Beneficial elements

The roles of sodium (Na), silicon (Si), cobalt (Co), selenium (Se), and aluminum (Al) are described. These elements are termed beneficial because they stimulate growth only in certain plant species, or under specific conditions, but do not meet the criteria for essential elements. Sodium can stimulate growth of halophytes and some other plants, particularly C4 species. In C4 plants, Na aids the movement of substrates between the mesophyll and the bundle sheath. Sodium can also to some extent replace K in its role as osmoticum. Silicon has a number of beneficial effects in many plant species, especially in rice. It can improve leaf erectness and mitigate various biotic and abiotic stresses. Cobalt is essential for N2-fixing plants because it is part of the coenzyme cobalamin (vitamin B12) that is important in nodule metabolism. Therefore, Co deficiency results in poor nodulation and low N2 fixation rates. The chemistry of Se is similar to that of sulfur (S), and Se can replace S, to some extent, in proteins, particularly in Se-hyperaccumulating plants. Selenium is essential for animals; therefore Se fertilization may be beneficial for human and animal health in areas with low Se soils. Aluminum is beneficial to some plants such as tea, but mechanisms of this beneficial effect are poorly understood. It may alleviate proton toxicity and increase the activity of antioxidative enzymes.