Dynamics of water and nutrients in closed, recirculating cropping systems in glasshouse horticulture : with special attention to lettuce grown in irrigated sand beds

Due to the high leaching fractions commonly practised in glasshouse horticulture, environmental pollution is clearly a major concern. Switching from soil-based to closed, recirculating substrate-based cropping systems potentially offers a good solution to this problem. As an alternative to the usual trial-and-error method in designing these new systems and optimizing fertigation strategies, this thesis uses the approach of modelling processes in the root zone supported with some detailed experiments. For this purpose a two-dimensional simulation model is developed.

Water movement is described by the mixed volumetric water content-pressure head Richards equation, which is solved numerically using the control volume finite element method. When the entire flow domain is unsaturated, the solution is obtained by the alternating direction implicit method. Otherwise, the incomplete Cholesky conjugate gradient method is used. Nutrient transport occurs by convection, dispersion and diffusion. Water and nutrient uptakes by the roots are described by scaled-up versions of microscopic steady-rate solutions for uptake by a single root.

As a test case, large bedding cropping systems filled with coarse sand (median diameter 0.6 mm) were used. The hydraulic properties of the sand were determined by the equilibrium hanging water column and transient multi-step outflow methods. Hysteresis was taken into account. The main validation data were volumetric water content measured by the TDR method, pressure head, and drainage outflow. Water use, transpiration, root length density distribution and root radius were obtained in experiments as well, while other parameters were obtained from the literature. Additionally, data on lettuce growth and nutrient uptake are presented and in some cases compared with data obtained in nutrient film technique experiments and from the literature. Dry matter productions in the two systems were comparable, but nutrient uptake differed.

A reasonable agreement between simulated and measured data was obtained. The model was applied in some case studies. Coarse porous media usually have enough water available for long periods of evapotranspiration. However, high solute concentrations may develop near the surface of the substrate since solutes are left behind when water evaporates and the flow through these regions is limited. Therefore, it is advised to schedule fertigation based on salinity control rather than on water supply control. Saturated conditions at the bottom occur only for short periods, and aeration problems in coarse porous media are unlikely.