Two field experiments were carried out during 2016 and 2017 consecutive seasons at
a Private Farm at Mit-Ghamr Distrect, Dakahlia Governorate, Egypt, to study the effect of humic acid
rates (0.0, 325.60, 651.2 and 976.80 g/fad.), NPK fertilization rates [0.0, 25, 50, 75 and 100% of the
recommended rate (RR)] and their interactions on growth, leaves and herb yield and some chemical
constituents as well as active ingredients of stevia plants. The obtained results referred that the
maximum value for each of plant height, branch and leaf number/plant, total dry weight/plant, dry
weight of leaves and herb/fad., total N, P, K and total sugar percentages as well as total chlorophyll
content (a+b) were detected when plants were applied with the highest rate of humic acid (976.80
g/fad.) and fertilized with 75% RR of NPK, in most cases. The main constituents of Stevia rebaudiana
leaves as detected by high performance liquid chromatography-mass spectrometry (LC-MS) were
Rebaudioside A, Rebaudioside B, Rebaudioside C, Dulcoside and Steviolbioside which increased with
treatment of 100% RR of NPK fertilizers combined with the highest rate of humic acid (976.80 g/fad.)
compared to control.