1) Direct sowing of seed without making nursery cause bolting & doubling 

2)  Transplating nursery of more than 45 days is major cause of boling 

3)    If the distance between  two seed ling  is more and fertlizer doses are even slight excess bolting 

          & doubling will arise 

• Onion requires well drained loamy soils, rich in humus, with fairly good content of potash
• An ideal soil should have pH in between 6.5 to 8
• It prefers moderate temperature in summer as well as in winter