This study was conducted to evaluate the effect of humic acid and biofertilization by Nitrobein
and rhizobacterin on flowering, yield and fruit quality of Valencia orange (Citrus sinenses) trees budded on sour
orange rootstock and grown in a loamy sand soil under drip irrigation system at El-Kassasin Horticultural
Research Station Farm, Ismailia Governorate, Egypt during two successive seasons 2017 and 2018. Four
different doses of humic acid (0, 50, 100 and 150 g/ tree) and two types of biofertilizers i.e., nitrobein and
rhizobacterin were added as soil application through drip irrigation (on distance 100-150 cm from tree trunk and
30 cm depth around each tree) at three equal doses in February, April and June at the rate of 2 L/ fed. for
factorial experiment under two factors. The results indicated that all application of different levels of humic acid
alone or in combination with biofertilizers had a positive effect on increased flowering characteristics, fruit set
percentage and total yield as well as improved fruit physical and chemical characters. In addition, interaction
treatment between rhizobacterin and humic acid at 150 g/ tree was the most effective treatment in enhancing
flowering parameters (leafy inflorescences and flowering percentage on leafy and woody inflorescences) as well
as total yield, average fruit weight, total soluble solids (TSS), TSS/ acid ratio, vitamin C content and total sugars
and decreased the woody inflorescences %, percentages of both the total acidity and reducing sugars in fruit
juice. In addition, fruit set percentage, fruit juice weight and volume were the best on trees biofrtilized with
nitrobein and humic acid at 150 g/ tree in both seasons.