Dynamics of water and nutrients in closed, recirculating sand bed systems: modelling water flow, nutrient transport, root water and nutrient uptake

摘要

The general equations for water movement and nutrient transport are presented, to enable simulation of plant growth using substrate physical properties and root nutrient uptake in closed cropping systems. The boundary conditions are described, withspecial reference to root water and nutrient uptake. Root water uptake depends on root length density, hydraulic conductivity and radius, and water potential of plant, rhizosphere and bulk substrate, and substrate volume from which the root can extract water. The actual transpiration is a function of plant water potential and potential transpiration. Maximum possible nutrient uptake depends on root length density and radius, bulk substrate concentration, water uptake rate, diffusion coefficient, and substrate volume from which the root can extract water. Actual water uptake equates to whichever is lower out of the required and maximum possible water uptake. Two worked examples are presented for a thin layer cropping system utilizing sand beds. In the first example, it was concluded that, since saturated conditions can limit oxygen supply to the roots, thin layer cropping systems need to use a coarse substrate. In the second example, limited uptake of a nutrient by the tomato crop resulted in its accumulation on the substrate surface, due to root water uptake and evaporation.