The fertility status of surface soils in the humid forest zone of Southern Cameroon
A. MOUKAM and D. NGAKANOU^1
Institute of Agricultural Research CRA/Nkolbisson, BP 2067 Messa-Yaounde,
(Received 28 October, 1996; accepted 10 July, 1997)
Code Number: CS97034 Sizes of Files: Text: Graphics: Tables and line drawings (gif) - 76KABSTRACT
Soils under humid forest vegetation of Cameroon are generally classified as "Ferrallitic soils". With the increasing population in the country, agriculture is becoming more intensive in this humid forest zone but there is little information available on the fertility status of these soils. In order to better describe these soils, 313 soil samples were collected in the Southern Cameroon. Laboratory analyses including pH, exchangeable aluminium, cation exchange capacity, macro-nutrient levels, organic matter, total nitrogen and texture, were conducted. The results obtained indicate the following tendencies: low pH, low organic matter content, low level of effective cation exchange capacity and bases; nitrogen, phosphorus and potasium deficiencies and a relatively high degree ofaluminium saturation of the exchange complex. Generally the nutrient level are nigher where organic matter is more abundant. Though there is a wide variation in texture, it seems not to be a limiting factor for the intensive agricultural use of these soils.
Key Words: Ferrallitic soils, humid forest, Southern Cameroon, topsoils
Les sols sous foret humide du Cameroon sont generalement consideres comme des sols ferrallitiques. A cause de la poussee demograpnique dans le pays, on pense de plus en plus a l'intensification de l'agriculture dans cette zone forestiere humide, mais on ne dispose pas assez d'informations tables sur la fertilite naturelie de ces sols. Pour evaluer cette fertilite, 313 echantillons de sols superficiels ont ete collectes dans toute la zone forestiere du Sud Cameroun. Les analyses au niveau du laboratoire ont porte sur le pH, l'aluminium echangeabie, la capacite d'echange cationique, les elements majeurs, la matiere organique et la texture. Les resultats obtenus ont montre une forte proportion des sols acides, des valeurs basses de matiere organique, de la capacitie d'echange cationique, des bases echangeables; des deficiences en azote, phosphore et potassium, une saturation en aluminium du complexe d'echange relativement elevee. On note en general une correlation significative entre le niveau eleve des elements nutritifs et l'abondance de la matiere organique. Malgre l'existence d'une grande variation en texture, elle ne semble pas constituer un facteur limitant pour l'utilisation intensive de ces sols.
Mots Cles: Sols ferrallitiques, forets humides, Sud du Cameroun, sols superficiels
Considering the population pressure on land in the Western, North Western Province and the predominance of marginal lands in the Northern part of Cameroon, research on soil resources for agricultural production under humid forest is a priority.
About 22 millions ha of Southern Cameroon representing 40% of the country is covered by forest (Fig. 1). Climate varies within this zone. In the South Plateau, the climate type is sub-equatorial with four unequal seasons: two dry and two rainy seasons. In the South-West area the climate is a humid tropical one.with pronounced dry and wet season. The mean annual temperature ranges from 17 to 34 C in the whole area. Rainfall varies from 1200 to 4000 mm and the annual evapotranspiration deficit varies from 0 to 125 mm. Most of the soils under humid forest vegetation of Cameroon belong to the orders of Oxisols, Ultisols, Inceptisols and Entisols (FAOUNDP-IRA EKONA, 1976; 1977; 1978;1985), but the most dominant are Oxisols and Ultisols, known here as "Ferrallitic soils". These soils are believed to have low inherent fertility. The clay mineral fraction consists mostly of a mixture of kaolinite and halloysite, iron oxides and gibbsite (Ngakanou, 1987). In the South-West area, some allophane are present in the Inceptisols.
Past studies (FAO - UNDP - IRA EKONA, 1977; 1978; Moukam and Matapit, 1986) show phosphorus deficiency in the South Plateau soils, while in the South-West area, despite the high P fixation of the pyroclastic materials from volcanic activities, no P deficiency was reported. This study provides an overview of the fertility status of the humid forest zone of Cameroon, for use in land planning and in the design of future soil fertility studies.
MATERIAL AND METHODS
A total of 313 surface soil samples (0-20cm depth) were selected from different areas as shown in Figure 1 and were analysed in the IRA laboratory at Ekona according to Tchuenteu and Schalk (1988) as briefly described below:
The samples were air dried and screened through a 2mm stainless steel. Particle size was determined according to the U.S.D.A. standard sieve method using a Na-hexametophosphate as dispersing agent. Organic carbon was colorimetrically determined, after Walkley and Black wet combustion. Total nitrogen was digested by Kjeldahl method, then colorimetricaly determined on a technicon auto-analyser. The pH values were potentiometrically determined at a ratio soil/solution 1 : 2.5. The CEC was determined by ammonium acetate at pH 7 and K, Mg, Ca were determined in the leachate, using either flame photometry (K, Ca) or atomic absorption (Mg). Exchangeable aluminium was extracted with 1 N KCl and determined by titration. Available phosphorus was extracted with O.1 N HCl and 0.03 N NH4F solution and determined colorimetrically (Bray 2). Effective CEC was computed as a sum of Kcl exchangeable aluminium and hydrogen plus bases. The percentage of aluminium saturation was calculated in relation to the effective CEC. Delta pH was computed as pH (KCl) - pH (H2O). All elemental analytical results were reported on oven-dry basis.
RESULTS AND DISCUSSION
As shown in Figure 2, the pH in water of these soils ranged from 3.7 to 7.0. Approximately 63% of the samples have a pH ranging from 3.7 to 5.0, 33% from 5.0 to 6.0, and 4% above 6.0. Seventy eight percent of the samples had a pH less than 5.5, an indication that these soils are strongly acid. Except at pH values less than 4.2, where the hydrogen ion concentration may stop or even reverse cation uptake by roots, it is known that pH per se has no direct effect on plant growth (Sanchez, 1976). The values obtained for pH in KCl were less than those in H2O. The charge pH (pH KCl-pH H20), ranged from 0.0 to - 1.6 pH units. For 70% of the samples, it was between 0.6 and - 1.8 units, indicating the predominance of net negative charge in these soils, mainly in Inceptisols.
Exchangeable bases and aluminium.
obtained (Table 1) show that 72% of the sample were under the values of 0.15 meq/100g soil considered as low, 17 % were between 0.15 to 0.50 meq/100g soil considered as medium, and 11 % higher than 0.50 meq/100g soil considered as high exchangeable potassium.
The exchangeable Ca ranged from 0.41 to 7.3 meq/100g soil. Values less than 2.4 meq/100g soil, represented 71% of the samples, medium from 2.4 to 4 meq/100 g soil represented 10 %, and high values above 4 meq/100g soil, represented 19%.
The exchangeable Mg ranged from 0.34 to 3.2 meq/100g soil. Forty eight percent of the samples were classified as low, with values less than 0.5 meq/100g, 34 % as medium with values between 0.5 and 2 meq/100g soil, and higher values above 2 meq/100g of soil, represented 18 %.
The range observed for Al was from 0.05 to 3.6 meq/100g soil. In general, critical levels for exchangeable Al interpretation are related to the type of crops. Absolute level of 2 meq Al/100g soil are recognised as excessive for many crops (Landon, 1984). A study carried out in the South Plateau showed a correlation between exchangeable aluminium, pH and clay percentage (Aelterman, 1983). In this survey, more than 15% of the samples had Al level of more than 2 meq/100g soil and more than 50% had more than 1 meq/100g soil. Knowing the low availability of bases in these soils, it is of no doubt that soil acidity as related to Al toxicity represents one of the major limiting factors to crop production in this area (Sanchez, 1976).
Effective C.E.C and Al saturation.
The cation exchange capacity (CEC) of the samples at pH 7, ranged from 1.41 to 39.50 meq/100g. Rating of CEC results for topsoils considers values under 7 meq/100g soil to be low, between 7 and 16 meq/100g to be medium and above 16 meq/100g soil to be high (Table 1). Thus, 63 % of the samples were found to have low CEC, 20 % medium and 17 % high. Samples with high CEC values were mainly encountered in the South West region, where Inceptisols are found.
The base saturation percentage (BSP) ranged from 2.4 % to 100 % (Fig. 7). BSP does not distinguish between different bases. Inbalances in their relative proportions can cause severe plant nutrition problems. For some authors (e.g. Sys et al., 1991) a desirable ratio of bases Ca/Mg/K is 75/18/7 and must be reached in a soil in order to overcome major cation imbalances and to have sustainable good crop yield. As shown in Table 1, 45 % of the samples had values of BSP, between 10 and 20 % considered as low, 36 % between 20 and 60 % considered as medium. and 19 % above 60 % considered as high.
Organic carbon and total nitrogen.
The organic carbon content ranged from 0.25 to 5.61% (Fig. 8). Organic carbon content is often taken as a measure of the quality of organic matter in soils, which in turn is taken as a crude measure of fertility status. As shown in Table 1, 67 % of the samples had values of organic carbon less than 1.5% considered as low and 11% had values above 2.5 % considered as high (Sys et al., 1991). Those with high values were encountered mainly in the volcanic areas. On soils of these areas, the organic matter was closely linked to the CEC. For soils similar to those of the South Plateau, in Brazil, Lopes and Cox (1977) reported that for pH values higher than 5.5, there is a significant correlation between ECEC and the organic matter. This emphasises the importance of liming these soils to increase their pH and also the number of their negative sites for more cations fixation.
Total nitrogen measurements in soils are difficult to interpret, because the levels of N are susceptible to change with the storage time, temperature, and moisture content. Nevertheless, reports from the literature compiled by Landon (1984) considers total N values to be low when below 0.2 %, medium 0.2 % and 0.5 %, and high above 0.5 %. The observed range was 0.05 to 0.7 % (Fig. 9). Considering those given limits, 89 % of the samples had low values of nitrogen, 6 % medium and only 5 % high.
The sand fraction ranged from 2.6 to 86.9 %. The silt fraction ranged from 1.0 to 75.0 % but almost 90 % of the samples contained less than 10 % silt. The clay fraction ranged from 35 to 85.6 % (Fig. 10). In the South Plateau, the clay percentage is significantly correlated to the liming requirements of these soils (Aelterman, 1983). There is a wide variation in surface texture, ranging from clay to loamy sand with quite a lot of many other intermediate classes. This wide variation in texture will surely have an effect on their water holding capacity and will also influence their management types for sustainable agricultural production.
Soil management. From the above results, one can realise that the majority of soils in the Southern Cameroun have low inherent fertility. These soils are strongly weathered and represent about 42 % of the total area of Cameroon. It is estimated that approximately 4 % of this area is actually under slash and burn agriculture. At the level of small farmers, after the land clearing mostly by hand tools and burning, the soils is cropped for two to three years, then allowed fallow for several years. The fallow period ranges from four to nine or more years depending on the population pressure on the land and the market accessibility. Land abandonment is mainly attributed amongst others to the soil fertility decline and the weed encroachment (especially by Chromolaena odorata and Imperata cylindrica). The decline in soil fertility after a few years of cropping is often related to the depletion of the nutrient rich pool of the organic matter. Therefore, if one intends to use these soils for sustainable crop production, it would be necessary to improve their chemical status. Several scientists (Bindzi, 1987; Tchuenteu and Moukam, 1989) have obtained high and sustainable yields of maize (Zea mays L.), cassava (Manihot esculenta Crantz), and sugar-cane (Saccharum officinarum L.) by using incremental amounts of NPK fertilizers and lime. Other scientists (Moukam and Tchato, 1982; Moukam and Tchuenteu, 1991 ) have considerably improved the soil fertility status and the yield of maize and cassava on these soils by using manures, crops residues and brewer's wet grains.
Duguma (1994) also obtained good yields of maize with alley cropping using promising species such as Calliandra calothyrsus and Paraserianthes falcataria. These findings are in agreement with the works of Smyth and Cassel (1995) in the Amazon on the management of Ultisols and Oxisols.
Finally, to arrive at a sustainable crop production in the humid forests zone of Cameroon it is necessary to encourage as "best bet" alternatives to slash and burn agriculture, agricultural practices such as the recycling of crop residues, the use of manures, mineral fertilizers (NPK), improved fallows and liming. These types of management will be mainly applied to food crop production. For perennial crops such as cocoa (Theobroma cacao), oil palm (Elaeis guineensis) and rubber (Hevea brasiliensis) other "best bet" such as pure tree crop plantation, cocoa with tree upper story, oil palm / fruit tree systems, single home gardens and multistrate home gardens should also be encouraged.
Soils under humid forest vegetation of the Southern Cameroon are mainly acid soils. Soil acidity, nitrogen, phosphorus and potassium deficiencies seem to be the major constraints for their sustainable agricultural use. Their chemical nutrient reserves are generally low, especially in the South Plateau area. High nutrient reserves are found only in the South-West area due to the volcanic influence, although phosphorus and potassium deficiencies are common in the South Plateau area on soils formed on the basement complex; no deficiency is observed on young volcanic soils of the South-West area. The soils of the humid forest zone present a good textual distribution which is a good indice for their potential agricultural production. Their sustainable agricultural use can be achieved through appropriate management, taking into account the "best bet" alternatives to slash burn agriculture suitable for the local small farmers.
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Copyright 1997 The African Crop Science Society
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