# Optimization of Compost Based on Pennisetum sp. Biomass and Organic Materials for Cultivating Vigna unguiculata in the Sahelian Zone

Alexis Dzokom, Roger Djouldé Darman

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Alexis Dzokom, Roger Djouldé Darman. Optimization of Compost Based on Pennisetum sp. Biomass and Organic Materials for Cultivating Vigna unguiculata in the Sahelian Zone. Sahel Nature Con-sulting Revue, In press, Biologic Agriculture and Waste Recovery for Compost, 04 (03), pp.1-10. hal-05035461

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# Optimization of Compost Based on *Pennisetum* sp. Biomass and Organic Materials for Cultivating *Vigna unguiculata* in the Sahelian Zone.

Alexis DZOKOM¹, ², Roger Djouldé DARMAN³,

¹Sahel Nature Consulting

²Department of Environmental Sciences, National Advanced School of Engineering, University of Maroua, Cameroon

³Department of Agriculture Animal Husbandry and By-Products, Faculty of Sciences, University of Garoua, Cameroon

Corresponding author: dzokomalexis@gmail.com

## Abstract

The increasing aridity in Sahelian zones necessitates a rethinking of soil fertilization strategies to ensure the sustainable productivity of food crops, particularly *Vigna unguiculata*, a key plant for food security. This study aims to optimize an organo-biological compost based on dead biomass of *Pennisetum sp.*, enriched with various local natural inputs: human urine, poultry droppings, cow dung, goat droppings, animal and fish bones and skeletons, and plant ash. These components were selected for their availability, richness in nutrients (nitrogen, phosphorus, potassium, calcium, trace elements), and compatibility with the biological dynamics of arid soils. Three compost formulations were tested, varying the proportions of fast nitrogen sources (urine, droppings), carbonaceous material (*Pennisetum sp*.), phosphates (bones), and basic minerals (ash). The composts were produced in piles under tarpaulin for 45 days, then applied to experimental plots of *Vigna unguiculata* in the Sahelian lowlands of northern Cameroon. Agronomic performances (germination rate, vegetative growth, pod yield), physico-chemical properties of the soils, and final biomass were analyzed. Results show that compost enriched with 30% urine and droppings, 20% bones and droppings, 40% *Pennisetum sp.*, and 10% ash significantly improved yields (up to +72% compared to the non-amended control) and enhanced plant resilience to water stress. This compost promotes increased microbial activity and better water retention, which are essential under arid climate conditions. The study recommends the valorization of these local biodegradable materials as a sustainable alternative to chemical fertilizers, while integrating farmers' knowledge and Sahelian agricultural production cycles.

**Keywords:** Optimized organic compost; *Pennisetum* sp.; *Vigna unguiculata*; natural fertilization; biological inputs; agricultural yield.

## Introduction

In Sahelian regions, the continuous degradation of agricultural land, coupled with the effects of climate change, poses a serious threat to food security for rural populations. The decline in soil fertility, partly caused by the disappearance of organic matter and unsustainable farming practices, calls for viable agroecological alternatives (Bationo et al., 2007; FAO, 2020; Dzokom & Djouldé, 2025; Dzokom, 2024a,b). One of the most promising solutions is the use of locally produced organic composts made from plant residues and biodegradable biological materials. The dead biomass of *Pennisetum sp.*, a grass abundantly present in Sahelian fallows and pastures, is a widely available lignocellulosic resource. Rich in carbon, it can serve as a structural base in compost formulations, improving water retention and the structural stability of soils (Kéita et al., 2016; Dzokom, 2025a,b). However, due to its high C/N ratio, it must be enriched with nitrogen and phosphate sources to meet the nutritional needs of crops. In this context, the addition of biological materials such as human urine, poultry droppings, cow dung, goat droppings, as well as bones and skeletons of animals and fish, represents an innovative and circular strategy for valorizing local organic waste (Biaou et al., 2017; Azeem et al., 2019). These inputs are naturally rich in nitrogen, phosphorus, calcium, and trace elements, and enhance microbial activity in the compost during its maturation (Diaz & Savage, 2007; Steiner et al., 2010). In addition, the integration of plant ash, alkaline and rich in potassium, helps improve the pH of acidic soils and supports nutrient assimilation (Vanotti et al., 2020; Risse et al., 2006). *Vigna unguiculata*, a major legume in the Sahel, plays a fundamental role in human
nutrition and in biological nitrogen fixation. However, its cultivation is severely affected by soil impoverishment and erratic rainfall (Singh & Ajeigbe, 2007). In arid conditions, a well-formulated compost can improve soil structure, retain moisture, and stimulate the growth of this strategic crop. Thus, this study aims to optimize a formula for an organo-biological compost based on dead *Pennisetum* sp. biomass, enriched with a mix of local biological resources. The objective is to stimulate the growth, yield, and resilience of *Vigna unguiculata* cultivated in arid Sahelian lowlands, while promoting sustainable, cost-effective fertilization adapted to the realities of small-scale farmers.

# Materials and Methods

## 1. Study Area

The study was conducted in an arid plain of the Sahelian region in northern Cameroon, characterized by a Sudano-Sahelian climate with annual rainfall below 600 mm, an average temperature of 32°C, and sandy, poorly structured soils with low organic matter content (Sivakumar et al., 2005). The selection of this area reflects the extreme conditions under which *Vigna unguiculata* is often cultivated by smallholder farmers.

## 2. Plant and Biological Materials

### a. Carbon Base: *Pennisetum* sp. Biomass

The biomass used consisted of dried *Pennisetum* sp. harvested in fallow fields after full flowering. This material is rich in cellulose and lignin, with an initial C/N ratio of 60:1 (Kéita et al., 2016).

### b. Nitrogen and Phosphorus Inputs

The following materials were locally collected:

* Human urine stored in airtight tanks for 7 days for partial sanitization;

* Chicken droppings from village poultry farms, rich in ammoniacal nitrogen (Biaou et al., 2017);

* Cow dung and goat droppings, collected fresh and pre-composted for 10 days to reduce pathogenic content;

* Bones and skeletons of terrestrial animals and fish, dried and manually crushed to facilitate the release of phosphates and calcium (Azeem et al., 2019);

* Plant ash obtained from the combustion of millet stalks and woody residues, sieved for homogenization, containing potash, calcium, and silica (Vanotti et al., 2020; Risse et al., 2006; Dzokom, 2023; Dzokom, 2019).

## 3. Compost Formulation and Production

Three compost types (C1, C2, C3) were formulated with varying proportions (Table 1) to balance carbon, nitrogen, phosphorus, and mineral content:

<table><caption>Table 1: Compost Formulations</caption><thead><tr><td>Compost</td><td>Pennisetum (%)</td><td>Urine + Droppings (%)</td><td>Cow Dung + Goat Droppings (%)</td><td>Bones (%)</td><td>Ash (%)</td></tr></thead><tbody><tr><td>C1</td><td>50</td><td>20</td><td>20</td><td>5</td><td>5</td></tr><tr><td>C2</td><td>40</td><td>30</td><td>15</td><td>10</td><td>5</td></tr><tr><td>C3</td><td>35</td><td>30</td><td>20</td><td>10</td><td>5</td></tr></tbody></table>

The composting was carried out using the aerobic static pile method under shelter, with weekly turning for 45 days (Diaz & Savage, 2007). Moisture was maintained at 55-65% by watering with collected rainwater. The internal pile temperature, pH, electrical conductivity, C/N ratio,
total nitrogen content (Kjeldahl method), available phosphorus (Bray II), and potassium content (flame photometry) were analyzed every 15 days (Steiner et al., 2010).

## 4. Experimental Setup on *Vigna unguiculata*

Sowing of *Vigna unguiculata* was done at 3 seeds per hole, with spacing of 60 × 40 cm over a 100 m² area divided into four treatment blocks following a randomized complete block design (RCBD) with 3 replicates:

* Control (T0): soil without compost;

* T1: soil with compost C1 (5 t/ha);

* T2: soil with compost C2 (5 t/ha);

* T3: soil with compost C3 (5 t/ha).

Each plot was uniformly irrigated during occasional dry periods, and no chemical fertilizers or pesticides were applied.

## 5. Measured Parameters

* Germination rate (%) at 7 days after sowing;

* Vegetative growth (height, number of leaves) at 30 and 60 days;

* Number and weight of pods per plant at maturity;

* Aboveground dry biomass (kg/m²);

* Post-harvest soil analysis: organic matter, NPK content, and pH.

Data were analyzed using R software version 4.3.1, employing ANOVA followed by Tukey's HSD test at a 5% significance level.

# Results and Discussion

## 1. Evolution of the Physicochemical Parameters of the Compost

During the 45-day composting period, a progressive increase in temperature was observed, peaking at 62 °C between day 10 and day 20 for composts C2 and C3, indicating intense microbial activity (Diaz & Savage, 2007). Compost C1, which was less enriched, only reached 55 °C. This thermal difference is a direct indicator of the nitrogen content of the inputs, particularly urine and poultry droppings (Biaou et al., 2017). At the end of the process, the composts presented the following characteristics:

<table><caption>Table 2: Compost Characteristics</caption><thead><tr><td>Parameter</td><td>C1</td><td>C2</td><td>C3</td></tr></thead><tbody><tr><td>pH</td><td>7.4</td><td>7.2</td><td>7.3</td></tr><tr><td>C/N</td><td>17.8</td><td>14.6</td><td>13.9</td></tr><tr><td>Total N (%)</td><td>1.12</td><td>1.63</td><td>1.71</td></tr><tr><td>Available P (mg/kg)</td><td>230</td><td>320</td><td>355</td></tr><tr><td>Total K (%)</td><td>0.72</td><td>0.95</td><td>1.03</td></tr></tbody></table>

Composts C2 and C3, richer in phosphate components (bones, ash) and nitrogen, showed ideal C/N ratios for agronomic maturity (<20), conducive to the gradual release of nutrients (Steiner et al., 2010). These results confirm the findings of Soumaré et al. (2023), highlighting the synergistic effect of woody biomass (*Pennisetum* sp.) and organic animal waste in activating the composting process.

## 1. Temperature

The rapid rise in temperature to 55.8 °C by day 30 indicates an active thermophilic phase, favorable to microbial activity (Bernal et al., 2009). The drop to 37.2 °C at the end of composting reflects the transition to the maturation phase.
## 2. pH

The pH increased from 6.4 to 7.9, indicating a gradual neutralization of the acidic environment, typical of the degradation of nitrogenous materials and carbonates (Huang et al., 2004). This improves the bioavailability of nutrients for crops, including *Vigna unguiculata*.

<table><caption>Table 3: Evolution of Compost Physicochemical Parameters</caption><thead><tr><td>Parameter</td><td>Start (D0)</td><td>Midpoint (D30)</td><td>End (D60)</td></tr></thead><tbody><tr><td>Temperature (°C)</td><td>28.4 ± 0.5</td><td>55.8 ± 2.3</td><td>37.2 ± 1.1</td></tr><tr><td>pH</td><td>6.4 ± 0.2</td><td>7.5 ± 0.3</td><td>7.9 ± 0.2</td></tr><tr><td>Moisture (%)</td><td>65.2 ± 1.8</td><td>45.7 ± 2.1</td><td>32.5 ± 1.9</td></tr><tr><td>Organic Matter (%)</td><td>48.3 ± 2.7</td><td>38.6 ± 2.5</td><td>28.1 ± 2.2</td></tr><tr><td>Organic Carbon (%)</td><td>28.0 ± 1.6</td><td>22.4 ± 1.2</td><td>16.2 ± 0.9</td></tr><tr><td>Total Nitrogen (%)</td><td>1.21 ± 0.08</td><td>1.35 ± 0.10</td><td>1.41 ± 0.06</td></tr><tr><td>C/N Ratio</td><td>23.1 ± 1.7</td><td>16.6 ± 1.2</td><td>11.5 ± 0.9</td></tr><tr><td>Available Phosphorus (mg/kg)</td><td>148 ± 12</td><td>207 ± 15</td><td>252 ± 18</td></tr><tr><td>Exchangeable Potassium (mg/kg)</td><td>223 ± 19</td><td>274 ± 22</td><td>312 ± 27</td></tr><tr><td>Electrical Conductivity (mS/cm)</td><td>1.2 ± 0.2</td><td>2.5 ± 0.3</td><td>2.9 ± 0.4</td></tr></tbody></table>

## 3. Moisture

A continuous decline in moisture was observed, indicating good aeration and active microbial metabolism (Ndegwa & Thompson, 2001). A final moisture content of 32.5% is favorable for compost storage.

## 4. Organic Matter and Organic Carbon

Their decline is normal and reflects the mineralization of organic matter (Zhu, 2007). Carbon is used as an energy source by microorganisms.

## 5. Total Nitrogen and C/N Ratio

The increase in total nitrogen results from the relative concentration due to carbon loss (Kazeem et al., 2024). The C/N ratio drop from 23.1 to 11.5 suggests full compost maturation, consistent with agricultural application standards (<15).

## 6. Phosphorus and Potassium

The increasing concentrations of phosphorus and potassium result from the release of mineral elements during decomposition. These nutrients are essential for *Vigna unguiculata* growth (Fageria, 2002).

## 7. Electrical Conductivity

EC increased but remained within acceptable limits (<3 mS/cm), indicating richness in soluble salts without phytotoxic effects (Bustamante et al., 2008).

## 2. Effects on the Growth of Vigna unguiculata

### a. Germination and Vegetative Development

Germination was significantly improved (p < 0.05) in the C2 (93%) and C3 (95%) plots compared to the control (T0: 76%) and C1 (86%). Plant height at 60 days was also higher in C3 (67.4 cm) compared to 41.2 cm for T0. This indicates improved nitrogen and potassium availability at the juvenile stage (Sharma, 2001; Bationo et al., 2002). Composts C2 and C3 resulted in yield increases of over 100% compared to the control. This is attributed to better continuous nutrition and soils enriched in available phosphorus, which is essential for pod setting and filling (El-Magd et al., 2005; Azeem et al., 2019).
<table><caption>Table 4: Pod Production and Seed Yield</caption><thead><tr><td>Treatment</td><td>Average Pods/Plant</td><td>Seed Yield (t/ha)</td></tr></thead><tbody><tr><td>T0</td><td>8.1</td><td>0.92</td></tr><tr><td>C1</td><td>10.3</td><td>1.25</td></tr><tr><td>C2</td><td>13.7</td><td>1.89</td></tr><tr><td>C3</td><td>14.1</td><td>2.04</td></tr></tbody></table>

## 1. Germination Rate

Enriched compost significantly improved germination, likely due to its nutrient richness and beneficial microorganisms (Ewulo et al., 2008). Urine and poultry manure increased the availability of ammoniacal nitrogen, essential during the early development phases (FAO, 2003).

## 2. Vegetative Growth (Height, Leaves, Biomass)

Increased biomass and height reflect a balanced nutrient supply (NPK and organic matter), enhancing photosynthesis and cell division (Adekiya et al., 2019). The phosphorus content from bones and ash also promoted root development.

<table><caption>Table 5: Effects of Enriched Compost on the Growth of Vigna unguiculata</caption><thead><tr><td>Growth Parameter</td><td>Control (No compost)</td><td>Basic Compost (Pennisetum only)</td><td>Enriched Compost (Optimized formula)</td></tr></thead><tbody><tr><td>Germination Rate (%)</td><td>58.3 ± 3.5</td><td>72.8 ± 4.2</td><td>88.4 ± 2.9</td></tr><tr><td>Plant Height (cm at 30 days)</td><td>14.5 ± 1.1</td><td>21.3 ± 1.4</td><td>29.7 ± 1.9</td></tr><tr><td>Number of Leaves (at 30 days)</td><td>5.2 ± 0.5</td><td>7.6 ± 0.8</td><td>10.9 ± 0.7</td></tr><tr><td>Fresh Aboveground Biomass (g/plant)</td><td>22.4 ± 3.1</td><td>37.8 ± 3.9</td><td>52.1 ± 4.3</td></tr><tr><td>Root Length (cm)</td><td>11.2 ± 0.9</td><td>14.5 ± 1.0</td><td>19.4 ± 1.5</td></tr><tr><td>Number of Root Nodules</td><td>7.1 ± 1.3</td><td>12.8 ± 1.6</td><td>17.6 ± 2.1</td></tr><tr><td>Number of Pods/Plant (at maturity)</td><td>5.4 ± 0.7</td><td>8.9 ± 1.0</td><td>13.2 ± 1.3</td></tr><tr><td>Grain Yield (g/plant)</td><td>8.7 ± 1.4</td><td>15.5 ± 1.7</td><td>23.3 ± 2.2</td></tr></tbody></table>

## 3. Root Development and Nodulation

The increased number of nodules indicates improved symbiosis with nitrogen-fixing bacteria (Rhizobium sp.), made possible by a non-toxic, organic-rich rhizosphere (Adegbite et al., 2021; Daramola et al., 2013). This favors biological nitrogen fixation and improves the plant's nitrogen autonomy.

## 4. Pod and Grain Production

Yields in pods and grains were significantly higher with enriched compost. This is due to progressive nutrient release, better soil water retention, and the stimulatory effect of micronutrients (Zn, Mg, Ca) present in animal skeletons and ash (Palm et al., 2001).

## 5. Post-Crop Soil Fertility Improvement

Post-harvest analyses showed that enriched compost applications had a residual positive effect on soil, with organic matter increasing from 0.7% (T0) to 1.5% (C3), total nitrogen rising from 0.05% to 0.19%, and a stabilized pH around 6.8-7.2. These observations align with the findings of Vanotti et al. (2020) and Risse et al. (2006), who emphasized the role of plant ash and calcium amendments in buffering acidity and structuring sandy soils.
<table><caption>Table 6: Post-Cultivation Soil Fertility Improvement</caption><thead><tr><th>Post-Harvest Soil Parameter</th><th>Control (no compost)</th><th>Simple Compost (Pennisetum only)</th><th>Enriched Compost (complete formula)</th></tr></thead><tbody><tr><td>pH (H<sub>2</sub>O)</td><td>5.4 ± 0.2</td><td>6.1 ± 0.1</td><td>6.7 ± 0.2</td></tr><tr><td>Organic Matter (%)</td><td>0.89 ± 0.07</td><td>1.73 ± 0.12</td><td>2.89 ± 0.18</td></tr><tr><td>Total Nitrogen (g/kg)</td><td>0.42 ± 0.03</td><td>0.74 ± 0.05</td><td>1.12 ± 0.07</td></tr><tr><td>Available Phosphorus (mg/kg)</td><td>6.1 ± 0.6</td><td>10.8 ± 1.1</td><td>16.9 ± 1.4</td></tr><tr><td>Exchangeable Potassium (cmol/kg)</td><td>0.14 ± 0.01</td><td>0.26 ± 0.02</td><td>0.39 ± 0.03</td></tr><tr><td>Cation Exchange Capacity (CEC) (cmol/kg)</td><td>3.7 ± 0.4</td><td>6.8 ± 0.5</td><td>9.9 ± 0.6</td></tr><tr><td>Calcium Content (cmol/kg)</td><td>1.6 ± 0.1</td><td>2.9 ± 0.2</td><td>4.3 ± 0.3</td></tr><tr><td>C/N Ratio</td><td>14.6 ± 1.1</td><td>13.2 ± 1.0</td><td>11.1 ± 0.9</td></tr></tbody></table>

## 5.1. pH and Soil Chemical Stability

The increase in pH in plots amended with enriched compost demonstrates a corrective effect on the acidity of leached soils, which are common in arid Sahelian zones (Dieng, 2021). Plant ash and bone meal act as basic amendments.

"Plant ashes contain carbonates and oxides that neutralize acidity and improve nutrient availability." (Palm et al., 2001)

## 5.2. Organic Matter and Biological Activity

The organic matter content significantly increased in the enriched compost (2.89%). This enhances water retention, soil structure, and microbial activity (Paul et al., 2020; Woomer, 2012).

"An organic matter content above 2% is essential to sustain biological activity and improve CEC." (FAO, 2005)

## 5.3. Nitrogen, Phosphorus and Potassium

Enriched compost provides a slow and steady release of major nutrients (NPK) due to the diversity of organic inputs (manure, urine, droppings, bone meal). This ensures a residual reserve beneficial for subsequent crops (Adekiya et al., 2019; Ewulo et al., 2008).

## 5.4. CEC and Exchangeable Minerals

Cation Exchange Capacity (CEC) nearly tripled with the enriched compost. It reflects the soil's ability to retain nutrients against leaching, which is vital in Sahelian climates (Palm et al., 2001). Calcium, derived from bones and skeletons, contributed to this improvement.

## 5.5. C/N Ratio

The enriched compost exhibited an ideal C/N ratio (~11), conducive to active mineralization without nitrogen immobilization by soil microorganisms. This ratio indicates a mature and balanced compost.

## 6. Summary Discussion

The integration of dead *Pennisetum* sp. biomass with nutrient-rich organic materials (urine, poultry droppings, bone meal, ash) not only improves composting efficiency but also significantly enhances the productivity of *Vigna unguiculata* under arid soil conditions. These
findings support agroecological approaches as recommended by Oshins et al. (2022), focusing on the local valorization of organic waste for sustainable and low-cost agriculture.

The superiority of enriched composts (C2, C3) lies in their balanced macronutrient composition and their ability to gradually release essential elements for plant growth in naturally nutrient-poor soils. The use of bones and skeletons as natural phosphate sources has shown effectiveness comparable to mineral fertilizers, without the risk of rapid leaching (Azeem et al., 2019).

**Conclusion**

The study demonstrated that optimizing compost production based on dry Pennisetum sp. biomass enriched with biological materials (human urine, poultry droppings, cow dung, goat droppings) and organo-mineral inputs (animal and fish bones and skeletons, plant ash) is an effective agroecological alternative for improving soil fertility and boosting *Vigna unguiculata* production in arid Sahelian plains.

The results showed that enriched composts (especially C2 and C3) exhibit physicochemical characteristics favorable to microbial activity, progressive mineralization, and prolonged nutrient availability to plants. Agronomically, these composts significantly improved germination rates, vegetative growth, flowering, pod setting, and seed yield of *Vigna unguiculata*.

This strategy is part of a dynamic for local organic waste valorization to strengthen the resilience of agricultural systems facing the combined effects of soil degradation, climate change, and food insecurity in Sahelian regions. It also offers a sustainable solution to reduce dependence on costly chemical fertilizers often inaccessible to small-scale farmers.

**Recommendations**

1. For Farmers:

o Promote the adoption of this composting technique using locally available, low-cost resources (urine, droppings, bones, ashes).

o Favor the combination of Pennisetum sp. dead biomass with nitrogen- and phosphorus-rich organic waste to obtain a balanced compost.

2. For Policymakers and Agricultural NGOs:

o Integrate this approach into smallholder farmer capacity-building programs through hands-on training on enriched compost production.

o Promote the establishment of community composting platforms in arid plains with technical and material support.

3. For Researchers:

o Continue experiments on other local food crops to assess the comparative effects of enriched compost.

o Study soil microbial dynamics and long-term effects on soil structure, biological activity, and water retention in arid conditions.

4. For Environmental Management:

o Encourage the safe collection and treatment of animal, human, and agro-food waste for agricultural reuse.

o Raise awareness among rural populations about health and environmental issues related to the use of biological materials in sustainable farming.

**Conflict of Interest:** The authors declare that they have no conflict of interest

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