Toxic limits of replaceable zinc to corn and cowpeas grown on three Florida soils

Soluble zinc compounds are used under a variety of conditions in Florida to prevent or correct malnutrition of a number of plants caused by a deficiency of available zinc. The zinc accumulates in the soil either in the mobile replaceable form or in definite water-insoluble forms. The former is known to be toxic to plant growth when it accumulates in sufficient quantities; the latter are seldom toxic to plant growth. In order to establish the toxic limits of zinc for certain plants and to determine methods of preventing or alleviating this toxicity, three series of greenhouse cultures were employed. A simple but expedient method is described for preparing greenhouse cultures for the above purpose with a Norfolk sand, an Orangeburg fine sandy loam, and a Greenville clay loam, giving ranges of replaceable zinc in the soils. The principal feature of the technic is the use of soils which had been previously saturated with replaceable zinc and washed free from soluble salts, combined in varying proportions with the corresponding air-dried untreated soils to give a suitable range in concentration of replaceable zinc in each soil. Two crops of corn and cowpeas as test plants were grown for four weeks each in rotation. The weight of the dry plants from each culture was obtained. The results of the tests may be summarized as follows: 1. Replaceable zinc became toxic to corn on a Norfolk sand between the concentrations of 0.688 and 1.376 M.E. per 100 grams (451-902 pounds Zn per acre); between 0.758 and 1.137 M.E. (497-734 pounds Zn per acre) on an Orangeburg fine sandy loam; and between 1.615 and 2.153 M.E. (1,051-1,402 pounds Zn per acre) on a Greenville clay loam. 2. Replaceable zinc became toxic to cowpeas between the concentrations of 0.275 and 0.482 M.E. per 100 grams (181-316 pounds Zn per acre) on a Norfolk sand; between 0.379 and 0.758 M.E. (246-479 pounds Zn per acre) on an Orangeburg fine sandy loam; and between 0.538 and 1.077 M.E. (351-701 pounds per acre) on a Greenville clay loam. 3. The application of mono-calcium phosphate at the rate of 233 pounds per acre to the Norfolk sand and 1,800 pounds per acre to the Orangeburg fine sandy loam and the Greenville clay loam did not change the toxic limits of the replaceable zinc for corn or cowpeas. The presence of phosphate as a plant nutrient, however, stimulated the growth of corn and cowpeas on the Orangeburg fine sandy loam and the Greenville clay loam, while no effect was noted on the Norfolk sand. 4. The use of calcium carbonate at the rate of 1,000 pounds per acre on the Norfolk sand and 4,000 pounds per acre on the Orangeburg fine sandy loam and the Greenville clay loam definitely increased the concentrations at which replaceable zinc became injurious to corn and cowpeas and greatly alleviated the toxic condition.