Evaluation of chemical extractants for predicting lowland rice response to zinc in highly calcareous soils

Zinc (Zn) deficiency has been observed in lowland rice (Oryza sativa L.) in Iran. Relatively high PH and abundant calcium carbonate of soils are responsible for Zn deficiency in the plant. However, the Zn status in Paddy soils of Iran has not been reported. Therefore, available Zn in 20 highly calcareous soils with a wide range of chemical properties was evaluated with DTPA-CaCl2, EDTA-(NH4)(2)CO3, DTPA-NH4HCo3, EDTA, and Mehlich 3, using rice as a test crop. The results,showed that extraction of Zn decreased in the order: Mehlich 3, EDTA, DTPA-NH4HCO3, EDTA-(NH4)(2)CO3, DTPA-CaCl2. A significant positive correlation was found between Zn extracted by all chemical extractants with the exception of EDTA. The highest correlation was observed between DTPA-CaCl2 and EDTA-(NH4)(2)CO3 extractable Zn. Application of Zn significantly increased mean top dry weight, Zn concentration, and Zn uptake by rice. Apparent Zn recovery of applied Zn was quite low in all soils, probably due to conversion of Zn to unavailable forms. The critical level of soil available Zn with DTPA-CaCl2, EDTA-(NH4)(2)CO3, DTPA-NH4HCO3, EDTA, and Mehlich 3 was 0.30, 0;70, 0.76, 2.2 and 2.5 mg kg(-1), respectively. Plant dry matter and Zn uptake were significantly correlated with DTPA-CaCl2, EDTA-(NH4)(2), DTPA-NH4HCO3 and Mehlich 3 extractable Zn. However, Zn extracted by EDTA-(NH4)(2)CO3 and DTPA-CaCl2 best explained the variation in plant dry matter, whereas DTPA-NH4HCO3 extractable Zn was more appropriate to predict the variation in Zn uptake by rice. Moreover, inclusion of pH and ECe in a multiple regression equation led to the best prediction of top dry weight and Zn uptake by the EDTA-(NH4)(2)CO3 Soil test. It seems that EDTA-(NH4)(2)CO3 is the most suitable chemical extractant to differentiate the Zn responsive paddy soils from nonresponsive ones in view of its high statistical relationship with the rice growth parameters.