This work wasexecuted during two successive seasons of 2018 and 2019 in the nursery and laboratory of Fac. Agric., Minia Univ., Egypt to investigate the response of vegetative and root growthand some chemical composition of Delonix regia seedlings to four percentages of humic acid (0, 1, 2 and 4%) and four bio- and/or mineral NPK fertilization treatments (control, 100% NPK, Minia Azotein and 75% NPK plus Minia Azotein), as well as, the interaction between them. All used three percentages of humic acid significantly increased vegetative growth (plant height, stem diameter, the aerial part dry weight), root growth (root number and roots dry weight) and chemical composition (photosynthetic pigments, NPK%, total phenolic and total flavonoids contents) over control. Humic acid at 4% more was effective treatment. All used bio-and/or mineral NPK fertilization treatments significantly increased all studied characters over control. The best treatments were 100% NPK or 75% NPK plus Minia Azotein.
It could be recommended to add humic acid at 4% in combination with 100% NPK or 75% NPK plus Minia Azotein to improve growth of Delonix regia.