The variety of materials and mixes of materials used in horticulture as growing media is large. Peat products, wood products, composts, mineral fibres, mineral particles, synthetic foams, synthetic fibres and organic fibres; all these products are more or less suitable growing media.Selected physical aspects related to plant growth requirements, are used to discuss the suitability of growing media for plant growth. Aspects mentioned include bulk density, total pore volume, structural stability, root resistance, water retention, rewetting, hydrophobicity, water transport, and oxygen transport. Methods to measure these aspects and experience with the results are also discussed in this review.In conclusion, the physical properties used to assess the suitability of substrates for growth share a large dependence on pore architecture. This made it possible to relate the formulas for water retention, hydraulic conductivity and diffusivity to each other. It seems possible to extend this unification to resistance to rooting and rewetting. Bulk density, total pore space and structural stability exercise their influence on plant growth mainly through the key characteristics for growth assessment: resistance to rooting, water retention, rewetting and water and oxygen transport. Better definitions and possibly new methods to characterise rewetting, hydrophobicity and refreshment rate are advocated. When assessing the influence of water and oxygen transport, individual layers and gradients towards the roots are of importance because of the very fast transport rate changes with water or air filled pore space.

Soil-dwelling predatory mites can be very effective as biological control agents against larvae of the lepidopteral pest Duponchelia fovealis. Some growing media were reported to have natural high level and stable populations of predatory mite. The objective of this experiment was to define conditions to establish stable predatory mite populations in the rooting medium and to assess the direct effect of the rooting media on pest development. Eight rooting media were prepared, including a range of degradabilities as measured with the Oxygen Uptake Rate method (OUR). The OUR range was created by mixing peat products, coir dust, bark, perlite, compost and wood fiber. Each treatment was split: half with and half without a commercially used mulch to create a drier top layer. Kalanchoës were grown on these rooting media. After one week soil-dwelling predatory mites (Hypoaspis miles) were added. Adults of the pest Duponchelia fovealis were released during a number of weeks. Both populations were counted. Results show that the OUR range was successfully achieved. The commercial mulch, a cork based fine granulate, reduced the numbers of Duponchelia by 32%. The number of predatory mites was related to the oxygen uptake of the rooting media (R 2 =0.87). The predatory mite reduced the numbers of Duponchelia larvae on average by 58%. Thus, biological control by soil-dwelling predatory mites can be improved by offering rooting media with an increased degradability as measured by the oxygen uptake rate. The combined effects of using predatory mite and mulch layers are discussed.

The EU Water Framework Directive demands a sound ecological and chemical basis for ground and surface waters. This has motivated the Dutch greenhouse industry to seek more sustainable water management procedures which will enable a zero-emission of nutrients and plant protection products (PPP) in the year 2027. Although closed soilless growing systems are widely applied in The Netherlands, it appears that discharge of nutrients varies between 5 and 40%. Discharge based on salinity is only a minor part, up to 15%. In rose cultivation, growth inhibition is the major reason for discharge of the nutrient solution. Former research could not find a proper reason for growth inhibition, but it is most likely of organic origin. The water treatment method of advanced oxidation, at which first hydrogen peroxide is added, directly followed by an exposure to UV-C light (

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