Visual, Tissue, and Soil Factors Associated with Zn Toxicity of Peanuts

Zinc (Zn) deficiency of peanuts (Arachis hypogaea L.) has not been a production problem, but many areas show possible Zn toxicity where large amounts of Zn have been applied. Field and greenhouse studies and a survey were conducted to obtain data concerning visual symptoms, tissue concentrations, and soil test information about Zn toxicity of peanuts. The greenhouse experiment had treatments ranging from 0 to 50 mg Zn per kg of soil (Plinthitic Paleudult; fine loamy, siliceous, thermic). Zinc content of field soil (Arenic Paleudult; loamy, siliceous, thermic) to a depth of 90 cm, resulting from Zn fertilizer applications 5 years previously, were determined. Data concerning soil and tissue Zn concentrations were gathered, and visual toxicity symptoms were noted. The survey was undertaken to characterize soil and tissue Zn concentrations at locations where the problem is now occurring. Appearance of acute Zn toxicity occurred 14 to 120 days after emergence. Acutely affected plants exhibited chlorosis, stunting, purple coloration of the main stem and petioles, usually a lesion at the base of the plant, and premature necrosis in severe cases. Affected areas usually were only a fraction of a hectare, with necrotic or severely stunted plants in the center, and distinct borders. Tissue Zn concentration, in both the greenhouse and field, increased 15 mg/kg for each mg/kg increase in acid-extractable soil Zn. Tentative Zn toxicity critical values were 12 and 220 mg/kg for soil (acid-extractable) and tissue, respectively. Liming was effective in reducing the concentration of Zn in tissue and overcoming reduction in dry matter yield due to excessive uptake of Zn but lime did not change the acid-extractable level of Zn in soil. Zn uptake was reduced by increasing soil pH without changing extractable soil Zn. These data show that predicting Zn toxicity from values of soil Zn alone could give erroneous results. Also, an adequate liming program should reduce the problem. Acid Southeastern soils were found to retain appreciable Zn in the plow layer. Thus Zn should be cautiously applied to crops in rotation with peanuts or when planting peanuts in areas where large quantities of Zn may have been applied in the past.