Dilute hydrochloric acid as an extractant for available zinc, copper and boron in rice soils

Abstract

Because zinc deficiency is a widespread disorder of wetland rice and copper deficiency may occur with it, 0.05M HCl was compared with 0.1M HCl, EDTA and DTPA as an extractant for available zinc and copper. It was also compared with the reflux method for boron assay. Thirty-three wetland rice soils were analyzed for zinc and copper by the four methods. Rice was grown on the flooded soils, scored visually for zinc deficiency, and the plants analyzed for zinc and copper content. In the boron study, 53 soils were extracted by the reflux method and the 0.05M HCl procedure. Rice was grown on the flooded soils, scored visually for boron toxicity, and the plants analyzed for boron content. Fourteen of the 16 soils on which rice showed zinc deficiency gave <1.0 mg/kg Zn by the 0.05M HCl method but values far in excess of the critical limits by the other methods. The r values for available and plant zinc were: 0.05M HCl (0.88**); 0.1M HCl (0.55**); EDTA (0.43**); and DTPA (0.31ns). Twelve of the zinc-deficient soils gave<0.1 mg/kg Cu by the 0.05M HCl method but values exceeding the critical limits by the EDTA and DTPA methods. The r values for available and plant copper were: 0.05M HCl (0.74**), 0.1M HCl (0.64**), EDTA (0.28ns), and DTPA (0.20ns). The critical limit of 1.0 mg/kg by the 0.05M HCl extraction was confirmed for zinc deficiency and a tentative value of 0.1 mg/kg for copper deficiency proposed. The 0.05M HCl method separated boron-toxic soils from non-toxic soils and gave a better correlation (r=0.91**) between available and plant boron than the reflux extraction (r=0.84**). The toxic limit by the 0.05M HCl method was provisionally set at 4 mg/kg.