How Does Compost Amendment Affect Stevia Yield and Soil Fertility? ^†

Abstract

Stevia (Stevia rebaudiana) is a plant species belonging to the family Asteraceae. It contains natural intense sweeteners while presenting low carbohydrate content. Its insignificant effect on blood glucose makes its consumption possible for diabetic and hypoglycemic diets. Chemical fertilizers have a negative impact on the stevia leaves quality. Organic farming has become essential in producing medicinal plants such as Stevia to boost stevia growth with stevioside and rebaudioside-A content. The purpose of this study was to evaluate the effect of four compost doses, i.e., 0, 10, 20, and 30 t/ha on Stevia yield and soil fertility. This experiment was conducted in the Melk Zhar experimental domain, Belfaa of the National Institute of Agronomic Research (INRA), Agadir, Morocco during the period of February-June 2020. The obtained results revealed that increased compost doses significantly increased yield and soil fertility i.e., phosphorus; P, potassium; K and organic matter; OM contents. The highest yield (2.19 t/ha) was observed under treatment using 30 t/ha of compost followed by 20 t/ha (1.67 t/ha), and the lowest (1.50 t/ha) under control conditions. The highest OM content (1.02%) was found under treatment with 30 t/ha of compost while the lowest (0.85%) was under the control. The soil analysis also showed that the application of compost at 20 t/ha resulted in the highest P (28.68 ppm) and K (125.5 ppm). In the light of finding, it is concluded that the application of compost at 30 t/ha is the most effective recommended rate for improving Stevia yield and soil fertility.

1. Introduction

Stevia (Stevia rebaudiana Bertoni) is native of South American (Paraguay and Brazil), however, its cultivation as medicinal plant is spreading across the world. Worldwide, there are more than 100 species of Stevia are available. It produces sweet molecules called steviol glycosides (SG) [1]. The most important steviol glycosides in stevia leaf are stevioside (5–10% of dry leaf weight), which is approximately 300 times sweeter than sugar, and rebaudioside A (2–4%), which is better suitable for usage in food and beverage than Stevioside owing to its pleasant flavor [2].

In conventional agriculture systems, the application of agrochemicals has increased, which is resulting in increased residual content of those chemicals in therapeutic medicines. There is an increased danger of health risks and environmental issue because of the indiscriminate use of chemical fertilizers and pesticides. Compost and other organic fertilizers, when applied, can increase nutrient availability of the crops and assist to improve soil health. Organic fertilizers are good alternatives to inorganic fertilizers since they increase soil quality. One of the most recent breakthroughs is the adoption of organic fertilizers, which delays nitrification for an extended period and boosts soil fertility. The use of organic fertilizers in conjunction with integrated nutrition management has been shown to boost Stevia output [3]. Further, the use of organic fertilizers such as compost is beneficial in reducing environmental concerns caused by chemical fertilizers. Thus, the objective of this study was carried to determine the effect of various levels of compost on Stevia yield and soil fertility under arid climate of Souss Massa region in Morocco.

2. Material and Methods

2.1. Experimental Site and Design

This experiment was conducted in the Melk Zhar Experimental station, Belfaa of the National Institute of Agronomic Research (INRA), Agadir, Morocco (30°2′33″ N; 9°33′4″ W) The physico-chemical properties of the soil are presented in

Table 1. Physic-chemical characteristics of the soil of the experimental plots at the Melk Zhar experimental station.

Parameters	2020
Electric conductivity (dS/m)	0.10 ± 0.01
pH	8.24 ± 0.18
Clay (%)	8.80 ± 1.20
Fine silt (%)	4.73 ± 1.52
Coarse silt (%)	3.74 ± 1.75
Fine sand (%)	60.54 ± 1.65
Coarse sand (%)	22.45 ± 1.74
Organic matter (%)	0.86 ± 0.42
Potassium (ppm)	687.3 ± 765.25
Calcium (ppm)	23,548.5 ± 6258.25
Sodium (ppm)	1235.3 ± 525.68

. The study focused on the growth of Stevia under four different levels of compost (0, 10, 20, and 30 t/ha) during the period of February–June 2020. These treatments were implemented in a randomized complete block design containing 4 plots with an area for each of (62 m × 34 m = 2108 m²). The planting density is approximately 80,000 plant/ha.

2.2. Observations Recorded

2.2.1. Analytical Methods

Soil samples were collected from four plots at 25 cm deep using auger. The soil samples were spread and air dried for 48 h in the laboratory. Then, it was passed through a 2 mm mesh sieve to remove the coarse materials and retained only the fine soil to analyze available phosphorus (P), exchangeable potassium (K), and organic matter (OM).

Table 2. Methods used for soil fertility analyses.

Element	Method
Phosphorus	Olsen [4]
Potassium	Flame spectrophotometer [5]
Organic matter	Spectrometry [6]

shows the different methods used for the analysis of these elements.

2.2.2. Yield Parameters

To evaluate the effects of different doses of compost on yield (dry leaf yield), the plants from each plot were manually harvested 10 cm above the base of the stem at the end of the growth season (June 2020). Leaf samples were collected just before blooming to analyze steviol glycoside concentrations in the leaves, at this stage steviol content are at their highest [7]. Once cut, the fresh plants were dried immediately. The drying was carried out under natural conditions under the sun using a delta tunnel 9 for a period of 2 to 5 days depending on the climatic conditions.

3. Results

The analysis of variance statistical analysis showed a significant (<0.001) difference among the four compost treatments in the yield and P content, while no differences among the three doses and the control in OM and K levels were noted (p < 0.000). This indicates that compost affected yield and P content and did not affect K and OM contents. The results obtained (Figure 1a) showed that the highest yield (2.19 t/ha) was obtained after treatment with a compost dose of 30 t/ha followed by 20 t/ha (1.67 t/ha), and the lowest yield (1.50 t/ha) was observed under control conditions. Regarding soil fertility (Figure 1b), the highest OM content (1.02%) was found with 30 t/ha of compost, and the application of 20 t/ha resulted in the highest values of the remaining nutrients in the soil after harvest, equal to 28.68 and 125.5 ppm for P and K contents, respectively.

4. Discussion

Chemical fertilizers increased food production while posing a cost to the climate. Furthermore, Stevia produced through the application of chemical fertilizers has been linked to health issues according to some customers. As a result, research communities are searching for alternatives to agrochemicals [8]. Organic amendments such as composts, are an effective alternative to inorganic fertilizers and improve the production and development of Stevia in a sustainable way [9]. Compost enhanced the OM and nutritional content such as P and K of the soil, which had a good influence on Stevia yield, as had been reported in other crops [10]. Stevia has reduced nutritional requirements and can be easily adapted in poor soil quality [11]. On the other hand, a nutrient deficit can be harmful [12]. The higher yield of Stevia with compost might be due to better nutrient availability near the root zone. In fact, it is well recognized that increased productivity due to compost application helps to reduce the use of artificial fertilizers and hence boost the sustainability of stevia production [13].

5. Conclusions

The impact of compost on a crop is observed in the medium term and thereafter, concluding that recommendation of one of the three doses over the others is not supported for this first year of testing. The finding indicates clearly that Stevia responded positively to increased organic amendment dose and the optimal rate is 30 T/ha. As perspective, we recommend increasing further the amendment dose and minimizing the size of the experimental plot around 2000 m² and concentrate the factors studied while studying the economic impact of the tested solutions and estimating their cost.