A field experiment was carried out in the micronutrient experimental field, Soil Science Division of the Bangladesh Agricultural Research Institute (BARI), Joydebpur, Gazipur on 09 December, 2019. There were 16 treatment combinations comprising 4 levels each of boron (0, 0.01, 0.02 and 0.05%) and zinc (0, 0.01, 0.02 and 0.05%). Sixteen different treatments viz. T1 = B0Zn0, T2 = B0Zn0.01, T3 = B0Zn0.02, T4 = B0Zn0.05, T5 = B0.01Zn0, T6 =B0.01Zn0.01, T7 = B0.01Zn0.02, T8 = B0.01Zn0.05, T9 = B0.02Zn0, T10 = B0.02Zn0.01, T11 = B0.02Zn0.02, T12 = B0.02Zn0.05, T13 = B0.05Zn0, T14 = B0.05B0.01, T15 = B0.05Zn0.02, T16 = B0.05Zn0.05. The treatments were arranged in a randomized block design (Factorial) with three replications.
The tested variety was BARI Piaz-4. The unit plot was 2m×2m. Healthy and equal sized twenty eight days age seedlings of onion were transplanted in line with spacing of 15 cm row to row and 15 cm seedling to seedling. N120, P50, K110, S35 kg ha-1 were supplied from urea, TSP, MoP and gypsum respectively. All PKS and 1/3 of N were supplied at the time of final land preparation. The remaining two third of N were applied as top dress at 30 and 60 days after transplanting. Irrigation was applied in three times. Other intercultural operations were done as and when necessary. The source of micro nutrient for boron and zinc were boric acid and zinc sulphate, respectively. Onion plants were sprayed three times with micro nutrients at monthly intervals starting from 30 days after transplanting. Ten plants were selected from each plot as a unit for all observations on growth and yield. Based on the net plot yield, yield per hectare was calculated and expressed in tones per hectare. Total soluble solids (TSS) were determined by using hand held refracto meter and expressed in degree brix.
Methods of chemical analysis: Initial soil samples collected from 0-15 cm depth prior to fertilizer application, were analyzed for all important soil parameters using standard procedures. Standard methods were used in these determinations. Soil pH was measured by a combined glass calomel electrode (Jackson, 1958). Organic carbon was determined by the wet oxidation method (Walkley and Black). Total N was determined by a modified Kjeldahl method. Calcium (Ca), magnesium (Mg) and K were determined by NH4OAc extractable method, copper (Cu), iron (Fe), manganese (Mn) and zinc (Zn) were determined by DTPA extraction followed by AAS reading. Boron (B) was determined by CaCl2 extraction method. Available P was determined by the Bray and Kurtz method while S was determined using the turbidimetric method with BaCl2.
Foliar application:To study the effect of boron and zinc as foliar spray on yield of onion, four levels of boron (0, 0.01, 0.02 and 0.05%) and 4 levels of zinc (0, 0.01, 0.02 and 0.05%) were applied at different growth stage of onion. Foliar sprays were applied using a hand sprayer. All treatments were applied as a foliar spray on plants using hand operated compressed air sprayer at the 10 liter/plot. The time of foliar spray was 8:00A.M. Five plants were randomly selected from each and every treatment for recording necessary yield data such as plant height (cm), bulb length (cm), bulb diameter (cm), bulb weight (g) and yield (t ha-1). Fruit quality were tested for total soluble solid (TSS) and vitamin C. Data were analyzed statistically and means were compared by Tukey’s multiple comparison test using STATISTIX-10.
Total soluble solid and ascorbic acid content: Onion bulb from each treatment were cut into small slices and pooled. Samples were homogenized in a blender and portion of the homogenate were taken to determine the bulb quality. Total soluble solids content (TSS) was determined at 200 C with a refractometer and reported as 0Brix.Ascorbic acid contents (vitamin C) was measured by classical titration method using 2,6-dichlorophenol indophenols solution and express as mg 100 g-1of fresh weight (Miller, 1998).