chapter  13
- Chlorine
Pages 12

Of the earth’s 92 natural elements, chlorine (Cl) is ranked as the eighteenth most abundant (Graedel and Keene 1996), and thus far it is the most prevalent anion found in Martian meteorites (Sawyer et al. 2000). Chlorine, or more correctly the chloride (Cl−), is classified as a micronutrient because the chlorine requirement for optimal growth is between 340 and 1200 mg kg−1 (Marschner 1995). However, its uptake by crop plants is equal to that of macronutrients (2000-20,000 mg kg−1) (Fageria, Baligar, and Clark 2002; Fageria 2009). Because of these two contrasting properties, chlorine is known as a unique element. It also is a nonmetal micronutrient like boron (B). In 1954, the importance of chlorine for plants was established using a tomato plant for the purpose of testing (Broyer et  al. 1954). Later on, the same group of scientists proved the importance of this element for corn, dry beans, alfalfa, barley, and sugarbeets. All these studies were conducted in nutrient solutions. Chlorine and nickel (Ni) are among the latest micronutrients discovered to be essential for higher plants.