1. Introduction
Soil degradation is a significant global concern, particularly affecting developing countries where a large portion of the population depends directly on the soil for their livelihoods [1]. Various forms of physical, chemical, and biological land degradation, such as compaction, inorganic/organic contamination, and reduced microbial activity and diversity, have arisen due to excessive human pressures over the last century. These processes have severely impacted global natural resources [2]. Climate change has exacerbated salinity challenges in the root zone, especially in shallow water tables and coastal areas [3] [4]. Soil salinization is a major degradation process, particularly in semi-arid and arid zones [5]. Globally, over one billion hectares of soil are affected by salinity [6] [7]. By 2050, soil salinization is projected to impact more than half of the world’s agricultural land [8] [9]. According to the FAO [10], approximately 1.5 million hectares of cropland lose production, and 20 - 46 million hectares experience a decrease in potential production annually due to land salinization, resulting in an annual revenue loss of about $31 million.
Salinity affects 19.09 million hectares in Sub-Saharan Africa (SSA) [1], mainly in Eastern Africa, the coast of Western Africa, the Lake Chad Basin, and isolated areas of Southern Africa [10]-[12]. In Togo, salt-affected soils are documented along the lagoon from Lake Togo to the Mono River, at the edge of the Zio River valley, and in the village of Atti-Apedokoe [13] [14]. In agricultural ecosystems, soil salinity refers to the excessive accumulation of soluble salts within the root zone of plants, resulting in elevated pH (>8.5), sodium adsorption ratio (<13%), exchangeable sodium percentage (<15%), and electrical conductivity (EC > 4 dSm-1). This condition reduces crop yield and compromises soil health. In regions where soil salinity remains unaddressed, a significant portion of agricultural land is abandoned [15].
Soil salinity, arising from both natural and human-induced causes, is predominantly caused by inappropriate irrigation practices and seawater intrusion into coastal farming areas. This issue is further exacerbated by rising sea levels due to climate change and excessive groundwater extraction [9] [11] [16]. Salt-affected soils accumulate soluble salts such as NaCl, NaHCO3, and Na2CO3 in the root zone [11] [17]-[19], leading to adverse effects on soil fertility, stability, and biodiversity [20] [21]. The elevated concentration of soluble salts, or salinity, primarily impacts plant growth and crop production by increasing the osmotic potential of the soil solution [22] [23]. Consequently, salinity poses significant challenges to agricultural production, with far-reaching implications for rural livelihoods, economic sectors, environmental sustainability, and overall development [1] [6] [21] [24]. Notably, since 75% of the population across SSA relies on subsistence farming, addressing salinity-related issues is crucial for ensuring food security and sustainable agricultural practices [25] [26].
Mitigating and preventing salinity is crucial for advancing agricultural development, particularly in SSA. This region is transitioning from traditional rain-fed agricultural systems to intensive irrigated agriculture to combat food insecurity, poverty, and climate change. To achieve these goals, proper agricultural field management and adaptation for increased productivity are essential. Additionally, efficient food supply chains are needed to support a growing population in SSA.
A comprehensive set of adaptations and mitigation measures, including the use of salt-tolerant crop varieties [27]-[29] and effective soil and water management practices [22] [25] [26] [30], is essential to address the challenges posed by salinity. Among these measures, soil amendments play a crucial role. Salt-affected soils can be rehabilitated through inorganic and organic amendments. The physical, chemical, and biological properties of soil can be improved by applying organic matter, which accelerates salt leaching, improves aggregate stability, and increases water holding capacity [31], thereby enabling better plant growth in salt-affected soils. Additionally, adding organic matter (OM) can decrease the exchangeable sodium percentage (ESP) and electrical conductivity (EC) [32]. It can also enhance both cation exchange capacity and soluble exchangeable K+, which competes with Na+ in saline sodic soil, limiting its entry at exchangeable sites [31]. This adaptation and mitigation technology is cheaper compared to saltwater drainage and small-scale irrigation technologies. Furthermore, some studies [27] [33] used chemicals such as gypsum to reclaim salt-affected soils, which release Ca2+ to replace Na+ at soil exchangeable sites, followed by leaching with a good quality water supply.
The village of Atti-Apédokoe in Togo is renowned for its high market gardening production and its suitability for cultivating cereals such as rice, millet, and sorghum. However, salt accumulation in the water and soil has negatively affected crop yields, leading to land abandonment by vegetable growers [34]. The residents of Atti-Apédokoe depend on agriculture for their livelihood, and increasing salinization threatens their income and existence. Despite the critical situation, no specific studies have been conducted to explore cropping strategies that could alleviate soil salinity. The combination of salt-tolerant crops and soil amendments under drip irrigation may be a promising strategy to address the salinity of agricultural lands in Atti-Apédokoe. This study aims to assess the effect of various organic and inorganic soil amendments on millet and sorghum growth and yields in salt-affected soils.
2. Materials and Methods
2.1. Study Site
The study was conducted during two consecutive seasons (June to September 2021 and January to April 2022) at Atti-Apedokoe. The village is located 50 km northwest of Lome in the prefecture of Ave, in the maritime region of Togo (longitude 0˚54’ E and latitude 6˚26’ N). The area experiences a tropical sub-guinea climate with a bimodal rainfall distribution, featuring two rainy and two dry seasons. The main rainy season spans from March to June, and the second from August to October. Atti-Apedokoe receives an average annual rainfall of approximately 1200 mm, with temperatures ranging from 24˚C to 33˚C. Subsistence agriculture is the primary economic activity in the village, providing employment for approximately 80% of the population.
The experiment was conducted on-farm at a location chosen specifically for testing and demonstrating agricultural technologies as part of the RESADE project (Improving Agricultural Resilience to Salinity through the development and promotion of pro-poor technologies and management strategies in saline-affected agricultural areas). The soil at the site is characterized as silty-sandy near the surface (0 - 30 cm) and silty-clay-sand at greater depths (>30 cm). The initial physico-chemical parameters of the soil at the beginning of the experiment are detailed in Table 1. The experiment was repeated in the second season on the same plot.
Table 1. Physicochemical parameters of the topsoil (0 - 30 cm) before the experiment.
|
Sand (%) |
Silt (%) |
Clay (%) |
OMa (%) |
Nb (%) |
Pc (ppm) |
Kd (meq/100) |
pHe |
ECs (dS/m) |
CEC (meq/100) |
2021 |
81 |
8.6 |
10.4 |
2.30 |
0.064 |
5.00 |
0.11 |
6.3 |
2.17 |
13.47 |
2022 |
78 |
12 |
10 |
1.79 |
0.056 |
2.91 |
0.08 |
6.4 |
1.78 |
15.22 |
aWalkley and Black method as outlined by Nelson et Sommers [35]; bKjeldahl method; cOlsen and Sommers [36]; dHelmke et Sparks [37]; eDilution method, Jackson [38].
2.2. Plant Materials
The plant materials used comprised millet variety IP 19586 and sorghum variety ICSV-700. Genotype seeds were received from the International Center for Biosaline Agriculture (ICBA). These specific varieties were chosen for their demonstrated tolerance to saline soil and irrigation water. The characteristics of the millet and sorghum genotypes used in this study are presented in Table 2.
Table 2. Characteristics of crop genotypes used.
Crop |
Variety |
Origin |
Race/ Biological status |
Cycle duration (days) |
Salinity sensitivity |
Height (m) |
Use |
Yield potential (t/ha) |
Sorghum |
ICSV-700 |
ICRISAT |
Guinea-OPV |
120 - 130 |
Tolerant |
3.0 - 3.5 |
Dual purpose |
3 |
Millet |
IP 19586 |
ICRISAT |
Traditional cultivar/ Landrace |
75 |
Tolerant |
1.8 |
Dual purpose |
2.7 |
OPV: Open Pollinated Varieties.
2.3. Treatments Formulation
The treatment consisted of five sources of amendment, including the control (Amendment 1), green manure (Leucaena leucocephala, Amendment 2), biochar made from palm shells (Amendment 3), rock phosphate (Amendment 4), and lime (Amendment 5). Table 3 shows each amendment’s sources and application rate according to recommendations in Togo. The amendments were applied and incorporated into the soil by plowing each year before sowing. L. leucocephala (Amendment 2) was air-dried in the shade for two weeks before application. Each plot received mineral fertilizer at 150 kg/ha NPK 15-15-15 and 100 kg/ha urea (46%). The mineral fertilizer rate is recommended by the Togolese Institute of Agricultural Research (ITRA) for cultivating sorghum and millet. The effect of the amendments will be assessed through the difference between the control and the other amended plots.
Table 3. Source of amendments and application rates.
Treatments |
Source |
Rate |
kg/6m2 |
t/ha |
Amendment 1 |
Control (no application) |
0 |
0 |
Amendment 2 |
Leucaena leucocephala |
3 |
5 |
Amendment 3 |
Biochar (with palm shells) |
3 |
5 |
Amendment 4 |
Togo Rock phosphate |
1.5 |
2.5 |
Amendment 5 |
Dolomite (28% - 32% CaO) 17% - 18% (B, Cu, Zn, MgO, SO3−) |
2 |
3.3 |
2.4. Experimental Design
A randomized complete block design (RCBD) arranged in a split plot with three replications was used. Crops (pearl millet and sorghum) were assigned to the main plots, while the amendments were assigned to the subplots. The crops were sown in elementary plots of 6 m2 (3 m × 2 m). Sowing spacing was 80 cm × 30 cm, resulting in a plant density of 83,333 plants per hectare. Weed and pest management, disease control, and bird scaring were conducted as recommended by the Togolese Institute of Agricultural Research (ITRA).
Figure 1. Cumulative water quantity (rainfall and irrigation) and mean temperature during the experiment in 2021 (A) and 2022 (B).
The experiment’s total water input (irrigation + rainfall) amounted to 366 mm in 2021 and 283 mm in 2022. The irrigation water applied was 171 mm in 2021 and 114 mm in 2022. Irrigation was administered daily, except on rainy days, based on the water requirements of pearl millet and sorghum. Irrigation water was sourced from groundwater in the experiment area. The chemical characteristics of the irrigation water are detailed in Table 4, with an average EC of 5.6 dS/m, indicating moderate salinity [39], neutral pH, and moderate SAR according to Shainberg and Letey [40]. Figure 1 illustrates the cumulative water (rain + irrigation) and mean air temperatures (Tmean) per decade during the experiment.
Table 4. Chemical characteristics of used irrigation water.
Year |
pH |
ECw (dS/m) |
Na+ (mg/l) |
K+ (mg/l) |
Ca2+ (mg/l) |
Mg2+ (mg/l) |
SAR |
2021 |
7.33 |
5.46 |
25.63 |
19.3 |
36.0 |
12.4 |
5.2 |
2022 |
7.21 |
5.77 |
22.3 |
20.3 |
38.6 |
16.3 |
5.9 |
2.5. Data Collection
To assess the effect of salinity on plant growth, plant height and stem diameter measurements were taken at 30, 60, and 90 days after sowing (DAS). At physiological maturity, fifteen randomized hills from each subplot were harvested to assess grain yield (Y) and plant biomass (YB) after a controlled drying process.
To assess the plant’s adaptive response to salinity, three leaves were collected randomly from each plot for laboratory determination of biochemical parameters: total protein, proline, and malondialdehyde contents, whose accumulation in the plant indicates salt stress. Total protein and proline were determined according to Bradford [41] and Bogdanov et al. [42], respectively, as modified and adapted for plant material analysis. The extraction and determination of malondialdehyde (MDA) were conducted following the method described by Heath and Packer [43]. These biochemical parameters were calculated using the following equations:
(1)
where P is protein content; D is possible dilution; R is the value read on the spectrophotometer; V is the total volume of the extract; W is the weight of fresh material (here, leaves); Vs is the sample volume.
(2)
where Pro is proline content; Ae is the absorbance of the leaf extract; As represents the absorbance of the proline standard solution; Ws is the proline weight in the standard solution (μg); Wf stands for the fresh weight of the leaf (g); Qp is the amount of protein (mg/g fresh material).
(3)
where CMDA is the MDA concentration; Ae is the absorbance of the leaf extract; Є is the molar extinction coefficient; L is the width of the cell (1 cm).
The grain yield index (Gij) was calculated (Equation (4)) to evaluate the effect of amendments (leucaena, biochar, rock phosphate, and dolomite) compared to the non-application of amendments (control):
(4)
where Yij is the biomass/grain yield of crop j under amendment i (i = leucaena, biochar, rock phosphate, and dolomite); Y1j is the yield from the control treatment.
2.6. Statistical Analysis
Statistical analyses were performed using R, version 4.2.2. Data from the two years were pooled for each crop and parameter according to the randomized complete block design (RCBD). The data were subjected to analysis of variance (ANOVA). Treatment means were separated using the Least Significant Difference (LSD) at the 5% probability level.
3. Results and Discussion
3.1. Experimental Conditions
Figure 1 shows the cumulative water quantity (rainfall and irrigation) and the average air temperature during the experiments in 2021 and 2022. In 2021, a total of 366 mm of water was received, compared to 283 mm in 2022. During the first 30 days of the 2021 experiment, plants received more rainfall, reducing their exposure to pronounced salt stress. In 2022, plants received only irrigation water during the first 60 days, coinciding with the flowering and stem extension phases for millet and sorghum, respectively. As a result, the amount of salt water received during the experiment was higher in 2022. The average temperature during both years was similar, with a mean of 29˚C. Table 5 shows the nutrient content and chemical characteristics of the different amendments used.
Table 5. Chemical properties of the amendments during the evaluation of pearl millet and sorghum at Atti-Apedokoe.
|
N (%) |
P (ppm) |
K (meq/100) |
OM (%) |
Na (meq/100g) |
Mg (méq/100g) |
Ca (méq/100g) |
pH |
Leuceana leucocephala |
5.60 |
1.154 |
0.157 |
42.14 |
- |
- |
- |
7.05 |
Biochar* |
2.45 |
1.470 |
0.256 |
36.49 |
- |
- |
- |
7.20 |
Rock phosphate |
0.1 |
22.74 |
2.43 |
- |
0.27 |
2.77 |
1.73 |
7.01 |
Dolomite (28% - 32% CaO)
17% - 18% (B, Cu, Zn, MgO, SO3−) |
0.1 |
- |
- |
- |
|
87.5 |
86.7 |
9.91 |
-: non determined, *Biochar is made with palm oil shells, N: nitrogen, P: available phosphorus, OM: organic matter, Mg: Magnesium, Ca: calcium.
3.2. Effects of Amendments on Sorghum and Millet Growth
The impact of different amendment sources on plant height under saline soil and water conditions is shown in Figure 2. Millet height ranged from 29.5 cm (control treatment in 2022) to 320.0 cm with Leucaena in 2021, while sorghum height ranged from 43.3 cm (Leucaena treatment in 2022) to 386 cm (rock phosphate in 2021). Although plant height increased from tillering (30 DAS) to maturity (90 DAS) for pearl millet, the growth rate was lower than in the early growth stages (1 - 30 DAS). The growth rate for sorghum remained constant between the growth stages (Figure 2(C) & Figure 2(D)). No significant effect of amendment sources was observed on the height of both crops (P > 0.05), except at flowering (90 DAS) for sorghum (P < 0.05). Plant height was statistically similar for plots with different amendments but differed from plants under control (no amendment) at flowering. There was a significant difference in plant height at all growth stages between years (P < 0.05) (Table 6). Plants in 2022 were shorter than those in 2021 (Figure 2). Plant height was not affected by the interaction between amendment sources and year (Table 6).
Table 6. ANOVA results on pearl millet and sorghum plant height, stem diameter, protein, proline, malondialdehyde, dry biomass, and grain yield.
Crop |
Source of variation |
Plant height |
Stem diameter |
Salt stress indicators |
Dry
biomass |
Grain
yield |
30 |
60 |
90 |
30 |
60 |
90 |
Total Protein |
Proline |
MDA |
Millet |
A |
0.3668ns |
0.448ns |
0.273ns |
0.9326ns |
0.7129ns |
0.909ns |
0.827ns |
0.0191* |
0.000434*** |
0.591ns |
0.0012** |
Y |
07e−08*** |
1.37e−08*** |
3.33e−09*** |
1.25e−11*** |
3.47e−08*** |
1.87e−07*** |
0.281ns |
0.617ns |
0.233ns |
0.0381* |
0.5315ns |
M*Y |
0.2908ns |
0.726ns |
0.791ns |
0.9778ns |
0.4141ns |
0.883ns |
0.742ns |
0.836ns |
0.593ns |
0.1973ns |
0.5707ns |
Sorghum |
A |
0.257ns |
0.413ns |
0.0409* |
0.939ns |
0.515ns |
0.582ns |
0.22ns |
0.637ns |
0.00946** |
0.00237** |
0.0474* |
Y |
1.98e−09*** |
9.37e−06*** |
2.3e−08*** |
9.06e−13*** |
2.93e−08*** |
1.52e−08*** |
0.318ns |
0.441ns |
0.152ns |
0.365ns |
0.9533ns |
A*Y |
0.311ns |
0.911ns |
0.2067ns |
0.656ns |
0.260ns |
0.345ns |
0.612ns |
0.835ns |
0.97ns |
0.278ns |
0.1215ns |
A: amendment source, MDA: Malondialdehyde, Y: year, A*Y: Amendment and year interaction, ns: not significant, *Significant at the 0.05 probability level; **Significant at the 0.01 probability level; ***Significant at the 0.001 probability level.
Salinity negatively affects plant growth by increasing soil osmotic pressure and interfering with plant nutrition [22]. In this study, we found that applying different amendments under saline conditions did not statistically affect pearl millet plant height during the vegetative growth stages (Figure 2). This indicates pearl millet’s ability to maintain its growth rate in stressful environments, reflecting its tolerance or adaptation to salt stress. The results corroborate the work of Qaoud et al. [44], who demonstrated that pearl millet genotypes moderately tolerate salinity. The soil amendments used in this study appeared favorable for sorghum growth (plant height). During the 2022 cropping season, sorghum plants from the amended plots were taller than those in the control. The significant difference in effect between the control and amendment application on sorghum height at 90 DAS can be attributed to the combined effect of reduced salinity severity induced by the amendment and a decrease in cell division during abiotic stresses (salt and water) [45]. This result corroborates the findings of Punia et al. [46], who observed higher plant heights at later stages of crop growth in sorghum. Moreover, Benmahioul et al. [47] pointed out that the presence of NaCl in the culture medium leads to a significant decrease in stem length.
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Figure 2. Plant height (cm) measurements were conducted on pearl millet (A and B) and sorghum (C and D) grown in plots amended with biochar, dolomite, leucaena, and rock phosphate at Atti-Apedokoe.
Figure 3 shows the stem diameter at all growing stages for millet and sorghum. The stem diameter ranged from 0.64 cm (control in 2022) to 2.8 cm (Biochar in 2021) for millet and from 1.46 cm (control in 2022) to 2.7 cm (Dolomite in 2021) for sorghum. A highly significant effect of the year was observed for stem diameter at all growing stages. In 2022, plants showed the smallest stem diameter at all stages for both millet and sorghum (Figure 3(B) & Figure 3(D)). Salt stress did not influence the stem diameter at different stages. Neither amendment sources nor the interaction between amendment sources and year affected the stem diameter. This result aligns with Nasri and Benmahioul [48], who found no effect of salt stress on argan stem diameter when grown in a sand and peat mixture. However, it was observed that, irrespective of amendment sources, the stem diameter increased slightly between 30 to 60 DAS and between 60 to 90 DAS in 2021 for both millet and sorghum (Figure 3(A) & Figure 3(C)). In contrast, stem diameter increased significantly between 30 to 60 DAS (on average 0.54 to 1.49 cm) and slightly between 60 to 90 DAS (on average 1.49 to 1.7 cm) (Figure 3(D)), except for millet, where stem diameter decreased after 60 DAS in 2022 (Figure 3(C)). This decrease in millet stem diameter in 2022 might be due to plant senescence and/or osmotic effects. Under saline conditions, plants find themselves in a toxic environment and face osmotic pressure [22] [49].
Several studies have reported findings on the effect of salt on plant growth. These findings vary with crops, genotypes [15] [28], experimental conditions (laboratory vs. field) [27] [28] [33] [48], and salt concentrations (low, moderate, and high concentrations) [50]-[53].
Figure 3. Plant stem diameter measurement was conducted on pearl millet (A and B) and sorghum (C and D) grown on plots amended with biochar, dolomite, leucaena, and rock phosphate at Atti-Apedokoe.
3.3. Plants’ Responses to Salt Stress in Amended Soils
Sorghum leaves showed the lowest total protein content where leucaena, biochar, and rock phosphate treatments were applied (Figure 4(A)). However, the total protein content in the leaves of millet and sorghum did not differ statistically with the different amendments for both crops (P > 0.05) (Table 6). This indicates that soil amendments did not produce differences in the accumulation of total protein content compared to the control. Previous studies reported similar findings. Zahra et al. [54] reported insignificant differences between stresses (saline and non-saline pots) for protein content in roots of two maize genotypes. However, Tort and Turkyilmaz [55] found that protein content in barley leaves varied with salt concentration, with 120 mM of sodium chloride inducing higher protein content.
Histograms without letters are not significantly different.
Figure 4. Biochemical attributes in leaves at 30 DAS: total protein (A), proline (B), and malondialdehyde (MDA) (C).
Statistical analysis showed significant differences in millet leaves’ proline content (P < 0.05), whereas for sorghum, there was no significant difference (Table 6). The proline content for millet was significantly high under control conditions, where no soil amendments were applied. This suggests that amendments influenced proline content. Plants from plots amended with rock phosphate, leucaena, and dolomite showed the lowest proline content, while those under biochar application showed intermediate proline content (Figure 4(B)). Plants usually accumulate proline in response to stress [56]. Treatments with lower proline content relative to the control for millet indicate that those amendments contributed less to salt stress. However, proline content has been reported to increase in plants with salt stress by Savouré et al. [57] and Toumi et al. [58]. Our results for pearl millet suggest the contrary. In sorghum leaves, proline content did not differ among amended treatments, possibly due to a higher threshold of salt stress in sorghum [58] [59]. This salt insensitivity in sorghum plants could also be attributed to the intrinsic characteristics of the genotypes, such as salt tolerance and salt sensitivity.
The MDA concentration effectively assesses oxidative stress damage to the cell membrane [60]. In this study, no difference was observed in MDA accumulation between the control and rock phosphate treatments on pearl millet. Biochar, leucaena, and dolomitic lime significantly reduced MDA in both pearl millet and sorghum leaves (Figure 4(C)). There was no significant difference in MDA between the control and rock phosphate treatments in pearl millet and sorghum. Millet and sorghum showed the lowest MDA accumulation under leucaena and biochar amendments, respectively. The low level of MDA accumulation indicates better protection of cell membranes against oxidative damage induced by salt accumulation in plants. Our findings align with several studies [58] [61]-[63] that reported various MDA content depending on the stress level and the genotype of sorghum, halophyte (Atriplex halimus L. and Atriplex canescens (Pursh) Nutt), pearl millet, and colza. It can be concluded that soil amendment with leucaena and biochar reduces salt accumulation in the root zone, and millet and sorghum differ in their ability to respond to salinity in their leaf cells.
3.4. Effect of Soil Amendment on Crop Yields under Saline
Conditions
Figure 5. Effect of amendment sources on dry biomass yields measured from pearl millet (A) and sorghum (B) at Atti-Apedokoe. (Histograms without letters are not significantly different).
The results for the dry biomass yield are depicted in Figure 5. Average dry biomass yield varied from 5418.6 kg/ha (control) to 7424.5 kg/ha (leucaena) for millet (Figure 5(A)) and from 5912.5 to 16371.9 kg/ha for sorghum (Figure 5(B)). Amendment sources affected dry millet and sorghum biomass differently. Notably, there was a significant effect only on the dry biomass yield of sorghum (P < 0.01) (Table 6). The lowest average dry biomass yield was obtained from the control treatment, while the application of amendments induced high biomass production. Application of rock phosphate showed the highest dry biomass; however, this was not significantly different from the biomass recorded from leucaena, dolomite, and biochar applications (Figure 5). In general, the application of soil amendments increased dry biomass by 106 to 177%. Furthermore, there was a significant difference between years only for millet dry biomass, with 2021 showing the highest biomass. The amendment-year interaction was not statistically significant for dry biomass yield for both millet and sorghum (P > 0.05) (Table 6). Rekaby et al. [64] reported similar findings, observing that applying organic amendments (biochar, humic acid, and compost) significantly increased the dry biomass yield of barley compared to a control treatment.
The results showed that neither year nor amendment-year interaction significantly affects grain yield for millet and sorghum. However, soil amendments significantly affected grain yields (P < 0.05) (Table 6). For millet, the lowest grain yield (1100.3 kg/ha) was observed under the control (without amendment application), followed by rock phosphate (1640.8 kg/ha), biochar (1681.4 kg/ha), and dolomite (1691.7 kg/ha), which were statistically not different. The leucaena treatment showed the highest grain yield for millet (2214 kg/ha) (Figure 6(A)). The application of amendments improved grain yields for millet by 49% - 101%, with an average increase of 64%. For sorghum, the application of biochar and leucaena showed the highest grain yields (1153.3 and 1158 kg/ha, respectively), followed by rock phosphate (946.9 kg/ha) and dolomite (737.3 kg/ha), whereas the control showed the lowest grain yield (381.3 kg/ha) (Figure 6(B)). The application of amendments improved grain yields by 93% to 202% (an average increase of 159%) for sorghum. Applying amendments, regardless of the crop, resulted in better grain yields than not applying them, which is the farmers’ usual practice in the area. Millet performed better than sorghum, and grain yield responses to amendments under saline conditions varied slightly depending on the amendment source. Organic amendments (biochar and leucaena) responded better to salinity than chemical amendments (rock phosphate and dolomite), which produced intermediate grain yields for millet and sorghum. Leucaena, in particular, outperformed all other amendment sources (Figure 6). The amendments significantly improved salt-affected soil conditions by reducing salinity and sodicity, enhancing soil structure, and increasing nutrient availability. Organic amendments like leucaena and biochar likely improved the soil’s physical properties, water retention, and microbial activity more effectively than inorganic amendments like dolomite and rock phosphate. Increasing water infiltration and drainage helps leach out excess salts.
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Figure 6. Effect of amendment sources on grain yields measured from pearl millet and sorghum at Atti-Apedokoe. (Histograms with the same letter are not significantly different).
Organic matter (OM) improves the mineral nutrient status and growth of plants in saline soils by supplying nutrients, particularly N, P, and K [32] [65]. Moreover, mulching benefits saline soils by reducing evaporation from the soil surface and encouraging a downward flow of soil water [49]. The response of crops to amendments under saline conditions depends on the type of amendment (organic vs. mineral) and the type of organic matter (leucaena vs. biochar) (Table 5). The characteristics of each amendment used in the study were compared. Leucaena and biochar showed high OM and N content, making them crucial for improving soil structure, supporting plant nutrient provision, and limiting soil water evaporation. Amendment with dolomite did not perform as well as leucaena and biochar. However, this amendment contains calcium and magnesium, reducing soil sodium levels and increasing calcium and sulfur provision to plants. The low performance of dolomite compared to the organic amendments might be due to the timing of application (one day before sowing). There was likely insufficient time for reactions to occur in the soil. Our findings align with Litardo et al. [66], who found that the application of mineral and organic amendments on rice affects its reproductive growth. Among all their treatments, compost, sugarcane filter cake, and leonardite showed the highest yield values compared to control under saline conditions. However, the authors observed that the application of the amendments did not affect vegetative growth (plant height, tiller number, and straw dry mass weight). Other authors, such as Choudhary et al. [67], showed that in the case of saline-sodic irrigation, sugar yield from sugarcane under farmyard manure treatment was comparable to the combination of gypsum and farmyard manure treatment but was significantly higher than that of gypsum alone.
Additionally, the significant increase in millet grain yield with leucaena and sorghum yield with biochar and leucaena was associated with the plant’s MDA content. Treatments with lower MDA content produced more yield, likely due to better cell membrane protection from oxidative damage, which might influence photosynthesis and the accumulation of assimilates. Toderich et al. [63] found similar results for pearl millet in response to different soil salinity levels, with yield decreasing as salinity increases. Few studies have reported on the benefits of biochar and leucaena for plants under saline conditions. In a critical review, Ali et al. [68] noted that biochar application increased plant growth, biomass, and yield under saline conditions. Ibrahim et al. [69] found that biochar soil amendment effectively alleviated the effects of salinity on sorghum seedling growth. Among all biochar levels tested, a 5% biochar level had a significant impact on mitigating salt stress. Kul et al. [70] also found that biochar amendments at 5% and 10% (v/v) significantly decreased malondialdehyde (MDA) and proline contents while improving tomato plant performance, including plant height, leaf area, shoot fresh and dry weight, number of leaves, and root fresh and dry weight under saline conditions. L. leucocephala showed promise for afforestation of sodic soils due to its potential to produce higher biomass and improve the fertility of these soils [71].
The experimental conditions impacted the results. Crop response to the application of amendments was more positive in 2021 than in 2022 (Figure 2 and Figure 3). This can be explained by the experiment being conducted during the short dry season in 2021, which was not completely dry. The crops benefited from rain and probably leaching, which likely reduced the severity of salinity. However, the analysis of variance for the amendments and year interaction was not significant for all parameters measured. Therefore, the plants reacted similarly to the amendments during both years.
4. Conclusion
The findings of this study have potential implications for agricultural practitioners and professionals in soil science and crop production. The differential impact of amendments on the growth of pearl millet and sorghum in saline conditions, with sorghum showing positive effects and pearl millet showing no significant impact, suggests the need for crop-specific approaches to soil salinity management. The study also revealed that both pearl millet and sorghum experienced osmotic and oxidative stresses, as indicated by elevated proline and MDA levels in the control group compared to the amended plants. The application of amendments led to a substantial improvement in grain yield, with an increase of up to 101% (averaging 64%) for pearl millet and up to 202% (averaging 159%) for sorghum. Among the various amendments tested, leucaena and biochar were identified as the most effective in mitigating soil salinity. These treatments significantly enhanced the grain yield of both pearl millet and sorghum and were correlated with MDA content. Notably, the organic amendments, specifically biochar and leucaena, outperformed the mineral amendments, such as rock phosphate and dolomite, in mitigating soil salinity.
Acknowledgements
The authors thank Koffi Anaté and Essossina Piwili for their invaluable help and cooperation during fieldwork in their bachelor and master training periods. The authors are also grateful to ICBA, which leads the technical activities of RESADE, and all the farmers of Atti-Apedokoe who participated in the project implementation. The authors are also grateful to the Laboratoire de Physiologie et Biotechnologies Végétales of the University of Lome for its technical assistance in determining the biochemical attributes of millet and sorghum.