The rare earth element calcium is one of the most abundant elements in the lithosphere; it is readily available in most soils; and it is a macronutrient for plants, yet it is actively excluded from plant cytoplasm.
In 1804, de Saussure showed that a component of plant tissues comes from the soil, not the air, but it was considerably later that the main plant nutrients were identified. Liebig was the first person to be associated strongly with the idea that there are essential elements taken up from the soil (in 1840), although Sprengel was the first person to identify calcium as a macronutrient in 1828 (1). Calcium was one of the 20 essential elements that Sprengel identified.
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