Irrigation management in organic greenhouse: how to comply with sustainability goals

Irrigation in protected cultivation is essential due to the absence of natural precipitation. High evapotranspiration, due to higher temperature and prolonged cropping period, requires ample an adequate supply of water. The water supply in a greenhouse is solely carried out by irrigation and thus enables full control over the water management of the greenhouse soil. There are no specific prescriptions for organic greenhouse horticulture (OGH) regarding water sources or irrigation. However, according to the basic principles, growers are obliged to responsibly use natural resources such as water. Thus, efficient water use and/or water recycling are important issues for organic farming. Salinity should be avoided through proper fertilisation management and the use of water sources of adequate quality. Moreover, rainwater collection is strongly encouraged or requested for operation sites situated in climatic suitable areas. Optimal and tuned irrigation is quite challenging for OGH. Obviously, supplying the crop with sufficient water is an important precondition for optimum crop health and performance. In addition, sufficient soil moisture is required for optimal functioning of the soil biota to secure mineral delivery from the applied organic amendments. Also accumulation of undesired salts must be avoided. On the other hand, nutrient leaching should be avoided or limited to a minimum. The irrigation management should take heterogeneities due to irregular water delivery of the irrigation system as well as site variations in soil and plants into account Minimizing leaching should be one of the main drivers for optimization of irrigation in OGH. Since OGH in Europe are strictly soil bound cultures, leaching and nutrient emission to groundwater is a threat. For soils with groundwater within reach of the root zone, capillary rise may add to the water supply of crops. However, irrigation strategies aiming at deficit irrigation, in other words, relying on capillary rise, can be used for the short term only. Given the presence of a surplus of ions in groundwater not only salts like Na and Cl, but also Ca, Mg, SO4 and even K, salinity problems will occur on the long run. Irrigation management tuned to crop requirements is an important issue for sustainable organic greenhouse production. To achieve sustainable management of OGH production, irrigation should be tuned to the demand of the crop, taking into account also the variations in climatic zones, soil types, growing seasons, planting dates and cultivars. Consequently, tools are required for irrigation management with the flexibility to deal with such variation. This paper will review the important aspects of the crop water demand and tools for irrigation management in organic greenhouse vegetables.