Introduction

Stevia (Stevia rebaudiana Bertoni) is one of nearly 300 plant species belonging to the genus Stevia present all over the world, is known as the sweetest plant, sweet leaf, honey yerba, etc. It was originated from South America, officially discovered by Bertoni in 1905, belonging to the Asteraceae family (Das et al. 2007; Zaman et al. 2015b). Currently, the plant is cultivated in several countries including Japan, China, India, Korea, Taiwan, Philippines, Russia, Tanzania, Indonesia, Hawaii, Canada, Malaysia and Egypt (Rios and Recio 2005). It has become a popular natural source of high potency sweetener, dietary supplement and pharmaceutical products. For this reason, it has a significant impact on the economy of many countries (Debnath 2007; Huda et al. 2007; Zaman et al. 2015c).

The sweetness obtained from Stevia is 100 to 300 times sweeter than sucrose sugar which is extracted from sugar cane and sugar beet (Lemus-Mondaca et al. 2012). It has numerous health benefits against diabetes, hypertension, obesity, cancer, oxidative stress, and microbial infections (Ghanta et al. 2007). It is safe for diabetics and hypoglycemics being a healthy and natural sweetener (Ahmad et al. 2020; Kumar et al. 2013). Therefore, stevia represents a crop gaining much popularity among all types of sweeteners as a source of intensively sweet-tasting compounds, i.e., the steviol-glycosides. Stevia leaves contain steviol-glycosides mainly stevioside, rebaudioside A, rebaudioside B, rebaudioside C, and rebaudioside E, etc., and has not been observed any mutagenic, teratogenic, or carcinogenic effects, indicating it as an ideal substitute of sugar (Pól et al. 2007). Stevioside is one of the active constituents that have the largest share in stevia leaves (5–10% of dry weight basis) (Chumthong and Detpiratmongkol 2016; Das et al. 2008; Patil 2010).

Recently, researchers have been reported on using alternative nutrients to improve plant growth, development, and productivity under various environmental stresses. Elsheery et al. (2020a) have recommended that a combination of 100 mg L−1 nano-zinc oxide (nZnO) and 150 mg L−1 d nano-silicon (nSi) improves mango tree resistance, annual crop load, and fruit quality under salinity conditions. It has been recommended that a combination of 0.30% humic acid + 600 mg·L−1 boric acid to enhance the withstand environmental stresses and improve annual mango tree productivity and fruit quality (El-Hoseiny et al. 2020). The silicon dioxide (nSiO) showed higher amelioration effects and it can be used alone or in combination with other nanoribbons (zinc oxide, selenium, and graphene) to mitigate chilling stress in sugarcane (Elsheery et al. 2020b).

Stevia, being a vegetatively productive plant mainly needs nitrogen supplementation (Hasnain et al. 2020). Nitrogen is considered as an essential nutrient for foliage growth of plants (Hardjowigeno 2007). It is primarily available in the form of NO3− and NH4+, and plants show a strong preference for NO3− over NH4+ ions (Zhou et al. 2018). Oldfield et al. (2019) found that yields of maize and wheat crops were higher on average when the concentration of SOC was higher. These results are also in agreement with the findings of Zaman et al. (2015a) and Rashwan et al. (2017) in stevia plant, and Youssef (2016) in moringa plant.

The Ch-N along with EM significantly increased the P, K and chlorophyll contents of stevia. The reason might be the positive interaction of the combined application of Ch-N and EM with improved properties of stevia. Yousef and co-workers discovered that the use of a combination of inorganic NPK and biofertilizer was most effective in enhancing growth, yield and nutrient accumulation in jew’s mallow plants (Yousef et al. 2020). Similar results were reported by Das et al. (2007), Das et al. (2008), Liu et al. (2011), Kumar et al. (2013), and Enchev et al. (2018) in stevia plant.

The results showed that the content of stevioside in stevia leaves was significantly increased with the increase in biomass yield by applying Org-N along with EM. Stevioside accumulation in stevia leaf is the “mirror” to stevioside yield, which was considered as the final goal for every researcher (Hamad 2015). The combined application of Org-N and EM improved the vegetative growth of stevia, net photosynthetic ability, so that the photosynthetic product increased the amount of SOC and improved soil structure. Moreover, Chumthong and Detpiratmongkol (2016) found that the application of organic manure was suitable to increase different growth parameters of stevia and stevioside content in stevia leaves. Rashwan et al. (2017) found that the application of compost at 4.76 ton ha−1 + 95 kg ha−1 N as NH4NO3 with 3rd cutting was the best treatment for improving the yield and quality of stevia.

In this study, it was found that the combination of EM with Org-N or Ch-N fertilizer resulted in a significant reduction in soil pH compared to either EM or Org-N or Ch-N treatment alone (Fig. 5). The difference in pH response to different fertilization regimes could be explained by one of two mechanisms. First, organic fertilization could have affected soil pH due to the liming effect of organic matter and carbonates of organic fertilizer (Cooper and Warman 1997). Second, ammonium can lower soil pH by completing the exchange sites of the soil solid phases with base cations (Li et al. 1991). In addition, different fertilization regimes altered soil nutrient status to varying degrees in this study. EM with combined Org-N or Ch-N fertilizer applications as well as Org-N alone significantly improved SOC. We can attribute the improved SOC with organic fertilizer treatments to two mechanisms. Unlike other treatments, extracted soy protein has a high concentration of organic compounds that were readily biodegradable (Yanardağ et al. 2015). These results are in agreement with Yousef et al. (2020), who reported that the application of organic manure and its combination with biofertilizer increased the SOC at the end of the experimental period. In addition, organic manure could promote plant growth, resulting in increased input of SOC into the soil through the plants (Ding et al. 2012).

The results of the present study showed that the growth, yield and chemical composition of stevia plants were significantly affected by different nitrogenous fertilizers and effective microorganisms. Plant height, number of branches, fresh weight, dry weight, leaf area, dry leaf yield and stevioside content of stevia were found maximum under the influence of combined application of organic nitrogen and effective microorganisms. Soil properties like pH, EC, SOC, available N, P and exchangeable K content were also greatly improved by nitrogenous fertilizers and effective microorganisms. Considering all the parameters and treatments studied, it can be concluded that the application of organic nitrogen along with effective microorganisms is a promising approach to produce higher yield and improve the quality of stevia. Although the current study unfolded the performance ability of an isolated soybean protein as organic nitrogenous fertilizer to improving the productivity of the stevia crop, there is further need to understand the molecular mechanism behind it and improve the fertilization techniques and material according to the need for crops.