Rice Response to Zinc Fertilization On a High pH, Low Zinc Soil.

Zinc (Zn) deficiency is considered as one of the most common micronutrient deficiencies in commercial rice production throughout the midsouthern United States. Plant availability of Zn can be affected by native soil levels and changes in soil pH. Mehlich-3 extractable Zn levels below 1 mg kg-1 are considered to be low in soil test Zn using current LSU AgCenter recommendations. Solubility and plant availability of Zn decrease with increases in soil pH. Commonly, Zn deficiencies in rice are found on soils that have pH above 7. Zinc deficiency can also be aggravated after flooding as the soil changes from an aerobic to anaerobic condition. Further research is needed to refine Zn fertilizer recommendations for rice on silt loam soils that have a high pH and are naturally low in soil Zn. The objectives of the current study were to: 1) determine the optimum Zn fertilization rate when zinc-sulfate is surface broadcast at planting and the field is not drained after the flood is established at the 4- to 5-leaf stage of development; 2) determine if the nitrogen (N) source used prior to permanent flood establishment affects the rice yield response to Zn fertilization; and 3) to evaluate the rice nutrient content as affected by Zn fertilization on a high pH, low Zn silt loam soil with a history of severe Zn deficiency. A trial was established in 2009 on a Crowley silt loam soil just north of Crowley, Louisiana. Mehlich-3 soil test Zn was 1 mg kg-1 and soil pH was 7.9. Five rates of Zn (0, 5.6, 11.2, 16.8, and 22.4 kg ha-1) and two N sources (urea and ammonium sulfate) were used. The trial was arranged in a randomized complete block design with four replications. Zinc sulfate was used as the Zn source and was surface broadcast immediately after drill-seeding Neptune rice into a stale seedbed. Fertilizer N was applied at a rate of 168 kg ha-1 immediately before permanent flood establishment. Ammonium sulfate was applied at 112 kg ha-1 to all plots prior to seeding. Aboveground biomass samples were taken from a 1-m section of the middle drill row just prior to permanent flood establishment and again at the 50% heading stage of development. Nutrient content of the samples was determined using a nitric acid-hydrogen peroxide digest and ICP analysis. Zinc fertilization rate had a significant (P<0.0001) effect on rice grain yield. Rice grain yield increased with increasing Zn and was optimized at the 16.8 kg ha-1 Zn application rate. Grain yield was not affected by N source or by the Zn rate by N source interaction. Tissue aluminum (Al), iron (Fe), calcium (Ca), magnesium (Mg), manganese (Mn), potassium (K), phosphorus (P), and Zn concentrations were all affected by Zn fertilization at the 4- to 5-leaf stage of development. In general, concentrations of Al, Fe, Ca, Mg, Mn, and P were greatest when no Zn fertilizer was applied and decreased with increasing Zn fertilization, while K and Zn increased with increasing Zn fertilization. Toxic levels of Al (1222 mg kg-1) and Fe (1992 kg ha-1) in the aboveground biomass were observed at the 0 kg ha-1 Zn rate. Tissue concentrations of Al, Fe, Ca, Mg, Mn, K, and P at the 50% heading stage of development were not significantly affected by Zn application rate. However, tissue concentrations of Zn increased with increasing rates of Zn fertilization. Results from the current study indicate that a rate of 18.8 kg ha-1 is needed to eliminate yield losses associated with Zn deficiency at this location when zinc sulfate is the Zn fertilizer source. Further research using other Zn fertilizer sources on other high pH, low Zn soils is needed to improve soil testing derived fertilizer recommendations.