Integrated Effect of Plant Growth-Promoting Compost and NPK Fertilizer on Nutrient Uptake, Phenolic Content, and Antioxidant Properties of Orthosiphon stamineus and Cosmos caudatus

Abstract

The effects of plant growth-promoting (PGP) compost, conventional fertilizer (NPK), and their combination on fresh and dry herb yield, nutrient uptake, total phenolic content (TPC), total flavonoids (TF), and antioxidant activity (DPPH and FRAP) of Orthosiphon stamineus and Cosmos caudatus were determined. The highest values for each measured variable of O. stamineus and C. caudatus were obtained when both herbs were cultivated in soils supplemented with PGP compost and NPK at halved rates. Considering the effect of treatments on herb nutrient composition, a marginal but significant rise was detected in macro- and micronutrient contents (N, P, K, Ca, Mg, Fe, Zn, and Mn) in both herbs grown in soils supplemented with PGP compost and NPK. TPC ranged from 121–172.6 mg GAE g−1 DW, TF from 95.4–125.4 catechin CE g−1 DW, FRAP from 42.2–58.3 mmol Fe2+equivalent g−1 DW, and DPPH from 185.1–279.4 μmol Trolox equivalent g−1 DW in O. stamineus. TPC, TF, DPPH, and FRAP activities in C. caudatus were 176.1 mg GAE g−1FW, 152.49 catechin CE g−1 FW, 174.86 μmol Trolox equivalent g−1 FW, and 154.68 mmol Fe2+ equivalent g−1 FW, respectively. It is suggested that the usage of PGP compost in combination with NPK at halved rates can reduce the indiscriminate use of mineral fertilizers in greenhouse soil and that a combination treatment had better effects in terms of yields and phytochemical composition in both of the herbs compared with conventional mineral fertilization and unfortified or fortified compost alone.