these are related to the ability to adequately store and supply air and water to the plants. The fourth study determined water retention capacity and particle size distribution for compost based on different plant residues. The physical properties of compost are diverse and suboptimal compared to the properties of peat. However, peat is an inert material requiring large nutrient inputs to be suitable as growing medium, and it can be difficult to supply sufficient nutrients from organic sources. Knowledge on structural differences amongst plant species, and how the anatomical arrangement impedes decomposition, improves the ability of producing growing media with suitable structural properties.
In conclusion, these studies contribute to a general and basic understanding of how to manage composting processes and how plant material is decomposed. The mineralisation processes was shown to be altered when postponing part of the nutrient rich material 3 weeks during composting, hence increasing the nutritional quality of the compost when used as growing medium. In addition, the structural studies revealed differences in actual decomposition of different plant materials, not only dependent on lignin content, but more importantly on anatomical arrangement of tissues. This is fundamental knowledge and must be considered when producing a growing medium based on composted plant residues.