Perspectives: Can we apply sodium in tropical plantations to reduce the need of potassium fertilizer?

While soil salinization is a major concern for agriculture worldwide, the application of sodium (Na) can stimulate growth in many plant species, due to its ability to replace potassium (K) in some physiological functions. We present an overview of the potential and risks associated with replacing KCl fertilizer with a mixture of KCl and NaCl in tropical plantations, with a focus on Eucalyptus. A wide range of responses of Eucalyptus species to the salinization of the soil is well documented and the planting of Eucalyptus rows in salinity-damaged landscapes has been used in Australia to lower the water table and reduce soil salinization. A positive effect of NaCl application on Eucalyptus growth in K-deficient environments has been demonstrated in hydroponics, which is consistent with comprehensive studies conducted on entire rotations in Brazil. The addition of NaCl to K-deficient soils increased above-ground biomass at harvest in E. grandis plantations by a factor of 1.4, whereas the addition of the same amount of KCl increased biomass by a factor of 2. A comparison between KCl and K2SO4 applications suggested a positive response of E. grandis to Cl, in addition to the response to Na. The effects of Na application on the physiology of E. grandis support the hypothesis of a functional role for Na in K-deficient soils. Consistent results have been reported on the physiology of olive tree, coconut palm and cocoa tree. Atmospheric inputs of Na depend on the distance from the ocean, ranging from approximately 30 to 5 kg ha-1 yr-1 in coastal and inland forests, respectively. Na is an essential nutrient for animals, maintaining homeostasis and playing a crucial role in neural networks. Substantial quantities of Na are routinely fed to animals in extensive livestock farming. A growing body of evidence suggests that a shortage of Na can limit the activity of insects, particularly plant decomposers. The application of Na in tropical plantations could therefore accelerate the release of nutrients during litter decomposition, thereby promoting tree growth. However, Na application could also stimulate herbivory and lead to leaf area losses. We recommend setting up multi-location experiments covering a wide range of soils and climates in tropical regions, for perennial species of commercial interest. In-depth studies should be carried out on certain sites to understand how tree physiology, soil functioning and herbivory are influenced by the addition of NaCl.