Magnesium is a trace element that is a key component of chlorophyll (the green colouring of leaves) and deficiencies subsequently cause leaf bleaching. It has a high mobility, meaning it travels around the plant easily and is responsible for plant energy, accelerating enzymatic processes, accelerating plant growth and phosphate transport. There is an inverse relationship between magnesium and both calcium and potassium because an abundance of these elements prevent magnesium from being absorbed. Magnesium deficiency in plants appears as chlorosis (leaf yellowing) and dark spots where the leaf meets the stem. The most common way of adding additional magnesium to plants is with magnesium oxide (magnesite) or magnesium sulfate (epsom salts), both of which are generally a powder.
Salmon, R. (1963). Magnesium relationships in soils and plants. Journal Of The Science Of Food And Agriculture, 14(9), 605-610. https://doi.org/10.1002/jsfa.2740140901
Plant nutrients in the soil. Department of Primary Industries. (2022). Retrieved 10 February 2022, from https://www.dpi.nsw.gov.au/agriculture/soils/soil-testing-and-analysis/plant-nutrients.