For environmentally conscious and highly profitable agriculture, it is necessary to optimize the nutrient application to crops. Ion absorption of roots is regulated by the function of transport proteins of cell membranes, which largely depends on ion concentrations in the root zone. Furthermore, environmental factors such as radiation can affect the ion absorption through change in transpiration (i.e. root water absorption). In this study, we propose a model of root ion absorption integrated with water absorption by modifying the theory of enzyme kinetics (i.e. Michaelis-Menten equation). Ion absorption rate in intact roots of Welsh onion (Allium fistulosum L.) was measured under different conditions of ion concentration, light intensity and consequently under different rates of root water absorption. The integration model proposed can represent well the rates of ion absorption affected by ion concentrations and water absorption. Furthermore, this integration model will be helpful for optimizing nutrient application.