The development and testing of a general purpose soil-moisture-plant model is presented. The model incorporates soil, plant and hydrometeorological factors, and simulates daily variations in both leaf suction and the soil moisture deficit in each of three soil layers. A fourth soil layer permits the simulation of deep percolation and the rise and fall of a shallow water table. Two important features of the model are its unsaturated flow and root abstraction algorithms which are essential to the accurate modelling of moisture changes within layers. Actual transpiration is automatically reduced below the potential rate by the root abstraction algorithm when flow through the soil-plant system is unable to meet the demand. The physical characteristics of the soil layers are represented by mathematical functions relating soil suction to moisture content and unsaturated hydraulic conductivity. The model is calibrated and tested against field data obtained using the neutron probe technique. It is proposed that the computed values of leaf suction provide a better index of irrigation need than soil moisture deficit.