Hi Paul

A physiological disorder can cause the leaves of citrus plants to turn yellow. Physiological disorders relate to inadequate watering, mineral deficiency, growing a plant in too humid or too dry conditions, or at the wrong temperature, or insufficient light. Physiological disorders are often due to a combination of these factors and can only be corrected by improved growing conditions, such as feeding, mulching, watering or drainage and spacing. You could try to improve your soil with compost, and also feed and water. Its broad spectrum of nutrients makes seaweed an excellent fertiliser, and it can be applied either as a mulch or as a foliar spray (buy this from an organic supplier). Sprays of foliar feeds, at ten day intervals during the growing season, are beneficial for helping a plant recover from a physiological disorder.