Residual Availability of Fertilizer Zinc

Residual availability of fertilizer Zn was measured over a period of 5 years. Three variables were evaluated: Zn rate, free carbonates, and wheat straw residue. Zinc was applied at 0, 5.6, 11.2, 16.8, and 22.4 kg Zn/ha as ZnSO 4 · H 2 O to a noncalcareous soil and a subsoil containing 4.3% lime. The residue variable was imposed by making three annual applications of wheat straw of 11.2 metric tons/ha. Measurements were made annually for extractable soil Zn and the utilization of Zn by sweet corn ( Zea mays L., ‘Golden Cross Bantam’ hybrid). Percentagewise, conversion of fertilizer Zn to forms unextractable with 0.005 M DTPA solution or 0.1 N HCl was comparable for all rates of Zn application. There was a relatively rapid conversion of Zn to nonextractable forms during the first year, followed by a declining rate during the second and third year. The percentage of Zn extractable was essentially unchanged during the fourth and fifth years. At equilibrium, 85 to 90% of the fertilizer Zn was unextractable with 0.005 M DTPA. In terms of a desirable Zn soil test of at least 0.8 ppm, a Zn application of 5.6 kg/ha was adequate for not more than 2 years, whereas 11.2 kg/ha was satisfactory for about 4 years. Neither extractable Zn nor Zn uptake by sweet corn indicated an adverse effect of free carbonates on residual availability. The effect of straw residue on residual availability of Zn was too minor to have significance in terms of fertilization practices.