MICRONUTRIENT DEFICIENCIES IN RAIN-FED CALCAREOUS SOILS OF PAKISTAN .4. ZINC NUTRITION OF SORGHUM

Zinc (Zn) deficiency in crops is a major micronutrient disorder partic ularly in alkaline-calcareous soils like those of the rainfed Potohar plateau in Pakistan. A nutrient indexing of sorghum (cv. Potohar 4-8) by sampling less than or equal to 30 cm tall whole shoots and associat ed soils from 255 random field locations revealed that the crop was de ficient in Zn in 54% fields in Jehlum district and 64% in Chakwal. In a greenhouse experiment using a Zn-deficient calcareous Typic Ustorthe nts, maximum increase in grain yield with Zn fertilizer was 177% over control in improved sorghum variety (cv. PARC-SS-1) and only 10% in lo cal sorghum (cv. Potohar 4-8). Although biomass production of cv. PARC -SS-1 was much greater compared with cv. Potohar 4-8, fertilizer Zn re quirement for the two cultivars was not much different, 8.3 mg Zn/kg s oil for improved sorghum variety and 7.3 mg Zn/kg for local sorghum va riety. Contrary to its higher sensitivity to Zn deficiency, the improv ed sorghum variety was more efficient in utilizing fertilizer Zn. Desp ite low Zn availability in the Potohar fields, local sorghum is not ex pected to respond to fertilizer Zn. However, adequate Zn fertility mus t be assured for cultivating improved sorghum in these soils. Zinc con tent in mature grains of sorghum proved a good index of soil Zn fertil ity status. Internal Zn requirement in foliar plant parts of cv, PARC- SS-1 (whole shoots, 33 mg/kg; leaves, 22 mg/kg) was greater than in cv . Potohar 4-8 (whole shoots, 27 mg/kg; leaves, 20 mg/kg). In contrast, critical Zn content in grains of the improved sorghum variety (10 mg/ kg) was lower than of local variety (14 mg/kg), Three soil tests were equally effective in determining soil Zn fertility, Critical soil Zn l evels for cv. PARC-SS-1 were: DTPA, 3.4 mg/kg; AB-DTPA, 3.7 mg/kg; and Mehlich 3, 8.0 mg/dm(3). Similar to internal Zn requirement in foliar plant parts, soil test critical Zn levels were lower for cv, Potohar 4-8, i.e., DTPA, 3.1 mg/kg; AB-DTPA, 3.5 mg/kg; and Mehlich 3, 7.2 mg/ dm(3). Because of their better efficiency, 'universal' soil tests appe ar superior to the DTPA test for routine Zn analysis.