Early Research on Zinc Nutrition of Crops

Discovery of zinc as an essential element for higher plants was made by Sommer and Lipman (1) while working with barley (Hordeum vulgare L.) and sunflower (Helianthus annuus L.). However, Chandler et al. (2) stated that Raulin, as early as 1869, reported zinc to be essential in the culture media for some fungi, and speculated that zinc was probably essential in higher plants. Skinner and Demaree (3) reported on a typical Dougherty county pecan (Carya illinoinensis K. Koch) orchard in Georgia. Pecan trees that were placed in a study that started in 1918 increased in trunk diameter, but their tops had dieback each year, and their condition ‘appeared hopeless’ in 1922. Fertilizers (N, P, K), cover crops, and all known means were of no avail. Rosette, or related dieback, had been recognized since around 1900, but it was in 1932 before zinc was found to be the corrective element (4,5). The common assumption among pecan growers was that a deficiency of iron was responsible for rosette as pecans were brought into cultivation in the early 1900s. Alben used 0.8 to 1.0% solutions of FeCl₂ and FeSO₄ in his rosette treatments in 1931 and obtained conflicting results.

The 1932 treatments included injections into dormant trees, soil applications while the trees were dormant and after the foliage was well developed, and foliar spraying and dipping. The only favorable results were obtained when Alben mixed the iron solutions in zinc-galvanized containers. Analysis proved that the solutions contained considerable quantities of zinc. These experiments led to the use of ZnSO₄ and ZnCl₂ solutions, which permitted normal development of new leaves. Satisfactory results were obtained with trees located on alkaline or acid soils. The most satisfactory results were obtained with a foliar spray of 0.18% ZnSO₄ and a 0.012% ZnCl₂ solution. Roberts and Dunegan (6) also observed a bactericidal effect when using a ZnSO₄-hydrated lime mixture that controlled bacterial leaf spot (Xanthomonas pruni), which later became a serious pest for susceptible peach (Prunus persica Batsch.) cultivars like 'Burbank July Elberta' in the 1940s, 'Sam Houston' in the 1960s, and 'O-Henry' in the 1990s (personal experience). Hydrated lime was necessary to prevent defoliation of peach trees by ZnSO₄ toxicity.