ABSTRACT Farmer’s perception of nutrient management for onion bulb production often ignores the soil test basis (STB) which provides essential base line information on the nutrient status of soils. A field experiment on onion (cv. BARI Piaj-4) was conducted during three consecutive years 2016-17, 2017-18 and 2018-19 at the research field of Spices Research Centre (level Barind Tract, AEZ 25), BARI, Shibganj, Bogura, Bangladesh to determine the optimum doses of Nitrogen (N), Phosphorus (P), Potassium (K), Sulphur (S), Zinc (Zn) and Boron (B) for maximization of bulb yield of onion. The experiment was arranged in a Randomized Complete Block Design (RCBD) with three replications having six different levels of fertilizer doses i.e. T1 (Control), T2 (STB dose), T3 (STB + 15% extra of STB), T4 (STB + 30% extra of STB), T5 (STB + 45% extra of STB) and T6 (Farmer’s practice). The nutrient management treatments positively influenced plant height, leaf length, number of leaves plant-1, bulb diameter, marketable yield and total yield per hectare. The best results with regard to number of leaves plant-1 (5.98), bulb length (7.21 cm), bulb diameter (5.52 cm), single bulb weight (43.07g) and bulb yield (19.83 tha-1) were obtained from the treatment T5. Considering the yield contributing parameters in onion production, the most suitable nutrient management package was STB + 45% extra of STB but the highest marginal benefit cost ratio (7.60) was obtained in T2 (STB dose). N, P, K, S, Zn, B doses of 120, 54, 78, 30, 4.2, 1.5 kg ha-1, respectively is recommended for maximum onion bulb production in this location.

Key words: Yield, onion, nutrient.

Introduction
Onion (Allium cepa L.) is one of the most important spices as well as vegetable crops in Bangladesh. The crop was originated somewhere between western China and deserts lying east of the Caspian Sea (Jones and Mann, 1963). In Bangladesh, the area of onion is 1.76 lakh hectare and the total production is 17.35 lakh metric tons with an average yield of 9.86 t/ha (BBS, 2018) which is low compared to several major onion growing countries. The average yield of onion in the USA is 41.12 t/ha and India is 12.5 t/ha. At present the shortage of onion in Bangladesh is about 8-9 lakh metric tons per year. So, huge amount of onion is needed to import in each year to meet up the demand investing foreign currency. Nutrients supplied in the form of fertilizers are one of the most important inputs for increasing the productivity of onion crops (Anon., 1997). In order to obtain good yield, high responsive modern varieties of the onion crop require relatively high quantity of fertilizer compared to the traditional cultivars. Generally farmers in Bangladesh use a higher amount Nitrogen (N), Phosphorus (P), Potassium (K), Sulphur (S), Zinc (Zn) and Boron (B) fertilizers for onion bulb production at Meherpur, Chittagong and Rajshahi regions. The current average dose of these nutrients in a representative farmers field is N=345, P=150, K=135 kg/ha supplied from fertilizers, which is much higher than the general recommended dose (N=120, P=54, K=75 kg/ha). Moreover, the socio economic conditions of the farmers and benefit cost scenario of fertilizer usage are often ignored during nutrient dose recommendation. So, the present study has been designed to evaluate the response of onion to higher doses of nutrients and to determine the profitability of the same.

Materials and Methods
The experiment was carried out at the research field of Spices Research Centre, Shibganj, Bogura, Bangladesh during three consecutive years 2016-17, 2017-18 and 2018-19 to determine the optimum dose