1 Introduction

Climate change and soil poverty are the main problems of agriculture development in Africa. Soil poverty is the cause of the low production of crops in most countries. It manifests by the loss of soil structure, permeability, and nutrients. Among the causes of soil degradation, erosion, nutrient leaching, and bad cultural practices are the most important factors. In Togo, less than 10% of agricultural soils are moderately rich [1]. Kanda et al. [2] showed that many vegetable farmers cited soil degradation as the most constraining in crop production. Soils were exploited continuously without fallows due to demographic pressure [3, 4]. Some studies have focused on the utilization of mineral and organic fertilizers for fertility restoration. Agricultural practices such as waste composts amendment, fallows, and green manure were used and integrated perfectly with the principle of sustainable development [5, 6]. Among these practices, soil amendment with compost presents more advantages on soil stability and fertility improvement [7, 8]. The reported positive effects include soil structure and chemicals properties improvement. The soil fertilization with compost allows the enhancement of soil parameters such as structure, porosity, humidity content, pH, redox potential, and ionic conductivity [9,10,11,12,13]. These studies focused more on the effects of fertilizers on soils and crop production and are mainly the experimental tests. Currently, farmers move toward precision agriculture which uses several tools and methods as this of reasoned fertilization.

Reasoned fertilization formulas were compared to evaluate the fertilization needs of every crop through the estimation of some parameters that determine the three subcomponents of a predictive fertilization model [14]. The aim of reasoned fertilization is to satisfy the nutrient need of crops by completing the offer of soil nutrients considering the economic profit and environmental protection. It presents many advantages at several levels. Primary, it allows optimization of economic profit by controlling the quantities of fertilizers and secondary, the reduction of chemicals substances limiting their impact on the environment. Thus, reasoned fertilization must be considering factors such as the nutrients needed for crops, their availability in the soil, their restitution by crop residues, quantities and chemical forms provided by fertilizers, and the fate of the nutrient soil–plant–environment (air, water) system as well as the economic profitability [14,15,16]. In effect, fertilizers provided by farmers contribute efficiently to maintaining the levels of nutrients in the soil [17, 18]. The disproportionate quantities of nitrogen in the soil can decrease plant growth, production, and soil sustainability conservation. Crops fertilized with excessive nitrogen or example, are sensible to pathogens [19]. The reasoned fertilization is then applied to avoid excessive or deficit negative effects on soils and crops. This study assessed the influence of reasoned organic and mineral fertilization on okra (Abelmoschus esculentus) productivity and economic efficiency on low nitrogen sandy loam soil. The aim of the study was (1) to calculate mineral (N15P15K15) and organic fertilizer (wastes and biomass) compost quantities by a reasoned method; (2) to evaluate the effects of fertilizer quantities on the growth and production of okra; (3) to identify the impacts of fertilizers quantities on economic efficiency and cost-value ratio.

2 Materials and methods

2.1 Reasoned fertilization method

The reasoning consisted to calculate the quantities of fertilizers by considering the initial nutrient contents of different soils tested and the nutrient need of okra based on the previous work in literature. Fertilizer Rate Calculator online software was used to calculate the quantities of fertilizer that could satisfy the nutrient need of okra. For the soil’s nutrient contents lower than the need of okra plants, the doses of fertilizer were calculated in order to satisfy all the requirements of the crops.

The first phase of the reasoning is the soils samples chemical’s analysis to determine the initial nutrients contents such as nitrogen (kjeldahl method), phosphorus [20], potassium, calcium, magnesium (atomic absorption spectrophotometry) contents, chemical properties (pH, Electrical conductivity, organic matter (NFU 44–160) and organic carbon (NF ISO 14–235).

The second phase focused on fertilizers’ nutrient content determination. Two fertilizer were used. A mineral fertilizer commonly used by smallholder farmers (N15P15K15) and compost produced by the farmers with wastes and biomass were used. N15P15K15 mineral fertilizer contains 15% of N, 15% of P2O5, and 15% de K2O. Composts elaborated on the different sites were also analysed. Their nutrients and chemical properties were presented in Table 1.

Table 1 Chemicals characteristics of composts on the different sites
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The last phase of the reasoning concerned the determination of dose using a Multiple Fertilizer Rate Calculator, developed by the University of Kentucky in Lexington. The nutrient requirement for okra proposed by Nisha et al. [21] was considered. They showed that 100 kg/ha of nitrogen, 60 kg/ha of phosphorus, and 60 kg/ha of potassium are the nutrients needed for Okra’s best growth and production. This reasoning method led to establishing three formulas of fertilizer based on mineral fertilizer and composts. Figure 1 summarizes the reasoning method.

Fig. 1
figure 1

Reasoning method description 

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2.2 Agronomic test

The agronomic test was carried out on two sites located in south Togo. The first site is Tabligbo located at 6.6028°N, 1.5063°E and the second is Djikame located at 7°15ʹ0ʹʹ N, 1°34ʹ60ʹʹE for 4 months from September to December 2019. The rainfall and average temperature during the tests are respectively 27 °C and 695 mm for Tabligbo site, 27 °C and 692 mm for Djikame. Site 1 is sheltered by weakly ferralitic soil while site 2 is a tropical ferruginous soil [22]. Table 2 presents the nutrient content and chemical properties of the study soils.

Table 2 Chemical characteristics of experimental soils
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The experimental design is that of Fisher blocks completely randomized with three replicates per treatment. Treatments based on fertilizers quantities obtained from the reasoning (T2, T3, T4), control treatment, (T0), and indigenous farmer treatment (T1). Each elementary plot occupies a surface of 6m2.

2.3 Fertilizers’ impact on okra growth and production

Fertilizer formula impacts on okra growth were measured through plant height, and number of leaves on 20 plants per elementary plot with a frequency of 14 days (2 weeks) until the end of the vegetative phase (2 month). At the physiological maturity, fruit number, circumference, and weight were measured on each plot. At one month after maturity, the length of the main root was measured and the number and weight of grain in fruits accounted for 15 representative fruits per plot.

2.4 Fertilizers’ impact on economic efficiency and ratio cost-value

Costs of the formulas were estimated considering the price in the market. A price of a kg of N-P-K fertilizer is 500 FCFA (Franc CFA) while the price of 100 kg of compost is 3000 FCFA. Economic efficiency (EE) and Ratio Cost-Value (RCV) were calculated using the following formula [23]:

$$EE=\frac{\mathrm{PVPFTF}-\mathrm{CAIFF}}{\mathrm{PVPFF}} X 100$$

PVPFF = Sale price of harvest product on formula; CAIFF = Cost of Purchases or cost of production of formula inputs.

$$RVC=\frac{\mathrm{Total \,Income \,from \,harvest }}{\mathrm{Total \,cost \,of \,fertilizer \,formula}}$$

2.5 Statistical analysis

Firstly, data were submitted to a normality and homogeneity pre-test using box-plot. Simple analysis of variance (ANOVA) was used to analyse the data using SPSS 22.0 (IBM, 2013). Significant differences between means were compared using Fisher’s least significant difference (LSD) test at a probability level of 95% (P ≤ 0.05).

3 Results

3.1 Fertilizers quantities from reasoned fertilization

The studied soils are sufficiently rich in potassium according to standards and the reasoning process concerned only nitrogen and phosphorus nutrients (Table 3). At Tabligbo, it needs, 667 kg/ha of N15P15K15 (T2), 10.210 kg/ha of compost (T3) and 335.5 kg/ha NPK + 5110 kg/ha for a mixed formula (T4). At Djikame, 667 kg/ha of N15P15K15 (T2), 12.100 kg/ha of compost (T3) and 335.5 kg/ha NPK + 6050 kg/ha of compost for a mixed treatment (T4) (Table 3). It noted that Djikame soil needs more compost than this of Tabligbo. The chemical formula (N15P15K15) quantities (200 kg/ha) are high than that used commonly by farmers in south Togo (50 kg/ha).

Table 3 Fertilizers quantities from reasoned fertilization in the different formula
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3.2 Fertilizers effects on okra growth and production

3.2.1 Effects of treatments on height, leaves number, root number, and length of the main root

Box-Plot homogeneity test of the height and number of leaves at the end of growth show generally a good repartition of measures per treatment except T3 and T4 on the site of Djikame (Figs. 2, 3).

Fig. 2
figure 2

Height and number of leaves dispersion at the end of the growing period on the site of Tabligbo

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Fig. 3
figure 3

Height and number of leaves dispersion at the end of the growing period on the site of Djikame

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After two weeks, plant’s growth is homogenous for all treatments. At 21 days after sowing, plants on reasoned fertilizers (T2, T3 and T4), quickly grow more than those on control and indigenous treatments (T0 and T1). At the end of the vegetative phase (56 days after sowing), plants grown on fertilizers T2 and T3 presented the highest height with a mean of 65.82 ± 2.51 cm compared to 31.16 ± 0.70 cm for the control treatment at Djikamé. On Tabligbo site, the highest height was obtained on fertilizers T3 and T4 with a mean of 74.50 ± 3.53 cm compared to 48.50 ± 04.94 cm for the control treatment (T0).

Concerning leaves number, there is no difference between treatments at the 14th and 28th days after sowing (Fig. 4). Between 28 and 56th days, the lowest number of leaves is obtained on T0 and T1 and the highest number on T2 and T4 on site 2. At 56th days after sowing number of leaves vary from 8 ± 2.24 (for control treatment T0) to 15 ± 3.00 (for T4). On site 1, the number of leaves is higher on the plots treated with compost and a value of 11.5 ± 1.24 was obtained and compared to 6.8 ± 0.28 for the control treatment (Fig. 5).

Fig. 4
figure 4

Effect of reasoned fertilization on plant’s height (a Djikame; b Tabligbo; T0: Control; T1: Smallholders farmers practices; T2: mineral fertilizer N-P-K; T3: Compost; T4: mineral fertilizer N-P-K + compost)

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Fig. 5
figure 5

Effect reasoned fertilization on number of leaves (a Djikame; b Tabligbo; T0: Control; T1: Old fertilization technic; T2: mineral fertilizer N-P-K; T3: Compost; T4: mineral fertilizer N-P-K + compost)

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Roots data exploring through box plot showed that except T4 treatment the values dispersion is homogenous. However, ANOVA showed any significant difference between treatments for the length of the main root (Fig. 6). However, treatments impacted significantly root number and the highest number was obtained on reasoned treatments T3 and T4 with an average of 31.42 ± 4.83 cm compared to 18.4 ± 4.41 cm for T0, T1, and T2 treatments (Fig. 7).

Fig. 6
figure 6

Mean of root number per plant and length mean of principal root dispersion after harvest on the two sites

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Fig. 7
figure 7

Effect of reasoned fertilizers on root number and principal root per plant (T0 Control, T1 Smallholders farmers technic, T2 mineral fertilizer N-P-K, T3 Compost; T4 mineral fertilizer N-P-K + compost, ns none significant; a, b, c and d = homogeneous groups. The treatments with the same letters are in the same group. ns = not significant; MRNPP = Mean of Roots Number Per Plant; MLMRPP = Mean Length of Main Root Per Plant)

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3.2.2 Effect of reasoned fertilizers on fruit number, weight, circumference per plot, grain number per pod, and weight of 200 grains

With the exception of fruits mean and circumference, the measurements are homogeneous for yield parameters (Figs. 8, 9).

Fig. 8
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Yield parameters values dispersion on site of Djikame

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Fig. 9
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Yield parameters values dispersion on site of Tabligbo

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ANOVA showed significant differences for the number and weight of fruits, the number of grains per pod, and the weight of 200 grains on the two sites (Table 4). On site 1, the highest number of fruits per plant was obtained on treatment T4 presenting 102 ± 09.00 fruits compared to 85 ± 07.50, 78.5 ± 14.25, and 53.5 ± 06.25 respectively for T3, T2, and T0. The same formula presented the highest fruit weight with an average of 1322.5 ± 106.00 g compared to 647.5 ± 136.00 g for T0. Plots of T3 and T4 recorded the highest number of grains per pod with a mean of 99.75 ± 01.00 compared to 70 ± 0.50 for T0 and T1. The same treatments presented the highest weight of 200 grains with a mean of 17.95 ± 1.28 g compared to 6.31 ± 0.50 g for T0 and T1.

Table 4 Effect of reasoned fertilizers on fruit number, weight, circumference per plot and grain number per pod, weight of 200 grains
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On site 2, number of fruits per plot was high on T3 with a mean of 72 ± 03.50 compared to 58.5 ± 01.87, 62.5 ± 02.36, and 38.5 ± 03.62 respectively for T2, T4, and T0. Plants on T3 and T4 treatments presented the highest fruit weights. An average of 745 ± 57.5 g was obtained these treatments compared to 296.6 ± 35 for T0, T1, and T2. According to the number of grains per pod, the soils received the mixture of inorganic and organic fertilizers (T4) presented the highest number, 71.5 ± 0.75 compared to 48.38 ± 0.94 for treatments T0 and T1. Plants on T3 treatment have the highest 200 grains weight, 12.76 ± 0.88 g compared to 2.45 ± 0.37 for treatments T0 and T1 (Table 4).

3.3 Formula effects on economic profit

On Tabligbo site, the highest fertilizer cost is 222.52 F CFA (T4) and the least expensive is 183.78 F CFA (T3) while at Djikamé, the most expensive is T3 formula (217.8 F CFA) and the least expensive is T2 treatment (200 F CFA) (Table 5). Socioeconomic evaluation allowed to quantify and monetize the economic costs and benefits of the formula from reasoned fertilization. Value-Cost Ratio (VCR) and economic efficiency were used to assess the profitability of these formulas. On the Tabligbo site, the highest economic efficiency was obtained on treatment T4 (53.48) while treatment T3 presented the best VCR (01.97). On site 2, treatment T2 recorded the highest economic efficiency (24.62) and VCR (01.35). Economic efficiency and VCR were very low on site 2 (Djikame) than site 1 (Table 6).

Table 5  Cost of different formula from the reasoned fertilization
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Table 6 Price, economic efficiency and ratio cost value of the harvest for each formula
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4 Discussion

4.1 Organic and inorganic fertilizers dose

The high dose of N15P15K15 for T2 treatment from the reasoned method compared to that commonly used by smallholder farmers could be explained by the lowest chemical fertility of the studied soils. In effect, the use of inappropriate and extractive farming practices leads to the loss of soils nutrients and their capacity to satisfy the needs of crops [17, 18]. Human plays important roles in soil degradation in smallholder farms in sub-Saharan Africa [24]. Thus, soils were exploited without restoration and compensation of nutrients loosed by erosion, leaching, and plant nutrition. Soils more exploited become degraded and are incapable to allow crop production. Quantities of compost resulting from the reasoned (6050–12000 kg/ha) are similar to the recommended (5000 to 10.000 kg/ha). On the other hand, the unavailability of fertilizers for rural farmers is an significant impediment to the agricultural development in Togo. Many farmers do not use mineral fertilizers due to financial constraints. The high cost (more than 15,000 FCFA per 50 kg) poses a problem, significantly reducing inputs (7-50 kg/ha) contributing to soil poverty.

4.2 Reasoned formula effects on okra growth and production

The growth of okra was influenced by fertilizers formulas and the highest height obtained on compost treatment (T3 and T4) could be explained by the improvement of soil physical and chemical properties as structure, nutrients, and organic matter contents [25, 26]. Soils studied are poor in organic matter and moderately acid. The amendment of these soils with basic compost rich in nutrients could improve their chemical properties, plant nutrition, and growth. The highest leaves number recorded on the T4 formula could depend on the effects of mature organic matter on soil properties and nitrogen uptake by plants. The application of compost and mineral fertilizer increase plants’ nitrogen uptake and growth [27] through N and P accumulation in plants [28]. The length of the main root wasn’t affected by fertilizers formulated because of differences between the chemical properties of fertilizers tested. Several factors influence rhizogenesis and soil structure is the main which could be influenced roots number. In effect, composts have improved soil structure (texture and porosity) allowing roots elaboration. The highest number of fruits and weight recorded on treatments T4 (Tabligbo), T3, and T4 (Djikame) would be depend on the effects of organic matter on plants’ nutrient uptake. The addition of organic matter to agricultural soil increases the water holding capacity and nutrient availability [29] but mineral fertilizer enhanced only chemical properties [25]). The water holding and nutrient availability in compost soils increased okra nutrition, growth, and production [30]. Organic fertilizers promote microbial degradation and nutrients, improving pH and growth and yield parameters ([29, 31]. The low yields obtained on the plot with mineral fertilizer T2 on Tabligbo site could be depend on acidity increasing causing a disparity of nutrients and their leaching towards deep layers [32]. The addition of organic manure should not exclude the application of mineral fertilizers [33] and FAO [34] recommends the association of mineral fertilizer and organic manure to avoid minerals leaching. The beneficial effect of the combination of organic and inorganic fertilizers on crops profitability has been proven by [35] for cabbage crops and Ullah et al. [36] for brinjal. Indeed, inorganic fertilizers allow crop growth while organic fertilizers create the conditions necessary for the quick uptake of nutrients. The two types of fertilizers are therefore complementary as a synergy to allow better growth and production of crops. A study on 12 agricultural soils in southern Togo has shown that 92% are poor in nitrogen, 8% in phosphorus, 0% in potassium and 25% in organic matter [1]. Soil fertilization has been limited to inorganic fertilizers and the high costs of these fertilizers do not promote good crops fertilization, especially in the smallholders’ farms.

4.3 Reasoned formula effects on crops economic efficiency and ratio cost-value

The highest Economic Efficiency (EE) obtained on T4 treatment on site 1 (Tabligbo) could be explicated by the effects of organic and inorganic fertilizers on the soils. The Association between inorganic and organic is shown to be beneficial for crop production and decreases fertilizers quantities needed for both fertilizers formula [37, 38]. On site 2, treatment T2 showed the highest economic efficiency and economic parameters are very low. It confirms the work of Hammad et al. [25] which showed that synthetic fertilizers increased grain yield and farm profit while organic manure enhanced grain quality. Soil of site 2 is poor in nutrients with a high C/N ratio. This soil is a degraded soil demanding more nutrients, particularly, nitrogen in order to reduce the C/N ratio. Inorganic fertilizer would play this role to allow nutrient uptake by plants. The economic efficiency of treatments T3 and T4 are the results of carbon accumulation in soil increasing the C/N ratio. The increasing of C/N ratio could perturb nitrogen uptake by plants. This ratio affects frequently soil reactions and the nutrients mineralization process. The perfect value is between 10 and 20 indicating the availability of nitrogen for plant nutrition [39]. However, the ratio obtained is very high and expresses low nitrogen content as opposed to the accumulation of carbon. These conditions decrease nitrogen uptake by plants, yield and profit. Qian and Schoenau [40] showed a significant negative correlation between cattle manure organic C/N ratio and N mineralization. Undeniably, according to Lavigne-Delville [23], formulas having RCV values high than 1 are the best commonly adopted by producers. For Aune et al. [41], novel technology is economically profitable if the RCV is higher than 2.

This study does not allow to predict the long-term effects of the tested fertilizer formulations. However, composts are well known for their long-term beneficial effects on soil fertility. In addition, the tests were restricted to formulas calculated considering only essential nutrients. However, it is well known that soil chemical properties have a significant impact on the biogeochemical functioning of fertilizers in soils, particularly the transfer of nutrients to crops [42, 43]. Okra is a nutrient-demanding crop, increasing the fertilizers doses determined in this study and the extension of agronomic tests on three season would help to optimize its economic profitability.

5 Conclusion

The objective of this study was to evaluate the influence of reasoned organic and inorganic fertilization on okra (Abelmoschus esculentus) productivity and economic efficiency on two sandy loam soil from Togo. Reasoned fertilization showed that contents of nitrogen and phosphorus are the lowest in studies soils. Inorganic fertilizers (N15P15K15) quantities commonly used by farmers in Togo are very low than those from reasoning due to advanced soil degradation. The results of the agronomic tests showed a positive effect of the combination of inorganic and organic fertilizers on the growth, production, and economic efficiency of okra on low soil organic content. On soil with high C/N, inorganic fertilizer contributes to improving economic efficiency. Data from this study will aid in better fertilizing of gardening soils of smallholders in the study areas to optimize their economic efficiency. Furthermore, the reasoning method merits to be optimized by considering the physical (texture and structure) and chemical characteristics (pH, electrical conductivity, organic matter, and Redox potential) of soils. The effective fertilizer formulas discovered in this study will be shared with farmers for sustainable garden crops production in the study areas.