The characteristic color of alluvium and water of the River Nyong appears as an exceptional phenomenon in Southern Cameroon. Usually, the water color of rivers changes with seasons and raining events. This is linked to the geological environment. The curiosity is that, among many rivers sharing the same area, the River Nyong is the only one which contains dark alluvium and water. This is the reason why the origin of that characteristic color should be understood in order to extend this comprehension in others cases. The significance of this study is to determine the origin of the dark color of alluvium and the dark water in River Nyong in Akonolinga.

The field studied extended from latitude 3˚27' to 4˚47'N and from latitude 11˚36' to 12˚43'E (Figure 1). The coordinates of pedological excavations, in which samples of clay have been taken, are as followed: P1: N03˚46'08" and E012˚14'41"; P2: N03˚46'07" and E012˚14'32"; P3: N03˚46'02" and E012˚14'3. The river covers an area of 27800 km² and his bed is deep at 650 m of altitude in marshy valley, in the equatorial forest: Ndam, 1997 [1] ; Bineli, 2009 [2] and Samba, 2008 [3] . This field is submitted to the equatorial climate with four seasons.

To open pedological excavations (P1, P2 and P3), in which samples were collected, manual work materials were usually used. A digital camera, a laptop and the GPS were used. The sample were packaged in plastic bags and sent to the laboratory (IRAD-Yaoundé) to determine the quantity of organic matter, the granular composition, and the acidity of the soil. Four levels of test samples were defined notably at 0 cm, 20 cm, 50 cm and 80 cm.

The determination of the various parameters was made after drying, grinding and sieving of samples. The organic carbon is determined according to the method of oxidation to the dichromate of potassium of Walkley and Black. The comparable phosphor is measured according to the method of Bray2. It is extracted by a mixture of HCl and NaF while the soluble phosphor is extracted according to the method Olsen in the NaHCO₃ 0.5 mol/l in pH 8.50 ± 0.02. The pH_water is measured in a solution ground/water solution ratio of 1/2.5, with a pH-meter provided with a glass electrode and the pH_KCl is obtained by realizing in the same proportions a mixture normal ground/KCl. The total nitrogen is determined according to the method Kjedhal. The cation exchange capacity is estimated by colorimetry at 655 nanometer of wavelength spectrometry. The granulometry was performed according to the method of Robinson pipette.

The results were separated in three groups: the organic matter; the granulometry and the acidity of the soil.

The works of several authors have studied the origin of the color of the water. According to Davis and Milligan, 2011 [8] , the blue color of lakes and oceans is closely related to that of heaven. This is due to reflections mechanisms and absorption of light radiation. It is a differential absorption of some radiation in the electromagnetic spectrum of white light: Pope and Jery, 1996 [9] ; Hoge, Vodacek, Swift, Yungel and Blough, 1995 [10] . For cons, the difference established by Jansen 1974 [11] ; between the white and black water rivers and streams only considers their mineral composition. Thus, the black waters are rich in aluminum, while the clear waters are rich in calcium ions, potassium. According to the same author, the abundance of ferric ions can make ocher the color of water. However, in forests, several authors show that the presence of organic matter in solution is responsible for the black color of river waters: [11] ; Sioli and Harald, 1975 [12] ; Duncan and Fernandes, 2010 [13] . These last results are in agreement with those obtained in the Nyong because the alluvial samples in the floodplain of the river revealed the presence of large amounts of organic matter. The peculiarity in the Nyong is that, the black color of the river water also affects the alluvium of the valley. Two major facts may justify this combination: the accumulation of decaying organic matter in the valley and the regular flooding of the latter by the black waters of Nyong River which may contaminate the alluvium.

Due to the accumulation of important quantities of organic matter in the valley of River Nyong in Akonolinga, it has made the water and the alluvium to be black. This black color of the alluvium and water is also facilitated by the acidity of the valley, the absence of oxygen, the poor activities of microorganisms and the presence of impermeability clay layer. The dissolving of the organic matter in these conditions justifies the black color of the water and associated alluvium.