A Decision Support System to Optimise Fertigation Management in Greenhouse Crops

Optimal operation of greenhouse crops requires an appropriate management of fertigation, especially when the availability and/or the quality of irrigation water are limited and/or there are environmental constraints, such as those derived from Nitrate Directive in Europe. The paper illustrates the main features of a decision support system (DSS) for the fertigation management in soilless culture, which is still under development in the framework of the European research project EUPHOROS. The DSS modules provide information on the optimal irrigation regime (based on the static hydrological properties of the growing medium) and on crop water and nutrients requirements (including both plant uptake and leaching requirement in open and semi-closed growing systems). An easy-to-use nutrient solution calculator and an extendable crop database have been integrated in the DSS. The DSS can assist the grower in daily operational management of fertigation as well as for off-line (prior-to-planting) simulation, for instance to compare in terms of water and nutrients use efficiency different fertigation strategies, growing system layouts, water qualities and/or crop species (analysis of the scenario). An example of such simulation is provided for tomato grown under saline conditions. INTRODUCTION The advantages of soilless culture over conventional soil cultivation (e.g., higher use efficiency of water, nutrients, labour etc.; better product quality) are fully achievable only in closed systems where nutrient effluents are recirculated (Stanghellini et al., 2007). However, the management of closed systems is more difficult due to the risk of rootborne diseases and the possible accumulation of non-essential ions in the recirculating Nutrient Solution (NS) (Pardossi et al., 2006). When poor quality irrigation water (e.g., saline water) is available, the progressive accumulation of ballast ions (such as Na and Cl) scarcely absorbed by the crop makes it necessary to discharge the recirculating NS (flushing) with consequent waste of water and fertilizers (i.e., semi-closed systems). In these systems, the knowledge of the recirculating NS composition is a key factor to optimize the fertigation management, in particular to decide when flushing is necessary, based on physiological and/or environmental criteria, e.g. taking in account of the limits imposed to the concentration of pollutants (e.g., NO3) in the wastewater discharged to surface water by the legislation derived from European Nitrate Directive (The Council of the European Communities, 1991). In the framework of the European research project EUPHOROS (http://www. euphoros.wur.nl/UK/), work is in progress to develop a decision support system (DSS) for the management of fertigation of greenhouse crops (Hydrotool). This paper illustrates the general structure of Hydrotool and reports the results of the validation of the models implemented in the DSS in order to simulate crop water and mineral relations. An example of the possible use of DSS for analysis of scenarios (different strategies for fertigation) is also reported. a [email protected] Proc. XXVIIIth IHC – IS on Greenhouse 2010 and Soilless Cultivation Ed.: N. Castilla Acta Hort. 927, ISHS 2012 116 MATERIALS AND METHODS DSS Structure and Algorithms Hydrotool is a modular system developed in C# Microsoft .Net Frameworks 3.5. Default language is currently English (US), but the system will support other languages such as Italian and Spanish. The DSS is composed of a central database containing all input and output data to or from the different subroutines (Fig. 1). Graphical and statistical tools have been implemented for data analysis and reporting. The DSS incorporates three main modules (Fig. 1): 1) nutrient solution calculation; 2) evapotranspiration (ET) simulation; and 3) fertigation management (onand off-line). 1. Nutrient Solution Calculation. The module (Fig. 2) calculates the composition and the cost of the nutrient stocks based on: i) the ion composition of irrigation water; ii) the crop fertigation recipe (i.e., pH, EC and the concentrations of all nutrients delivered to the crop); iii) technical characteristics and prices of fertilisers, acids and bases; iv) technical characteristics of the fertigation device (i.e., volume of stock tanks and dilution ratio). The module contains a pre-compiled database with the characteristics of the main fertilisers used for soilless culture and several nutrient solution recipes for the main greenhouse crops. Obviously, the database can be extended by the grower, who may include his/her own recipes and/or fertilisers. The occurrence of salt precipitations due to an excessive salt concentration in the nutrient stocks is also verified by the NS calculator. 2. ET Simulation. ET can be provided directly by the user or estimated from weather data using different simplified equations such as those proposed by De Villele (1974), De Graaf (1988) and Baille et al. (1994). These models require the knowledge of Leaf Area Index (LAI) evolution, which can be provided by the user or simulated as a function of thermal time calculated automatically using weather data and the base temperature for the crop of interest. 3. Fertigation Management. This module is based on some algorithms originally developed and reported in detail in previous works (Carmassi et al., 2007; Incrocci et al., 2008; Massa et al., submitted). It simulates the variation of the ion concentration in the NS that is drained out from open systems or recirculated in semi-closed systems. In the latter systems, the user can select the strategy for nutrient replenishment among three possible options (Fig. 3; Massa et al., 2010), which are briefly described below: Strategy A1. ET is compensated in the mixing tank with refill NS at pre-established EC and ion composition. Due to the possible accumulation of ballast and nutrient ions contained in the irrigation water at concentrations higher than uptake concentrations (CU; it is the ion/water uptake ratio), the EC of the recirculating NS tends to rise up starting from a pre-fixed EC set point. When the maximum EC value tolerated by the crop is reached, the NS in the mixing tank must be discharged. Strategy A2. ET is compensated with refill NS as in Strategy A1. However, when the ceiling EC is reached, the mixing tank is refilled with only (acidified) raw water till N-NO3 concentration is lower than a physiological or environmental limit (e.g., maximum nitrate concentration of 1.0 mol m, according to Italian legislation on the wastewater discharge in the environmental). Afterwards, NS is discharged as described for Strategy A1. Strategy B. ET is compensated with a refill NS that has a variable ion composition adjusted in order to maintain a determined EC of the recycling NS. Due to progressive NaCl accumulation in the recirculating water, the macronutrient content tends to decrease until N-NO3 concentration drops below a critical concentration; afterward, NS is discharged as described for Strategy A1 and A2. The DSS can be run in two different modes:  Off-line (prior to planting): to predict crop water and nutrient requirements based on water quality, climate, possible legislative constraints and a number of parameters regarding crop physiology (i.e., plant mineral CU), the layout of growing systems (i.e., the volume of recirculating NS in the growing system) and fertigation regime (i.e., the strategy for nutrient replenishment, as previously described);