In both field and greenhouse production, there is an increasing pressure to improve the policies of irrigation and fertilisation that should both satisfy the objectives of production and quality and avoid losses of nutrients in the environment. At the time of writing, empirical methods are used; they should be improved with mechanistic models that are being developed.
The supply of water to the crop must fit its water requirements. In soil-less culture, irrigation is usually calculated based on radiation measurements. Several relationships have been established between the crop water uptake and the incident radiation for tomato and as well as for other vegetable crops (formulae reviewed by Jolliet25). The VPD should also be taken into account when radiation and VPD are uncoupled, for example in changing climatic conditions and when using systems of climate control such as thermal screens or fog systems.32 The water demand depends on a crop coefficient that increases with the leaf area development. In soil culture, the availability of water in the soil compartment must be considered: it depends on the hydraulic properties of the soil and on the root development. In the field, the rain flux must enter into the water balance.
In greenhouses, computers are used to monitor radiation and to control the quantity of water that is provided for open systems (on soil or soil-less), that is, the calculated evapotranspiration plus about 25% run-off to avoid salt concentration in the root substrate. In closed soil-less systems, the water input must fit the crop demand to maintain the total volume of circulating nutrient solution. In the field, new DSS are designed to calculate the proper water supply. For example, the IRRIGERE software, designed for field tomatoes, estimates the daily evapotranspiration from climate and crop development and the soil water reserve from the soil characteristics and the root depth.102 Irrigation will not meet crop demand when water stress is needed to increase the quality (dry matter content) of fruits. In that case, the objective is to exhaust the water available in the root zone at fruit harvest. With these constraints, irrigation is proposed when the watering dose gets higher than a threshold value of 3 mm.
Few attempts have been made to build fertilisation strategies using models of crop requirement, even in soil-less culture. In this cultivation system, nutrients are usually supplied in excess together with water. Therefore there is no way to control the crop growth or product quality through the regulation of fertigation. Marcelis et al.103 proposed the combination of models and sensors to optimise the nutrient supply in closed systems.
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