Micronutrient deficiencies in rainfed calcareous soils of Pakistan. III. Boron nutrition of sorghum

Boron (B) deficiency in crops is a major micronutrient disorder particularly in alkaline‐calcareous soils. A nutrient indexing of rainfed sorghum [Sorghum bicolor (L.) Merr; cv. Potohar 4–8] in the Potohar plateau of Pakistan revealed B deficiency in 50% of the 140 sampled fields in Jehlum district and 115 fields in Chakwal district. Hot water extractable (HWE) B in the associated soils, however, indicated more widespread B deficiency. The 255 sampled fields represented 21 soil series: 100 belonging to Alfisols, 63 to Entisols, 50 to Inceptisols, and 42 to Aridisols. Boron fertility varied in various soil types; the minimum HWE B (0.25 mg/kg) was noted in Calci Ustochrepts and the maximum (0.69 mg/kg) in Fleventic Ustochrepts. Boron fertilization of an alkaline calcareous Typic Ustorthents (HWE B, 0.11 mg/kg) increased grain yield up to 32% in improved sorghum (cv. PARC‐SS‐1) and 29% in local sorghum (cv. Potohar 4–8) over the respective control yields. Fertilizer requirements for near‐maximum (95%) grain yield was 1.2 mg B/kg for the improved and 0.6 mg B/kg for local cultivar; higher fertilizer requirement of improved sorghum may be attributed to its greater biomass production. Critical plant tissue B concentration in improved sorghum was 17 mg/kg in whole shoots and 25 mg/kg in most recently matured leaves. Contrary to its lesser fertilizer requirement, internal B requirement of local sorghum was greater, i.e., 18 mg/kg in whole shoots and 31 mg/kg in leaves. The three soil tests for B, hot water, hydrochloric acid (HC1), and mannitol were almost equally effective in evaluating soil B fertility. Soil test critical B levels (mg/ kg) for improved sorghum were: hot water, 0.48; HC1, 0.43; and mannitol, 0.37. Contrary to its internal B requirements, soil test critical B levels (mg/ kg) were slightly lower for local sorghum, i.e., hot water, 0.45; HC1, 0.40; and mannitol, 0.36. Thus, local sorghum appeared more efficient in utilizing soil B. As soil test critical levels as well as fertilizer requirement were greater for improved sorghum, a much widespread B deficiency is expected in this cultivar if grown on alkaline calcareous soils of the Potohar plateau.