Yields increasing in agriculture has led to excessive use of chemical fertilizers, with negative environmental consequences clearly established in previous studies [1] . It urges the development of innovative approaches to create new types of ecological fertilizers, highlighting the role of biodegradable vegetal organic matter. Natural phosphates (apatite), like the one exploited in Togo, are main sources of phosphorus, one of three key elements for plant growth. This mineral has a P₂O₅ content of approximately 28.20% [2] . However, the low solubility of these minerals is a challenge for their direct use as fertilizers [3] . Phospho-composting then appears as a practical approach to valorize them in agriculture [4] . This work aims to study the dissolution efficiency of natural phosphate from Togo during composting of organic matter and ore mixtures.

Biodegradable materials come from plant or animal sources, namely cow dung, poultry droppings, nymphea (nenuphar), and household waste. The choice of these types of biodegradable organic matter is due to their common use for agricultural soil amendment [5] [6] . The mineral material is natural phosphate from Togo, which is an apatite ( Figure 1 ).

Phospho-composts were developed for six (06) months by constituting ore-organic matter mixtures: phosphate and cow dung (A+P), phosphate and poultry droppings (B+P), phosphate and Nymphéa (water lily) (C+P), and phosphate and domestic biodegradable waste (D+P) in a ratio of 50 kg of ore and 450 kg of organic matter. 100% organic composts were also developed under the same conditions.

Organic matter content in our composts was calculated after calcining 10 g of each sample at 550˚C using the formula below:

$\%OM=\frac{m_1-m_2}{m_1}\times 100$

%OM: organic matter content;

m₁: mass of dry matter before calcination;

m₂: ash mass after calcination.

Total organic carbon content (TOC) in the different samples was calculated using the following formula:

$\%TOC=\frac{OM\left(\%\right)}{1.724}$

For the determination of nitrogen content, 0.5 g of compost was mixed with 10 ml of concentrated sulfuric acid and 0.5 g of Kjeldahl catalyst. Digestion was carried out hot until samples turned white. After cooling, the solution was diluted with 100 ml of distilled water and then subjected to distillation. A control sample was also prepared under the same conditions. During distillation using a nitrogen distiller, the diluted solution was combined with 50 ml of 40% sodium hydroxide in a dedicated flask, and 10 ml of 1N boric acid was poured into a 250 ml Erlenmeyer flask. Distillate was titrated with 0.02 N sulfuric acid (H₂SO₄) in the presence of a colored indicator The total nitrogen content was determined using the following formula:

$\%N=\frac{1.4\times N\times \left(Ve-Vt\right)}{P}$

%N: Total Nitrogen Content.

Ve: sample volume (ml).

Vt: control sample volume (ml).

N: normality of the titrant solution (0.02 N sulfuric acid).

P: weight of test sample (g).

The Olsen method has been used to extract available phosphorus from compost. 2.5 g of compost is placed in a container and 50 mL of 0.5 M sodium bicarbonate solution at pH 8.5 is added. The mixture is subjected to continuous stirring for 30 minutes to promote solubilization of available phosphorus. The solution is filtered to separate the liquid phase containing the available phosphorus from the solid residues [7] .

Determination of total phosphorus and available phosphorus content in samples was carried out using a HACH DR 3800 molecular absorption spectrometer, in accordance with the guidelines of the DR 3800 procedure manual. Measurement parameters of total phosphorus and available phosphorus are reported in Table 1 .

Extraction, characterization, and titration of humic substances are carried out after oxidation of the sample by potassium permanganate (KMnO₄) under alkaline conditions. Forty-five milliliters of ultrapure water were added to five (05) grams of compost, followed by slow stirring for two hours at room temperature. This operation promoted desorption of water-soluble compounds. The solid obtained, containing organic matter, was then centrifuged at 4000 rpm for 15 minutes at 4˚C. In a second step, a 0.1 N alkaline NaOH solution was introduced into the tube containing the solid. The mixture was subjected to the same conditions of stirring, allowing solubilization of the organic matter. Five milliliters of the filtrate were taken into a 50 mL beaker. To this sample, 10 mL of 0.1 N NaOH solution and 20 mL of 0.1 N potassium permanganate (KMnO₄) were added. The mixture was brought to a boil for 10 minutes to oxidize the organic compounds present.

After the solution had cooled completely, 20 mL of oxalic acid and 10 mL of concentrated sulfuric acid (6 N) were added to neutralize the excess permanganate and stabilize the solution before titration by gradual addition of 0.1 N KMnO₄.

For FTIR analysis, 2 mg of compost, previously dried at 105˚C for 24 hours, was collected. The sample was finely ground and then placed directly on the ATR of the spectrophotometer. Uniform pressure was applied using the integrated pressure arm. This configuration allowed us to record our infrared spectra between 350 and 4000 cm⁻¹ using the spectrophotometer FT-IR JASCO FT/IR-4700.

We have highlighted the dissolution of natural phosphate by organic matter during composting of ore and organic matter mixtures. Samples obtained from the analysis showed that the available phosphorus concentration in phospho-compost is higher than that in organic matter compost due to natural phosphate dissolution. This dissolution during the composting process depends on the nature of the organic matter and the time of maturation. Nymphea phospho-compost has a high concentration of available phosphorus (88%) after six (06) months of maturation.

Authors thank SIGMA Scientific Group and Mohamed VI Polytechnic University for their financial support.