In the beginning

Chloride has long been suspected as a nutrient which was important for plant growth.   In the mid-1800's (Totingham, 1919) reported that barley topdressed with NaCl (common table salt) was helpful in stiffening straw, and that Cl was the active ingredient in this fertilizer.  Lipman (1938) concluded that if Cl was not essential for buckwheat growth, it was certainly highly beneficial.

The 1950's

The essentiality of Cl for plant growth was finally confirmed during the 1950's.  Broyer et al. (1954) is generally considered to be the first study to demonstrate a requirement for Cl in higher plants.   Growing Marglobe tomato plants in a hydroponic culture depleted of Cl,  they found growth (dry matter production) was retarded up to 65% compared to plants receiving adequate Cl.   Deficient plants resumed satisfactory growth after Cl was added to the nutrient solutions, or injected into the plants via a hypodermic needle. Deficiency symptoms were characterized by wilting of leafltets in the early stages, followed by chlorosis, bronzing, and necrosis (see photograph below) in areas proximal to the wilting.   Following this initial study, Cl requirement were demonstrated in number of other plant species, including alfalfa, barley,  bean, buckwheat, cabbage, carrot, corn, lettuce, potato, squash, subterraneum clover, and sugarbeet (Ulrich and Ohki,1956;  Johnson et al., 1957;  Ozanne, 1958; Gausman et al., 1958).

Marglobe tomato Leaflets exhibting Cl deficiency symptoms, i.e. curling and chlorosis (left) and normal growth (right). Hydroponic/growth room studies at the Montana State University-Plant Growth Center.

Post 1950's

For many years it was believed that field grown crops would not benefit from application of Cl containing fertilizers, or that Cl deficiency was unknown in the field.  This belief was  based on the fact that particular precautions were required by earlier investigators to induce deficiency symptoms in many plant species.  This included minimizing Cl contamination from the air, water, chemicals, and seed.  In addition, the ubiquitious presence of Cl in the environment (including soil, irrigation water, rain, fertilizers, and air pollution) was believed to make it unlikely that deficiency symptoms could be found in the field.   Oregon studies (Taylor et al., 1981; Christensen et al., 1981) provided some of the first evidence that field grown crops (e.g. wheat) could benefit from Cl fertilization. Later, spring wheat studies in South Dakota demonstrated yield responses to KCl fertilization were due to the Cl component in this fertilizer (Fixen et al., 1986a), and that soil and plant analysis were potentially useful tools in Cl response prediction (Fixen et al., 1986b).  Today, there appears to be little doubt that field grown crops can benefit from Cl applications.   However, our understanding of the processes, crop species, and environmental conditions under which these responses occur is still evolving.