The salts of many essential plant nutrients are soluble in water and may be applied to plant leaves directly as a foliar fertiliser. This practice has become widespread in the American and Australian cotton industries over the past 20 years as a means of correcting crop nutrient deficiencies and supplying nutrients to plants during peak demands when root uptake may not be adequate to meet plant demands. The efficacy of these foliar fertilisers, and the yield and quality benefits that may be obtained is debatable. Variable plant responses to foliar

fertilisers have been recorded, ranging from yield increases of 30% to a reduction in photosynthesis and leaf functioning. Knowledge about the precise mechanisms of penetration of foliar applied chemicals is limited, and the factors contributing to the effective uptake of foliar applied nutrients are insufficiently understood to explain the highly variable yield responses recorded. This review will examine the factors affecting the penetration and uptake of foliar applied nutrients, and discuss the potential of this practice to supply nutrients to

developing crops and supplement soil fertilisation in a commercial system.