Optimal combination ratio of NPK and organic fertilizers for highest saponin and flavonoid contents in Ophiopogon japonicas

Objectives Proper fertilization is necessary for high-quality herb cultivation and yield. We studied the optimum combination of N, P, K, and organic fertilizer (OF) in the artificial cultivation of Ophiopogon japonicus.

Methods Field experiments were conducted using a four-dimensional quadratic orthogonal rotation combination design. All the N, P, K, and organic fertilizers were applied in three times during growth of O. japonicus. Ophiopogonin D (OSD), ophiopogonin D′ (OSD′), methylophiopogonanone A (MONA), methylophiopogonanone B (MONB), and ophiopogonanone D (MOND) were measured after harvest. A model was used to analyze N, P, K, and organic fertilizer’s influence on the main constituents. We used the statistical frequency method to integrate various indicators to obtain the optimal fertilization plan.

Results The application of N, P, K, and organic fertilizers had significant effects on OSD, OSD′ and MONA content, but not on MONB and MOND. However, fertilizers influenced active constituents to different extent. Potash fertilizers mainly affected OSD′, MONA, and MONB contents, OF mainly affect OSD, and N mainly affect MOND. With the increase of K fertilizer, OSD content gradually increased, while OSD′ decreased first and then increased. The interaction of N × OF was beneficial to the accumulation of OSD; K × OF did not improve OSD′; N × P and K × OF increased MONA; and N × K, P × K, P × OF did not increase MONA. Application of N 307–368 kg/hm², P₂O₅ 23–27 kg/hm², K₂O 189–190 kg/hm², OF 2419–2534 kg/hm² could improve OSD to ≥ 140 μg/g, OSD′ to ≥ 75 μg/g, and MONA to ≥ 100 μg/g.

Conclusions The practical application of N, P, K, and organic fertilizer can increase saponin and flavonoid contents in O. japonicus, especially MONA and MONB. The optimum N-P₂O₅-K₂O-OF application ratio was 332-25-204-2616 kg/hm² for OSD, 339-25-199-2352 kg/hm² for OSD′, and 339-25-173-2400 kg/hm² for MONA.