The Kobongoye rice valley lies between latitudes 14˚8' and 14˚10' N and longitudes 16˚46' and 16˚43' W, within the municipality of Fimela, Fatick department and region (west-central Senegal) (Figure 1). Its rice-cultivable area covers approximately 200 hectares. The geology of the area has been substantially shaped by marine transgressions and regressions during the recent Quaternary. As a result, the geomorphology is characterized by channels, mudflats, tannes (salt flats), and sand bars. The soil units of the valley consist of tropical ferruginous soils, non-saline hydromorphic mineral soils, and halomorphic soils. The climate, of the coastal North-Sudanian type [11], is characterized by an average annual rainfall of 601 mm over the period 1991-2020. Mean annual temperature is 28.7˚C, ranging from a minimum of 25.4˚C in January to a maximum of 30.4˚C in June. Vegetation in the valley is mainly herbaceous. The valley has a substantial potential for rice cultivation.

Figure 1. Location of the study area.

2.2.1. Data Collection Methods

Although the valley has a cultivable potential exceeding 200 hectares, the organo-mineral amendment—and therefore the monitoring of its effect—concerned an area of 30 hectares, covering the cultivated rice plots that constitute the actual study area. This part of the valley, once delimited (Figure 1), was amended with phosphogypsum at a rate of 0.4 t∙ha⁻¹, combined with cow manure at a rate of 0.5 t∙ha⁻¹, and ploughed to facilitate incorporation.

A systematic sampling design was applied using a 100 m × 100 m grid. Two soil sampling campaigns were carried out, each comprising 29 sampling points (58 observation points in total), at a depth of 0 - 30 cm. The first campaign took place in July 2023, before amendment and cropping, and the second, at the same location, in May 2024. Both months correspond to peak dry-season conditions (July normally marks the start of the rainy season, but no rainfall had yet occurred at the time of sampling); they were selected to minimize the confounding effect of the seasonal dynamics of capillary salt rise during the dry season and salt leaching during the rainy season. Soil samples were packaged, coded, and sent to the National Pedology Institute (NPI) laboratory for physico-chemical analyses.

It should be noted that the absence of an untreated control plot constitutes a methodological limitation of this study.

2.2.2. Soil Analysis Methods

After air-drying, grinding, and sieving, the soil samples were analyzed for the following parameters: particle-size distribution, pH, Electrical Conductivity (EC), organic carbon, total nitrogen, available phosphorus, Cation Exchange Capacity (CEC), and exchangeable bases. The trace metal elements Pb and Cd were also measured. The analytical methods used are summarized in Table 1 below.

Table 1.Analytical methods for soil parameters.

2.2.3. Data Processing and Analysis

Soil analysis data were compiled and processed using Excel and RStudio for descriptive statistics and multivariate analysis (PCA), and ArcGIS for spatial mapping.

The physical and chemical results were interpreted using classification thresholds drawn from the following references: particle-size distribution (Aisne texture triangle); pH (ISRIC, 1993; CRAAQ, 2003); EC (Durand, 1983); P (London, 1984); N (Piéri, 1989); CEC (CRAAQ, 2003; Doucet, 2006); BS (Doucet, 2006); Mg (London, 1984); K (Durand, 1983); OM, C/N, and Ca (Pasquier, 2000).

The physico-chemical results of the analyzed samples are presented in Table 2 below.

pH values indicate acidic to slightly acidic soils before amendment with phosphogypsum and cow manure (PGY and CM), and extremely acidic to neutral soils after amendment.

Electrical Conductivity (EC) values indicate soils ranging from non-saline to extremely saline both before and after amendment.

Regarding organic matter (OM), total nitrogen (Nt), and the C/N ratio, no substantial variability in soil characteristics was noted before and after amendment. However, elevated N contents and a rapid-to-optimal OM mineralization rate were observed after amendment.

For available phosphorus (P₂O₅), potassium (K), and magnesium (Mg), no substantial differences were observed before and after amendment. In contrast, calcium (Ca) contents decreased after amendment. Regarding the parameters characterizing the soil adsorbing complex (CEC, BS, and ESP), values before and after amendment were essentially similar.

Particle-size analysis shows a shift from a sandy-loam texture before amendment to a sandy-loam to light sandy-loam texture after amendment.

Table 2.Interpretation of soil physical and chemical parameters.

Water pH showed a modest but real increase (+1.3%), suggesting a limited corrective effect of the amendment on soil acidity, consistent with the weakly acidic character already observed before treatment. More notable is the decrease in electrical conductivity (EC), which dropped by 33.7%. This decline reflects a substantial reduction in the soluble salt load of the soil, a result of particular relevance in the context of these paddy soils, where salinization constitutes a major constraint on production.

Table 3.Comparison before/after amendment (paired Wilcoxon test, n = 29 pairs).

The amendment therefore appears to have exerted a leaching or complexation effect on soluble ions, limiting their mobility in the soil solution. Organic carbon (C), organic matter (OM), and total nitrogen (N) contents all increased consistently (+13.3%, +13.3%, and +5.8%, respectively), accompanied by a rise in the C/N ratio (+6.5%). This joint increase confirms an effective input of organic matter by the amendment, with a slight rise in the C/N ratio that nonetheless remains within a range favorable to active mineralization, without risk of excessive nitrogen immobilization. This result supports the hypothesis of an organo-mineral amendment rather than simple liming. The behavior of exchangeable cations is more contrasted. Calcium (Ca), magnesium (Mg), sodium (Na), and sulfur (S, comparable to the sum of exchangeable bases) all decreased significantly, with respective declines of 51.0%, 35.6%, 24.2%, and 44.2%. In contrast, potassium (K) showed a dramatic increase of +137.9%, suggesting a targeted potassium input from the amendment, possibly in the form of plant residues, ash, or an associated potassium fertilizer. This redistribution of exchangeable bases—a joint decline in Ca, Mg, Na, and S alongside a sharp rise in K—was logically reflected in the cation exchange capacity (CEC), which decreased by 31.6%, and in base saturation (V), which fell by 15.3 points. This decline in CEC could reflect a loss of exchange sites associated with the leaching of divalent bases (Ca, Mg), a phenomenon that would merit being related to the texture and clay mineralogy of the soils studied. Available phosphorus (P) content dropped by 32.3%, which could indicate either phosphorus immobilization through adsorption/precipitation mechanisms favored by the new physicochemical conditions of the soil, or increased uptake by the crop in response to the improvement of other fertility factors. The exchangeable sodium percentage (ESP), on the other hand, increased slightly (+14.9%), a change that, while remaining below the critical sodicity threshold (generally set at 15%), warrants monitoring insofar as it appears to contradict the decrease in exchangeable Na in absolute terms. The most concerning result concerns the significant increase in cadmium (Cd: +43.1%) and lead (Pb: +65.0%) contents after amendment. This increase is attributable to the phosphogypsum, which contains these metals. Overall, the amendment produced beneficial effects on salinity (decrease in EC), organic status (increase in C, OM, N), and potassium supply, but was accompanied by a decrease in cation exchange capacity, base saturation, and phosphorus availability, as well as an undesirable enrichment in cadmium and lead.

The boxplots below (Figure 2) show the variability of pH and EC values before and after amendment. Most pH values were concentrated between 5.7 and 6.2 before amendment, with extremes of 5.3 and 6.5 and a median of 5.9. After amendment, extreme values ranged from 4.3 to 7, with a clear predominance of values between 5.6 and 6.5, and a median of 6. EC values were more widely spread before amendment (41 to 9,280 µS/cm) than after amendment (92 to 5,521 µS/cm). The interquartile range, which encompasses 50% of EC values, also changed, from 294-2,020 µS/cm before amendment to 150-1,680 µS/cm after amendment. The median EC decreased from 456 to 430 µS/cm before and after amendment, respectively.

Figure 2.Variability of pH (A) and EC (B) before and after PGY + CM amendment.

Multivariate analysis of the soils’ physical and chemical parameters identified two principal components for each PCA (Figures 3(A)-(B)). The variances explained were 65.16% before amendment and 64.8% after amendment, indicating a satisfactory representation of the data.

Before PGY + CM amendment, Component 1 (36.68% of the variance) was mainly driven by the contribution of Ca, Mg, Na, K, S, BS, ESP, and EC, while Component 2 (28.48% of the variance) was determined by C, C/N, OM, and N.

After amendment, the same two components were still present but with a lower explained variance and weaker correlations between variables. A more significant contribution of particle-size parameters (clay, silt, and sand) and CEC was also observed, in contrast to ESP, which followed an opposite trend.

Both PCAs also revealed a negative correlation between pH and EC. The same pattern was observed between silt and sand, with a stronger correlation after amendment.

Figure 3.Principal component analysis of soil physicochemical parameters before (A) and after (B) PGY + CM amendment.

3.5.1. Spatial Variability of pH

Spatially, before amendment, soils were dominated by moderately acidic pH, followed by acidic soils located in the North and slightly acidic soils in the Center-East (Figure 4). After amendment, the pH distribution became more heterogeneous, with a predominance of slightly acidic soils in the Center and South, followed by moderately acidic soils in the Center-West and acidic soils in the North. However, extremely acidic and very acidic soils were noted in the Northeast, along with patches of neutral-pH soils scattered across the Center and South (Figure 4).

Figure 4. Spatial variability of pH before and after amendment.

3.5.2. Spatial Variability of Electrical Conductivity (EC)

Figure 5. Spatial variability of soil electrical conductivity before and after amendment.

Regarding the spatial distribution of EC, before amendment, non-saline soils were found from the South toward the Center, while saline soils—ranging from slightly saline, saline, and very saline to extremely saline—occupied more than half of the area toward the North-East (Figure 5). After amendment, a significant decrease in the area of extremely saline soils was observed, together with an increase in slightly saline soils at the expense of saline soils (Figure 5).

3.5.3. Spatial Variability of Soil Organic Matter (OM)

Before amendment, soil OM contents were moderate from the North toward the centre of the valley and low from the centre toward the South. Soils well and very well supplied with OM were located in the North-East (Figure 6). After amendment, soil OM contents improved across most of the study area (Figure 6).

Figure 6.Spatial variability of soil organic matter.

Figure 7 summarizes the post-amendment changes in the distribution of soil pH, EC and OM in the studied part of the valley.

Overall, pH increased by 0.2 to 1.24 units across most of the study area, except for a few pockets in the Northeast and West, and small patches in the Center and South, where increases of 0.41 to 1.30 units were noted.

Regarding EC, a decrease of 1,000 to 3,000 µS/cm was observed across most of the study area. Only a small portion of the area recorded an increase in EC, with values ranging from 200 to 1,000 µS/cm.

Figure 7. Dynamics of soil pH, EC and OM after amendment.

Figure 8.(A) Pb and (B) Cd contents in soils before and after PGY + CM amendment.

Pb and Cd contents are shown in Figure 8. The Pb and Cd contents of the applied phosphogypsum were 0.5 and 6.7 mg/kg, respectively.

For Pb, contents increased from 0.05 mg/kg (before amendment) to 0.45 mg/kg (after amendment).

For Cd, an increase from 0.02 mg/kg (before amendment) to 0.29 mg/kg (after amendment) was observed.

The highest values for both lead and cadmium were recorded at sampling point M29, located in the Northeast of the study area.