Meta-Analysis of the Impact of Soil Fertilizers on Quantitative and Quality Yield of Medicinal Plants: From Past to Now

Meta-analysis is a comprehensive statistical analysis method for multiple independent experiments under the same subject. In the agricultural research field, the technique is mainly used for the comprehensive study of trials to explore the response characteristics of the main treatments (11). Due to the dispersions and differences in the results of studies on the effect of fertilizers on the growth and quantitative and qualitative yield of important medicinal species, in this study, meta-analysis method was used.

We conducted a global meta-analysis to evaluate the effect of various fertilizers on quantitative yield and quality of some medicinal plants including psyllium, fennel, basil, savory, dill, black seed and cumin using data obtained from peer-reviewed publication during past 50-year based on a ROSES protocol. So, 78 studies were selected that they representing 18 studies on psyllium, 8 studies on fennel, 12 studies on basil, 11 studies on savory, 7 studies on dill, 9 studies on black seed and 13 studies on cumin. Hedges effect size, publication bias (by using Begg and Mazumdar correlation) and Kendall's Tau coefficient were computed and funnel graphs were drawn by using Comprehensive Meta-Analysis software, version 2.

Based on the results, the highest effect size was related to the impact of humic acid on dried weight of stigma (g=3.434). After humic acid, biofertilizer, chemical fertilizer and compost were computed in the next ranks. Based on the Hedges effect size, vermicompost (g=1.106) had the highest impact on psyllium yield of and the next rank was computed for integrated fertilizers (g=0.061). Different fertilizers had not significant effect on yield of fennel and dill. The results revealed that bio-fertilizer (g=0.553) had significant effect on yield of black seed. Also, cumin yield was significantly affected by integrated and chemical fertilizers. Effect size values for manure, vermicompost and integrated fertilizers on mucilage percentage of psyllium were computed significant impact with 0.6, 0.9 and 0.7, respectively. Manure, bio-fertilizer and integrated fertilizers were also significant on essential oil content of fennel, black seed and cumin. The results of Begg and Mazumdar correlation revealed that the impact of chemical fertilizer was heterogeneous on cumin and these fertilizers are not recommended for the medicinal plant species. The effect of chemical fertilizer was calculated as homogeneous on leaf dried weight of basil. In other cases, the application of biofertilizer, vermicompost and manure are homogeneous and recommended for these crops with 95% confidence interval.

Totally, there was high residual heterogeneity in meta-analyses of soil fertilizers on different species of medicinal plants, suggesting that a large amount of further research with detailed analysis of soil physical, chemical and biological properties is also required to fully understand the influence of fertilizer treatments on growth, quantitative yield, and quality oil yield. This meta-analysis quantified the growth and yield of medicinal plant affected as different fertilizers based on available scientific evidence, providing a basis for improvement of medicinal plants management.