New Technologies Developed for Conventional Growing Systems : Possibilities for Application in Organic Systems

In conventional greenhouse systems, recent innovations have contributed to increased production levels, and at the same time, to the decreased ecological footprint in terms of the reduction of water use, nutrients and CO2 emissions. Several examples of application of standard and innovative technologies to improve production, optimize utilization of solar energy and increase efficiency of energy and water use are presented. These technologies can also be applied in organic (soilbased) protected growing systems and the practical application is, as with conventional growing systems, primarily limited by the economic feasibility. The examples include the implementation of modern sensor technology, the use of models in environmental control and innovative covering materials for greenhouses, as well as the latest developments in the field of (semi-) closed greenhouses. INTRODUCTION Organic greenhouse horticulture is defined by ISHS as the production of organic horticultural crops using inputs from only natural, non-chemical sources, in climatecontrollable greenhouses and tunnels. This method of production differs in many aspects from conventional production and one of the key issues is the approach to soil management (Voogt et al., 2011). Although the differences are pronounced, the basic principle of both organic and conventional production is to optimize the use of all available inputs within the given restrictions (e.g., technical, economical or EU regulations) to achieve a more sustainable production with less impact on the environment. In conventional greenhouse production, yields are usually high and are reached using high inputs and at high investment costs. Currently, three major issues are of concern in greenhouse horticulture: energy use and CO2 emission, water and nutrients, and integrated pest management to reduce chemical use. Over the last decades, conventional growing has adopted biological control and crop protection technologies which originally tended to be used in organic growing systems. Conversely, technological developments for water use and energy efficiency in conventional growing may be applicable in organic greenhouse production to improve sustainability. However, the application of artificial substrates and recirculation of water and nutrients in organic growing systems is not commonly accepted and sometimes even prohibited by regulatory constraints in various countries. This limits the application in organic systems of a wide range of water and nutrient supply technologies developed in conventional growing systems. This paper therefore focuses on new technologies developed for conventional growing systems which are not linked to substrates or recirculation and also could be applied in organic production systems using climate-controllable greenhouses and tunnels. In general the production levels in organic systems are lower compared with conventional systems (e.g., in The Netherlands, the average yield for tomato is up to 65 kg m, while for organic soil-grown tomato, around 45 kg m is considered the standard level (Raaphorst, 2011). Depending on the price levels and impact on yield, economic a sjaak.bakker@wur.nl Proc. First IC on Organic Greenhouse Hort. Eds.: M. Dorais and S.D. Bishop Acta Hort. 915, ISHS 2011 48 feasibility of the implementation of new technologies in organic systems may differ compared with conventional systems. WATER USE EFFICIENCY The impact of water use can be expressed in different ways, for instance, Water Footprint, Virtual Water, Product Water Use (PWU) and Water Use Efficiency (WUE) (Nederhoff and Stanghellini, 2010). Water Use Efficiency (WUE) is the yield (kg) divided by the amount of water used for growing (L), and is usually expressed in kg L. Although the term WUE is often used, now PWU (L kg) seems to be a more preferred term. The Water Footprint Network has collected and published an extensive data set for a large range of products which shows that PWU for fruit vegetables is generally low (Hoekstra and Chapagain, 2008). More detailed data on PWU for tomatoes was presented by Van Kooten et al. (2008), showing that the PWU for tomato ranges between 300 L kg for field grown crops, to 15 L kg in greenhouses. Protected cultivation is generally much more water efficient than open field growing (e.g., Klohn, 2002; Nederhoff and Stanghellini, 2010). The higher water efficiency in greenhouses primarily results from a higher yield as the result of better and more optimal environmental conditions (CO2, temperature, humidity), an extended growing season, and protection from the elements and pests and diseases. The other main factor is that crop transpiration in greenhouses and subsequent water uptake by the crop is much lower compared with the open field due to lower radiation, higher humidity levels and reduced wind speeds in greenhouses. The overall effect of improved production and reduced water use is a significant decrease of PWU by a factor 4–5, on average. For tomato, the theoretical limit of PWU is about 1.25 L kg since 80% of the total fresh biomass in greenhouse tomato is fruit weight (de Koning, 1993). However, the lowest values, as claimed in closed environment greenhouses with full water recovery and recirculation (de Gelder et al., 2005), are around 4 L kg (van Kooten et al., 2008). Completely closed greenhouses (e.g., Opdam et al., 2005), or semi-closed greenhouses with reduced ventilation capacity were originally designed to combine improved production with energy savings (Bakker, 2009), and for southern regions, also to improve water efficiency (Buchholz et al., 2005). These greenhouses use (forced) cooling and no or limited natural ventilation. The reduced air exchange with outside air allows higher CO2 levels to be maintained, even under high radiation levels, thus achieving higher yields. Due to the absence or limited use of the vents, the water that evaporates from the crop does not escape the greenhouse but condenses against the heat exchangers. The first results in completely closed greenhouses with forced cooling have shown production increases up to 20% with tomato (de Gelder et al., 2005). In a recent study, three optimized designs of (semi-) closed greenhouses were tested year-round with different crops (cucumber, tomato, pepper and pot plants), but despite the high production levels (e.g., 76 kg m for tomato), none of the systems has shown to be economically competitive (de Zwart, 2010). The high investment costs are still not balanced by the (significant) production increase and savings on energy or water use, making them the major limitation for application of new greenhouse system designs in both conventional and organic production. Very low PWU’s generally can be reached in closed greenhouses using artificial substrates with recirculation of the nutrient solutions. However, the principle of closedenvironment greenhouses can also be applied to soil-grown crops. The potential of these systems, in terms of reduced PWU, is high, not only for more moderate climates, but especially for more southern and/or hot arid regions. In these regions, greenhouse cooling is traditionally achieved using pad and fan systems or direct evaporative cooling, which requires large quantities of water, resulting in PWU’s of up to 60 L kg or higher (van Kooten et al., 2008). Application of closed systems may significantly reduce the PWU, e.g., in the Watergy greenhouse, a closed greenhouse project in Spain, 75% of the irrigation water was recovered (Zaragoza et al., 2007). Although growing crops in a greenhouse reduces the PWU, good irrigation and