Immobilization of Zinc Fertilizer in Flooded Soils Monitored by Adapted DTPA Soil Test

Zinc (Zn) deficiency is a persistent problem in flooded rice (Oryza sativa L.). Severe Zn deficiency causes loss of grain yield, and rice grains with low Zn content contribute to human nutritional Zn deficiencies. The objectives of this study were to evaluate the diethylenetriaminepentaacetic acid (DTPA) extraction method for use with reduced soils and to assess differences in plant availability of native and fertilizer Zn from oxidized and reduced soils. The DTPA-extractable Zn decreased by 60% through time after flooding when the extraction was done on field-moist soil but remained at original levels when air-dried prior to extraction. In a pot experiment with one calcareous and one noncalcareous soil, moist-soil DTPA-extractable Zn and plant Zn uptake both decreased after flooding compared with the oxidized soil treatment for both soils. In the flooded treatment of the calcareous soil, both plant and soil Zn concentrations were equal to or less than critical deficiency levels even after fertilization with 50 kg Zn ha(-1). We concluded that Zn availability measurements for rice at low redox potentials should be made on reduced soil rather than air-dry soil and that applied Zn fertilizer may become unavailable to plants after flooding.