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Lutte antiérosive

Thème 1. Efficacité de la lutte antiérosive sur la restauration de la productivité des sols

Effect of improved fallow systems on soil chemical properties and crop yield in the rain forest of southeastern Nigeria

F. N. Ikpe et L. D. Gbaraneh


Dans la majorité des zones subsahariennes d’Afrique, les paysans ont réduit la durée de la jachère à moins de dix années. Mais dans le SE du Nigeria, la pression démographique est telle que la jachère a été réduite à moins de 5 ans. Des terres marginales ont été mises en culture: d’où la nécessité de définir des techniques de production capables d’améliorer la productivité des sols. L’étude eut lieu dans une zone de forêts humides, à 10 m d’altitude et 2400 mm de pluie monomodale, entre mars et novembre. Les sols sont des ultisols très acides, profondément altérés, à argile kaolinitique à faible capacité d’échange de cations et faible saturation en bases. Ces sols sont pauvres en matières organiques (0,5 à 3 % de MO) et ont par conséquent peu de réserve de nutriments. Deux essais ont été conduits de 1996 à 2000 comparant divers systèmes de jachères. Sous les jachères améliorées, les propriétés chimiques des sols ont été significativement améliorées en une courte période et les rendements en manioc et maïs largement augmentés. Bien que le paillage de Tephrosia a entrainé une tendance à l’acidification dans cet essais, la jachère à Lablab, avec ou sans pâturage par les moutons, a amené une augmentation du pH, du taux de MO, de l’azote, du P et K. L’adoption par les paysans de ces techniques va dépendre de la rentabilité et de la durabilité de ces techniques dans ces écosystèmes tropicaux très fragiles.

Texte intégral


1Soils of the humid tropics are highly weathered and acidic, with pH ranging from 3.5 to 6.0 and mineralogy dominated by low activity clays. The important characteristics of low activity clay soils are low cation exchange capacity (CEC), which is normally less than 8 cmol kg-1 of soil, and low base saturation (Juo and Adams 1986). The soils have a low organic matter (0.5-3%) and consequently low nutrient reserve, and imbalanced nutrient elements composition and toxic levels of microelements. Therefore, loss of organic matter in these soils causes soil compaction, low water and nutrient retention, low infiltration rate and accelerated run-off and erosion, leading to loss of the natural resource base and decline in soil productivity (Hulugalle et al. 1998).

2The dominant farming systems in the tropics for centuries have been based on bush or forest fallow. The potentials, problems and the structural framework for this land use system have been extensively discussed by Nye and Greenland (1960), Allan (1965) and Nair (1986). Over the years attempts have been made to use annual and perennial herbaceous legumes to enhance green manure input and thus recycle nutrients (Webster and Wilson 1980).

3This has led to the modification of fallow systems in quality and duration to suit farmers’ needs in the management of soil fertility. Consequently, the annual legumes are only suitable for use in relay fallowing (Ikpe et al. 2003) lasting for about six months. Jaiyebo and Moore (1964) reported that trees and shrubs, such as Tephrosia candida (Proxb.) D. C., which have deep roots, are more efficient agents of nutrient recycling and thus are more suitable for fallow periods of between 1 and 5 years and longer.

4During the fallow period, plant nutrients are taken up by fallow vegetation, either trees or shrubs, from various soil depths. These nutrients are later released during litter and biomass decomposition for the improvement of soil organic matter and sustainable yields of food crops. This study thus evaluates the potentials of an indigenous, leguminous, perennial, fast growing tree, Tephrosia candida, and lablab (Lablab purpureus) an annual herbaceous legume for nutrient recycling in fallow systems of southeastern Nigeria.

Materials and Methods

5The experiments were conducted on-farm and on-station on an acid Ultisol at Kpite, 50 km from Port Harcourt (4045’N, 7018’E) and at Onne (4051’N, 7003’E), respectively in southeastern Nigeria. Kpite and Onne are in the high rainfall, humid forest of the Niger Delta region in southeastern Nigeria. The mean annual rainfall at the experimental site is 2400 mm in monomodal distribution, lasting from March to November. Temperatures are moderate. Relative humidity in the areas remains high throughout the year, with mean values September ranging from 78% in February to 88% in July and September. Monthly mean temperatures of the coolest (July and August) and hottest months (February to April) are 25 and 270 C, respectively.

Field trial. Experiment I

6The experimental design was a split plot fitted into a randomized complete block. The main treatment was fallow age (1 vs. 2 yr) and sub-treatment was fallow species (Tephrosia candida vs. natural bush regrowth). The treatments were replicated four times. At the peak of the rainy season T. candida seeds estimated to give 10,000 plants per hectare were broadcast uniformly on the plots. These plots were kept free from weed. At the end of the first year, plots designated for 1 yr fallow were cleared as T. candida shoots were cut back at the soil surface. Wood, leaves and twigs were chopped and distributed evenly on the plot to serve as mulch. The same procedure was applied to plots designated for 2 yr fallow. Cassava was planted at 1 x 1 m (10,000 ha-1), and maize intercropped at 1 x 1 m as well (10,000 hills ha-1 and 20,000 plants ha-1). On each row, there was one cassava plant every meter, with one hill of maize plants at mid distance. Plots were had weeded three times per year. Maize was harvested at 90 days (maturity) while cassava was allowed to grow for one year. The cassava (Manihot esculenta Crantz) variety used was var. TMS 9193, while maize (Zea mays L.) was var. TZPB-SPW. One cassava cutting of about 25 cm was plated per hill, while two maize plants were left per hill.

Soil sampling and analytical methods

7At the onset of the trail, prior to sowing seeds of T. candida and after 1 yr of cropping in the main plots (1 yr and 2 yr of fallowing) soil samples were taken. An auger was used at 0-5, 5-15 and 15-30 cm depths. The soil samples were air-dried in the laboratory and ground to pass a 2 mm sieve prior to chemical analysis. Soil pH was determined on a 1:1 soil: H 2 O solution with a glass electrode pH meter according to the procedure of Tel and Rao (1982). Organic carbon was determined using a modification of the method of Walkley and Black (1934). Total nitrogen was determined using a Technicon autoanalyzer (Technicon AAII) after digesting the sample with a mixture of concentrated orthophosphoric and sulphuric acid in a Tecator Digester. Available phosphorus in soil was determined by the Bray-1 method (Bray and Kurtz 1945) using the Technico authoanalyzer. Exchangeable cations were extracted with 1 N NH 4) AC + 0.01M EDTA at 20:50 fresh soil: extractant ratio. The extracts were determined with an Atomic Absorption spectrophotometer (AAS), while the concentrations of potassium and sodium were measured with a flame photometer. The effective cation exchange capacity (ECEC) was calculated by sum of exchangeable cations and exchangeable acidity, expressed in cmol kg-1 soil.

Analytical methods for plant nutrients

8Dry matter of weed, litter and T. candida biomass were ground to pass a 1 mm sieve. These ground plant materials were digested with a mixture of concentrated sulphuric acid and hydrogen peroxide in presence of one Kjeldahl catalyst tablet in a Tecator Digester for the determination of total nitrogen (Tel and Rao 1982). Total carbon content was determined using the procedure of Amato (1983).

9Extracts for the determination of phosphorus, calcium, magnesium and potassium in plant materials were obtained after digesting samples using the procedure described in the IITA laboratory manual (1979).

Experiment II

10The experimental design was a split plot with three replications. Fallow systems and grazing methods were the main and sub treatments, respectively. The main treatments were improved (with lablab) and unimproved (natural) fallows and measured 30 x 15m each. The sub treatments consisted of grazing with sheep, and no grazing with the plots measuring 24 x 15m and 6 x 15m, respectively.

11After land preparation, both main plots were annually cropped with maize (var. (TZ DMR SR-W) to a population of 40,000 plants/ha in April, with a spacing of 1 x 0.25m and seedlings thinned to one plant per stand 2 weeks after planting. However, the improved plots were relay cropped with lablab (Lablab purpureus), 4 weeks after maize planting at a population of 1333 (Gbaraneh, 1997). The unimproved plots were not relay cropped with lablab. Rather weeds took over the plots after maize harvest to form a dense bush cover until the commencement of grazing, three months later. At maize harvest a 2m x 2m quadrat taken at random in four different locations within a paddock (sub plot) was used for maize yield measurement. Maize cobs within the quadrats were harvested, shelled and grain yield estimate at 13% moisture content. Other parameters collected were maize stover and weed DM yields. At the onset of the dry season, in December of each year, 3 West African dwarf (animal) sheep of average age and weight of 8 months and 12 kg, respectively, were introduced to the grazing plots. They were allowed to graze in situ for as long as biomass supply lasted (from early December to late January). Harness and bags were fitted to the animals one day per week for estimation of daily faecal output. Animal weight change was monitored weekly throughout the grazing period. Mineral salt block and water were provided ad-lib. Others parameters were forage available at grazing, forage disappearance (intake) at the end of grazing, dung production and soil nutrients status.

Statistical analysis

12Differences in the yields of cassava and maize, weed incidence, nutrient uptake and soil chemical properties due to fallow age and type of fallows were estimated and tested by analysis of variance (ANOVA) using the procedure GLM of SAS (SAS Institute 1998). LSD at p < 0.05 was used for mean comparisons.

Results and Discussion

Experiment I. Biomass yields of Tephrosia candida

13After 2 yr of fallow under Tephrosia candida, litter was 150% higher than under natural regrowth for the same period. After 1 yr of fallow under T. candida, litter was 33% higher than under natural regrowth for the same period. Total biomass (wood and leaves) cutback after 2 yr of fallow under T. candida was double that harvested after 2 yr of fallow under natural regrowth for the same period. Total biomass from T. candida fallowed for 1 yr was 30% higher than that harvested from natural regrowth of 1 yr (Table 1).

Table 1: The effect of fallow age and fallow type on biomass yield (t ha-1) in a humid tropical environment, Kpite, southeastern Nigeria

Table 1: The effect of fallow age and fallow type on biomass yield (t ha-1) in a humid tropical environment, Kpite, southeastern Nigeria

Nutrient uptake

14Greatest uptake of N, P, Mg, and K in litter was recorded in plots fallowed with T. candida for 2 yr (Table 2). These were followed by nutrient yield in litter of plots fallowed naturally for 2 yr, then those fallowed T. candida for 1 yr, and finally those fallowed naturally for 1 yr. Highest uptake of calcium was found in liter gathered from plots fallowed naturally for 1 yr, followed by that in litter of 2 yr fallow with T. candida, then in litter of 2 yr national fallow and lastly, in litter of 1 yr fallow with T. candida.

15Nutrient uptake in leaves from fallow plots differed considerably from those in litter. Of al the nutrients, N uptake was the highest and this was consistent across fallow age and type (Table 2). The uptake of N, P, Ca, Mg and K was highest in leaves of T. candida fallowed for 2 yr. Nutrient uptake in leaves from other fallow plots did not differ significantly between each other. A similar scenario is observed for nutrient uptake in the woody part of this fallow species under study.

Table 2: Effect of fallowing on nutrient uptake in a humid environment, Kpite, southeastern Nigeria

Table 2: Effect of fallowing on nutrient uptake in a humid environment, Kpite, southeastern Nigeria

Crop yields

16Cassava tuber yield ranged between 6.8 and 18.3 t ha-1. The highest yield was recorded in plots fallowed with T. candida for 2 yr, and the lowest in T. candida plots fallowed for 1 yr. Maize grain yield showed a similar trend with that of cassava.

17Results obtained in both experiments indicate that improved fallow systems gave higher biomass dry matter for mulching and suppressing weeds. However, there were no significant differences in soil chemical properties among treatments for all parameters measured. Larger biomass and crop yields were obtained while more nutrients were recycled after two years of fallowing compared with natural fallow.

18In a companion study using litterbags to determine the decay rate of T. candida biomass, consisting of wood, twigs and leaves, and the release of nutrients, it was found that T. candida biomass has a slow decay rate of 0.13 wk and a half-life of 6 wk (F. N. Ikpe), unpublished data). This makes the pruning of T. candida most suitable for mulching under a humid tropical environment such as the Kpite site.

19Consequently, the release of nutrients was also slow. After 98 days of decay, only 40% of the initial nitrogen was released, followed by calcium 50%, phosphorus 70%, magnesium 88% and potassium 90% (F. N. Ikpe, unpublished data). The comparatively yields of cassava tuber and maize grain after fallowing for 2 yr with T. candida biomass on soil physical properties (Gichuru 1991), (ii) soil faunal activities which improve the chemical and physical properties of the soil (Tian 1992), and (iii) the recycling of nutrients.

Changes in soil chemical properties

20After two years of fallowing and cropping, total nitrogen increased significantly over the initial T. candida fallow. Total nitrogen was higher in plots under T. candida than the values one year after natural and plated fallow and with two years of natural in surface soil (Table 3).

21The soil pH decreased significantly after two years of fallow when compared with that measured in plots one year after fallowing, fallow type notwithstanding. Organic carbon was significantly higher in T. candida plots than in those with natural fallow. The dynamics of available phosphorus followed the same trend as pH, decreasing significantly when compared with initial and one year of fallowing (Table 3).

Table 3: Effect of fallowing with natural regrowth Tephrosia candida soil chemical properties after two years of fallowing and cropping

Table 3: Effect of fallowing with natural regrowth Tephrosia candida soil chemical properties after two years of fallowing and cropping

22All the exchangeable cations increased in concentration over the values observed one year after fallowing. The concentrations of calcium and magnesium were, however, considerably higher in the plots under natural fallow than in those fallowed under T. candida plots than in the natural fallow plot. Values for ECEC and base saturation were higher in plots fallowed for one year and also in T. candida plots than in natural fallow plots. Soil chemical properties decreased with soil depth.

Experiment II. Maize Grain Yield

23Maize grain yield was significantly influenced by fallow and grazing management systems. In 1996, the first year of the experiment, yield did not differ significantly (p > 0.05) between the natural and improved fallow systems, although the improved fallow slightly increased yield by 5% over the natural fallow system. Grain yield significantly (p < 0.001) increased in the improved fallow treatment by 18% over the natural fallow system in the second year (1997) attributed to improved soil nutrients status by the previous lablab crop (Mohammed-Saleem and Otsyina, 1986). Grazing by sheep also showed no significant effect in the first year of the experiment since grazing was done after the maize harvest. There was a significant grain yield increase of 9% by the grazed treatment over the no-grazing in the second year of the experiment. This yield enhancement in the grazed plots could possibly be attributed to improved soil fertility by the dung and urine (faecal output) deposited by sheep while grazing ad lib in the plots (Powell and Ikpe, 1992; Ikpe et al., 1999).

24Grazing effect became prominent in the second year of the experiment (Table 4), as the grazed plots significantly increased total forage-on-offer by 1.20 t ha-1, representing 50%, over the yield in the un-grazed plots. Crude protein content of forage-on-offer also increased by 1.42 g kg-1, representing 32% in the grazed plots as against the un-grazed plots.

Table 4: Influence of grazing methods on yield of fodder-on-offer (t ha-1) and crude protein content of forage (g. kg-1) at Onne, southeastern Nigeria

Table 4: Influence of grazing methods on yield of fodder-on-offer (t ha-1) and crude protein content of forage (g. kg-1) at Onne, southeastern Nigeria

ns = not significant at 5% level of significance

Dung Production

25The dung production (faecal output) of sheep in the grazed plots is shown in Table 5. Dung production was highest in the improved fallow-grazed plots than the natural fallow -grazed plots across the years. Average daily faecal output sheep-1 day-1 was 67 and 81% higher in the improved fallow plots than the natural system in 1996 and 1997, respectively. The high output in the lablab-improved fallow plots may be attributed to the highly nutritious biomass and intake recorded as against the natural fallow plots. Faecal output also showed a direct and positive relationship with total forage on offer and intake.

Table 5: Forage intake (t/ha) and faecal output (g/sheep/day) by West African Dwarf sheep in natural and improved fallow systems at Onne, southeastern Nigeria

Table 5: Forage intake (t/ha) and faecal output (g/sheep/day) by West African Dwarf sheep in natural and improved fallow systems at Onne, southeastern Nigeria


26The results of this study show that T. candida and L purpureus can play a major role in nutrient recycling in the cropping systems of southeastern Nigeria. However, strategies to mitigate the soil acidifying tendency of T. candida need to be pursued. Furthermore, the integration of crops and livestock based on sown forage legume in the intercropping system can reverse the persistent decline in soil fertility under continuous cropping in these diminishing fallow systems as well as providing high quality fodder. There is need however, to adopt technologies that will recycle nutrients more efficiently for higher crop yields.



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Table des illustrations

Titre Table 1: The effect of fallow age and fallow type on biomass yield (t ha-1) in a humid tropical environment, Kpite, southeastern Nigeria
Légende Nutrient uptake
Fichier image/jpeg, 80k
Titre Table 2: Effect of fallowing on nutrient uptake in a humid environment, Kpite, southeastern Nigeria
Fichier image/jpeg, 180k
Titre Table 3: Effect of fallowing with natural regrowth Tephrosia candida soil chemical properties after two years of fallowing and cropping
Fichier image/jpeg, 240k
Titre Table 4: Influence of grazing methods on yield of fodder-on-offer (t ha-1) and crude protein content of forage (g. kg-1) at Onne, southeastern Nigeria
Légende ns = not significant at 5% level of significance
Fichier image/jpeg, 80k
Titre Table 5: Forage intake (t/ha) and faecal output (g/sheep/day) by West African Dwarf sheep in natural and improved fallow systems at Onne, southeastern Nigeria
Fichier image/jpeg, 81k


Department of Crop/Soil Science, Rivers State University of Science and Technology, Port Harcourt, Nigeria.

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