Mineral Fertilizer Use for Optimal Groundnut Production in the Sudano-Guinean and Sudanian Zones of Benin

This study aims to determine the optimal N, P, K, Mg and Zn rates for groundnut production on Ferric and Plintic Luvisol in the Sudano-Guinean and Sudanian zones of Benin Republic. Two years (2018 and 2019) experi-ment was carried out in the municipality of Ouessè in the Sudano-Guinean zone and Bembèrèkè in the Sudanian zone. The tested nutrient doses were N (0, 20 and 40 kg∙ha −1 ), P (0, 25 and 50 kg∙ha −1 ), K (0, 20 and 40 kg∙ha −1 ), Mg (0, 2.5 times the yield in the farmers’ field) pod yield and the best return on in-vestment per hectare. Nevertheless, for a sustainable groundnut production, treatment 13.1-25.07-11.47-20-1.82 is suggested as regular K input is required for the respect of the fertilization laws.


Introduction
Groundnut is one of the protein sources widely used for both human and animal nutrition in developing countries [1]. Leguminous crops represent high economic important crops in the traditional cropping system with a wide ecological adaptability [2]. Through the symbiotic association between legumes and rhizobium bacteria, atmospheric nitrogen is fixed in soil in mineral form [3]. Through this process, legumes improve soil nitrogen stock for the subsequent crops [4] [5]. In addition, some legumes, especially groundnut, contribute to the solubilization of insoluble phosphorus in the soils [6]. Legumes improve the physical environment and soil microbial activity as well as replenishment of soil organic matter stock [7].
Unfortunately in the traditional cropping system, groundnut is cultivated without fertilizer application, despite the low soil fertility level leading to crop yield decrease. For sustainable groundnut production, there is a need to develop appropriate fertilizer formula knowing that, as mentioned by [5], total amount of nitrogen fixed by a legume is not enough and cannot satisfy crop nitrogen requirement. Although nitrogen-based fertilization is regarded by several authors as a precursor of crop yield formation, not so important for groundnut yield [8] [9] when the soil fertility is at its appropriate level. Due to the oily nature of the seeds, the plants take up large amount of nutrients from the soil, leading to soil fertility depletion when nutrients are not replenished [10]. Furthermore, legumes with high harvest indices do not improve soil fertility, because a large amount of nutrients are in the seeds. But they rather deplete soil nitrogen [11]. It is reported that, soil nutrient deficiencies are the major cause of failure in legumes' root nodulation and affecting nitrogen fixation [10]. Thus, nutritional imbalance, mainly of macronutrients and micronutrients, contributed to yield losses with drawbacks on oilseed quality [12].
However, balanced mineral fertilization of groundnut considering both macronutrients and micronutrients would be a strategy to improve crop yield and soil fertility [13] [14]. Micronutrient application for leguminous crops plays an important role in improving crop yield. Due to intercropping practices, a single micronutrient deficiency can affect crop productivity [15]. For instance, Zn deficiency in the soil reduces yield by up to 13%. Zn and Mg largely increase crop yields. Zn is involved in many physiological functions, which positively impact groundnut crop yields at low concentrations [16]. [17] showed that Zn application results in a remarkable increase of groundnut yield. It is an active element in the biochemical processes [18] and interferes chemically and biologically with N and P [19].
Likewise, Mg also promotes the uptake of N and K fertilizers [20]. Mg also promotes assimilation and migration of P in the plant and seed grains to form phytine and lipid [21]. According to these authors, reasonable application of P and K fertilizers with an appropriate supply of magnesium sulphate has significant effect on plant growth and nutrient uptake and groundnut yield improvement. Furthermore, [21] holds that Mg combined with P increase groundnut yield by 69.3% over the control plot. Similarly, [22] also showed that application of Mg and P increased groundnut yield by 12.7% compared to P application alone.
The aforementioned results show that balanced nutrition involving micronutrients and macronutrients is important to significantly induce groundnut yields while maintaining soil fertility. Despite the importance of these nutrients for balanced groundnut nutrition, no study has documented groundnut fertilization, with regards to Zn and Mg in Benin soils. Groundnut is a crop of high economic importance, but it is cultivated without application of mineral fertilizer leading to soil mining, because a large part of nutrients is in the seeds [23].
The aim of the present study was to assess the response of groundnut to different doses of N, P, K, Mg and Zn for optimal grain yield in the Sudano-Guinean and Guinean zones of Benin. Specifically, the effect of these nutrient doses on plant growth (root length, number of gynophores, number of nodules and plant recovery) and yield components (pod and aboveground biomass production) are assessed and nutrient optimal doses are determined. We assume that synergistic interaction among N, P, K, Mg and Zn could optimize groundnut pod yield.

Study Area
The study was carried out in the municipalities of Bembèrèkè and Ouessè located in the southern Borgou agroecological zone (AEZ 3) and in the cotton agroecological zone (AEZ 5) of the centre of Benin respectively.
The AEZ 3 is located between 1˚10'E -3˚45'E and 9˚45'N -12˚25'N. This zone is characterized by a unimodal rainfall distribution, with an average annual rainfall less than 1000 mm and located in the Sudanian zone. The relative moisture varies from 18% to 99% while temperature varies between 24˚C and 31˚C.
The Ferric and Plintic Luvisol [24]   . Plots with previous maize crops were selected for the trial and managed by each farmer owner of the land. Groundnut variety sown was TS 32-1 called locally "Moto" (90 days of growth cycle with attainable yield in farmer's condition of 1.7 t•ha −1 ). The seeds were purchased at CRA-Nord (Centre de Recherche-Agricole du Nord) of Benin National Agricultural Research Institute (INRAB) located at Ina in the north. The seed viability was around 80%. Ridge ploughing was carried out with a 50 cm row spacing (in the Centre) and flat ploughing using animal traction with a depth of 15 cm (in the South Borgou zone). Sowing is carried out at a depth of 5 cm using two seeds per hole and 50 cm between rows and 20 cm between plants are the sowing spaces. Nutrients are applied in the form of urea (46% N), TSP (46% P 2 O 5 ), KCl (60% K 2 O), kieserite (23.5% MgO) and zinc sulphate (35% Zn 2+ ). Fertilizer application is carried out 15 days after sowing closed to each hole under the supervision of the research team and considering the doses calculated.

Experimental Design and Field Experiments
The collected data represent the recovery rate (D), root length, number of gy-

Statistical Analysis
The statistical analyses were performed using SAS v. 9.4 packages. The collected data in each agroecological zone were subjected to one-way analysis of variance considering the treatments. General linear mixed-effect model, considering farmers as a random factor and nutrient combinations as a fixed factor. Student Newman-Keuls test was performed for means separation at a significance levels of P < 0.05.
For the optimal dose determination, the response surface analysis method was used [26]. The overall equation of the regression models for the response surface analysis was: where y, x i and x j are the factors, β 0 is the constant or intercept, β i , β ii and β ij represent the first-order, quadratic and interaction term coefficients, respectively, and ε is the residual associated with the experiments.
The maximum fertilizer doses are obtained by cancelling out the first derivative of the above equation with respect to each fertilizer unit (the marginal productivity of the fertilizer) [26]. Thus, solving this system of equations helps to determine the values of x 1 , x 2 , x 3 , x 4 and x 5 which were the maximum doses of each nutrient. P Y was the price of the product and P N , P P , P K , P Mg and P Zn the respective prices of kilogram of N, P, K, Mg and Zn. The maximum profit was obtained by equating the marginal product to the fertilizer/product price ratio as follow:

Soil Physico-Chemicals Parameters
The soil particle sizes vary from sandy to sandy clayey texture. The pH (water) were 6.5 and 6.25 for the sites of Ouessè and Bembèrèkè respectively; soil organic C were 6.2 and 4.8 g•kg −1 for Ouessè and Bembèrèkè respectively; total N were 0.89 and 0.41 g•kg −1 for Ouessè and Bembèrèkè respectively; available P are 45.25 and 14.25 mg•kg −1 for Ouèssè and Bembèrèkè respectively and the exchangeable K + were 0.34 and 0.18 cmol•kg −1 for Ouessè and Bembèrèkè respectively. The cation exchange capacity (CEC) of both soils were low (<15 cmol•kg −1 ). In general, the soils of the study area are slightly acid with low organic matter content (with C/N ratios varying between 10 and 14). The consequence of this low C/N ratio is a low level of the total N which seems to be with P, the most limiting nutrients for both soils. Overall, the soils of the site are characterized by a low fertility level.

Effect of the Different Nutrients N, P, K, Mg and Zn and Their Doses Applied on Groundnut Growth Parameters
The growth parameters (plant recovery diameter, number of nodules and number of gynophores) induced by the treatments are shown in Table 1 and plant root lengths in Figure 1. The analysis of variance reveals that root length,

Effect of the Different Nutrients N, P, K, Mg and Zn and Their Doses Applied on Groundnut Pod Yield and Aboveground Biomass
The average pod yield values and the aboveground biomass in both sites are shown in Table 2 and Table 3

Response of the Groundnut Plant to Each Nutrient Type and Doses Applied
The response surface diagrams of each nutrient applied and the interaction be- The interaction between Mg and Zn did not induce pod yields above 2 t•ha −1 . Figure 3 and Figure 4 show the response curves of groundnut pod yields to each nutrient applied at different rates in both experimental sites. A normal response was observed for all nutrients, except for K, which revealed no response.
It was also noticed that, the responses were strong for N and P but weak for Mg and Zn. Similarly, Figure 4 shows strong responses for N and P but weak res-

Determination of the Optimal Doses of Each Type and Rate of Nutrients N, P, K, Mg and Zn Applied
Analysis of the response surface results showed that quadratic model (at the site of Centre Benin) and quadratic with interaction (at the site of north Benin) were highly significant (p = 0.000, R 2 = 0.98) estimating groundnut pod yields versus variation of N, P, K, Mg and Zn doses. The regression equations (Equation (1) for the site of Centre and Equation (2)  These optimal doses have induced an optimal groundnut pod yield of 2.1 t•ha −1 .

Groundnut Response to Mineral Fertilizer
Application of nutrients significantly improved groundnut growth parameters especially root length. This root elongation enables good soil depth exploration for nutrients uptake. The increase in root length was due to the supply of P which is known to develop more extensive plant rooting system [27] and probably to Mg and Zn. Indeed, Mg and Zn contribute to the solubilization of insoluble phosphorus in the soil and also contribute to improve P absorption.
In addition, groundnut recovery diameters were significantly affected by the types and rates of nutrient applied. This is an indicator predicting plant development and the aboveground biomass production [25]. Our result showed that different rates of P and Zn induced significant effects on groundnut recovery diameter. This could be explained by the important role of these nutrients in cell division, which actively participates in the rapid development of meristematic tissues resulting in a large number of leaves and plant height growth [28] [29] [30]. This high plant recovery diameter actively participates in the photosynthesis and plant aboveground biomass production.
However, more gynophores and number of nodules were recorded during the present experiment. The high number of nodules per plant could be explained by the presence of P in the nutrient applied which is important in the establishment of nodules in legumes [31]. However, P ensures energy transfer, a cellular constituent that ensures the synthesis of nucleic acids and increases the number of groundnut nodules per plant. This had increased probably the atmospheric nitrogen fixation [32]. The Gynophores are an essential component of groundnut yield as they are directly related to the number of pods production. The results of the present study showed that treatments with intermediate doses of P, K and Mg produced high number of gynophores. It is well known that N is involved in photosynthesis, and ensures also a good development of the gynophores. P interacts with N through its ability to provide energy to the photosynthesis process and N biosynthesis, which activates the production of gynophores. [33] reported that N, P and K doses significantly increase the number of gynophores in groundnut crops. In general, based on the results of this study, the nutrients applied contributed to the growth and development of groundnut through their role in the plant. This may have contributed to pod yields improvement.
Compared to the control, the different types and rates of nutrient applied have increased by 2.5 times the pod yields. The positive effects of nutrients on legume the efficiency of macronutrient (N, P, and K) utilization by groundnut plants [16]. Indeed, Mg promotes the assimilation and migration of P in the plant and in the seeds to form phytine and lipids [21]. It is therefore suggested to optimize plant mineral nutrition to ensure yield improvement in the traditional cropping system.

Relevance of Mg and Zn in Groundnut Mineral Nutrition Improvement
Groundnut pod yields and aboveground biomass were significantly improved regarding the types and rates of nutrient applied combined with Zn. Our results also showed that pod yields were influenced by nutrient rates and interaction of N, P and K with Mg and Zn. The N, P and K use efficiencies were thus improved due to the uptake of Mg and Zn. These results corroborated that of [36] who showed that micronutrients significantly increase crop yields and were the main pillars for soil fertility management. Indeed, macronutrients are better taken up by the crops in the presence of micronutrients [20]. However, Mg promotes the uptake and migration of P in the plant and seeds and form phytine and lipids [21]. The levels of micronutrients in the arable soils in most of the sub-Saharan Africa's soil are below the critical threshold, particularly Zn [37]. The combined application of macronutrients with Mg and Zn has induced significant effect on macronutrients utilization efficiency. [15] showed that the addition of micronutrients with N, P and K resulted in yield improvement by 50% compared with application of N, P and S without micronutrients. [38] also showed that Zn application had increase macronutrients uptake in the plant and improved N, P and K use efficiency. Supply of Zn has induced N use efficiency by 17% leading to crop productivity improvement. When micronutrients were applied at appropriate rate, it is also beneficial to the soil biodiversity, including soil microbial colonization, plant growth, mycorrhizal development, legume fixation and nodulation [39]. Our study suggests supply of Zn and Mg to improve groundnut nutritional status in the farmers' cropping systems for pod yields improvement and for sustainable soil fertility management.

Agronomic Implication of the Optimal N, P, K, Mg and Zn Rates Determined
The application of the different nutrient N, P, K, Mg and Zn and rates showed a normal crop response except K for which no response was observed in the site of the north. The optimal dose of 0 kg K ha −1 found for this site means that no external K input is required to achieve optimal pod yield. This may be due to the to the balanced and efficient nutrient use and also to minimize adverse effect on the environment [40]. This can be achieved by combining the optimal nutrient inputs with best crop management practices, hence the law of minimum. However, the optimal dose of 0 kg K ha −1 cannot be recommended for farmers regarding the sustainability of the cropping system [41]. We recommend the minimum dose of 20 kg K ha −1 in order to guarantee sustainable soil production.
According to [42] the doses of 50 kg•ha −1 of P 2 O 5 to 75 kg•ha −1 are sufficient to improve seed oil content in addition to the pod yields. The different optimal doses found in the present study could then allow seeds with high oil content though this needs to be tested. Also, these N, P, K, Mg and Zn doses determined can guarantee profitable economic production as well as good quality product and by-product (oil, cake, groundnut paste) and finally quality fodder for animal feed. The next step of the study is to formulate an N-P-K-Mg-Zn fertilizer for farmers and to assess the climate variability effect on the yields regarding the doses determined.

Conclusion
Mineral fertilization of groundnut with N, P, K, Mg and Zn improved the agronomic parameters of groundnut showing a positive response of groundnut to mineral fertilization. Our results indicated that the application of N, P and K combined with Zn improves nodule production and groundnut growth parameters. The pod yields and aboveground biomass were significantly improved regarding the treatments. This is relevant that legume crops in the farmers' crop- groundnut production. These optimal doses have induced an optimal groundnut pod yields of 2.1 t•ha −1 . In order to avoid K mining in the soils as suggested by the model, a minimal dose of 20 kg K ha −1 was suggested. For sustainable groundnut production perspective, it would be interesting to evaluate the effect of long-term climate variability on the pods' yields considering the determined optimal N, P, K, Mg and Zn doses. Fertilizer formulations based on these optimal N, P, K, Mg and Zn doses are the next step of the programme in order to make available specific fertilizer for groundnut production in Benin.