Classification of Minerals Based on Their Quantity

Based on how much of each element a plant tissue actually needs, essential minerals fall into three groups (Table 12.1). Macronutrients are needed in comparatively large amounts - more than about 10 mmole per kg of tissue, or 0.1 to 10 mg per gram of dry weight - and include carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium and sulphur. Micronutrients are needed in much smaller amounts - less than 10 mmole per kg, or 0.1 mg or less per gram of dry weight - and include iron, manganese, copper, molybdenum, zinc, boron, chlorine and nickel. A third group, unclassified minerals, does not meet the strict essentiality criteria for plants generally but is required by some plants in trace amounts for specific roles: sodium, cobalt, silicon and selenium fall here. Silicon is a good example of why the unclassified category still matters agriculturally - in horsetails (Equisetaceae), sed …