In this study, rainfall erosivity factor, R. of the Revised Universal Soil Loss Equation (RUSLE) was estimated from daily rainfall amounts of the coastal, humid forest, savanna, semi-arid and arid zones of Nigeria using data from 17 locations, which spanned 10-33 years. Two power law equations were applied to compute the products of kinetic energy, e, and (i) maximum 30-minute intensity, I(30) (EI(30)), and (ii) maximum 15-minute intensity, I(15) (EI(15)). The indices were used to develop monthly and annual isoerodent maps. Mean monthly EI(30) ranged from 600 to 3200 MJ mm ha(-1) h(-1) whereas the annual values ranged from 3000 to 27,000 MJ mm ha(-1) h(-1) from the arid to the coastal zones. The EI(15) index was 1.7 times greater than the EI(30). Trends of rainfall erosivity in the derived, southern Guinea and northern Guinea savannas or wet savannas were erratic and less predictable from the trends of rainfall amount, unlike in the coastal, humid forest, semi-arid and arid zones. Extrapolation of data for soil conservation planning did not appear feasible in the wet savannas. Monthly values of erosivity presented are recommended for conservation plans during the cropping seasons in the various agroecological zones. The EI(15) index is recommended for both monthly (seasonal) and annual soil loss computations because short-term intensities reveal rainfall erosivity better in the tropics.
Based on the power law equations developed by Salako (2006; 2007; 2008), daily rainfall data (generally 1988-2005) were collected from 17 weather stations in Nigeria (representing all agroecological zones in the country) to compute kinetic' energy of rainfall and IS-minute intensity, 1/5. The Wischmeicr and Smith (1978) kinetic index. EWS was similar to Brown and Foster (1987) index, E-BF. Isolines of rainfall kinetic energy and intensity were drawn with SURFER Version 8. Kinetic energy and rainfall intensity increased, generally, from the coastal region of the southeast in the northwestward direction. There were, however, abrupt changes in both characteristics between latitude 7 and 1ION, mainly in the derived, southern Guinea (SGS) and northern Guinea (NGS) savannas. Sholi-term rainfall intensities could be similar among regions in spite of significant differences in cumulative rainfall amount or kinetic energy. Kinetic energy measured at a monthly scale showed less spatial variation than that on daily scale, suggesting that long timescale could obscure useful details. Rainfall erosivity trends across the country suggest a possible influence of the migration of the Intertropical Convergence Zone (ITCZ). The maps presented can be used to decide spacing for locations of monitoring centers for soil and water management in the country.
Potential soil erosion, sedimentation and water pollution risks were assessed on the landscape of the University of Agriculture, Abeokuta, southwestern Nigeria. Potential soil loss was estimated using the Universal Soil Loss Equation (USLE) while sedimentation and pollution risks were assessed from sediment and water samples taken every 2 weeks from August-December, 2004 from five rivers. For the USLE, rainfall erosivity factor, R, was estimated using the modified Fournier index; soil erodibility was estimated using Wischmeier and Smith nomograph after analyzing surface (0-20 cm depth) soil samples from the watersheds. The topographic factor, LS, was estimated from topographic maps. Annual rainfall erosivity factor, R, was 9829 MJ mm ha -1 h -1, suggesting that Fournier index agreed with other annual R values calculated with intensity-based models for the agroecological zone. Potential soil losses ranged from 1.5-49 Mg ha -1 a -1 whereas the mean suspended sediment loads in river waters ranged from 442-1098 mg cm -3. Sediment enrichment ratio was between 0.48 and 0.64. Water quality for drinking was good based on pH, Cl -, Na + and Fe 2+ but Zn 2+ levels might not be acceptable. Also, based on these chemical properties, electrical conductivity and sodium adsorption ratio, the river waters were rated good for irrigation of crops. Watersheds in the built-up areas had the higher soil erosion and water pollution risks that those under forest. The need to adopt soil conservation measures on all watersheds was highlighted.
This study was carried out in 2004 and 2005 at Ajegunle Farm Settlement Scheme's site, near Abeokuta, southwestern Nigeria. A factorial experiment in randomized complete block design with 3 replicates was set up. The factors were poultry manure (0, 5 and 10 t ha -1), lime as CaCO 3 (0 and 250 kg ha -1) and NPK 15-15-15 (0 and 100 kg ha -1). Maize (TZSR-Y) was planted. Soil physical properties measured included dry bulk density, saturated hydraulic conductivity ( Ks), unsaturated water flow and clay dispersion ratio (CDR). The range of surface soil bulk density, from 1.27-1.42 g cm -3, observed for poultry manure application was significantly lower than the range of 1.39-1.48 g cm -3, when manure was not applied. The highest Ks value of 1.70 cm min -1 from plot applied with 10 t ha -1 poultry manure (PM) was significantly higher than 0.41-0.49 cm min -1 from all inorganically treated plots and the control. At the end of the second cropping season, application of poultry manure especially at 10 t ha -1 reduced the CDR from 17.1% to a range of 15.7%-9.3%, thereby increasing the soil aggregate stability. Application of 10 t ha -1 Poultry manure significantly improved the infiltration rate of the field. Integrated use of lime or 10 t ha -1 Poultry manure with NPK as well as application of 10 t ha -1 poultry manure only, improved the ability of soil to transmit water (sorptivity) under unsaturated condition. Combined application of poultry manure with inorganic amendments is therefore important in the improvement of physical properties of the soil.
A survey of soil management practices was conducted in the northern Guinea savanna of Nigeria. Fifteen villages were randomly selected from a geographical grid covering an area of 100 × 200 km located in the benchmark area of the Ecoregional Program for the Humid and Sub-humid Tropics of Sub-Saharan Africa. In each village the chief and several farmers were interviewed to assess their soil management methods and attitude towards the need to conserve soil. A total of 181 farmers were interviewed in late 1996. The most common crop rotation systems were food legumes with non-legume crops (40%), followed by monocropping (28%). Fifty-three per cent of the farmers who included food legumes in their rotations did so for soil fertility considerations, while 49% of the farmers who practised monocropping did so to maximise their output. These practices indicate that farmers were well aware of the importance of legumes for maintaining soil fertility. Only 2% of the farmers practised mulching with crop residue. The most common use of crop residue was for fodder, the remainder largely being used as building material or else burnt. As a consequence, soil surfaces were generally bare at the onset of the rainy season and hence prone to soil erosion. Ridging was the most important land preparation technique (88%), with farmers perceiving benefit in terms of improved crop emergence (56%) and water conservation (11%). Other benefits included weed control. Ridging was generally practised along contours, with most farmers citing soil conservation benefits, e.g. water conservation, and erosion control as the reasons for using contour ridging. Those farmers who purposely ridged up and down the slope did so for drainage purposes. All farmers used the same method to build ridgesthe ridge from the previous year was cut in the middle and the two halves of neighbouring ridges were combined to form a new ridge in the furrow from the previous year. According to the farmers, this method controlled weeds and improved emergence. None of the farmers practised rebuilding old ridges, similar to permanent ridges. Such a practice might be acceptable to farmers in that it may be less labour intensive to rebuild partially collapsed ridges compared to reridging completely. Soil physical benefits from semi-permanent ridging would include increased soil structural stability, reduced soil compaction and increased root proliferation into the subsoil. Negative side effects might include reduced crop emergence and increased weed infestation. The most commonly used tools for soil preparation were hand hoes (80%), followed by draft animals (16%) and tractors (3%).
A factorial experiment with a randomised complete block design (three replicates) was performed to determine the effects of poultry manure (PM), lime (L) and NPK 15-15-15 fertilizer on soil chemical properties, and to determine the effects of their combinations on soil productivity and maize yield. The factors were PM (0, 5 and 10 Mg ha(-1)), L as CaCO3 (0 and 250 kg ha-1) and NPK 15-15-15 (0 and 100 kg ha-1). The soil had a loamy sand texture. The application of L and PM increased the surface soil pH in a similar fashion. In both years of the experiment, the effective cation exchange capacity (ECEC) of the soil after the combined application of 10 Mg ha-1 PM, L and NPK was significantly higher than after the individual application of L or NPK or their combination (5.75-7.65 cmol kg(-1) compared to 3.36-4.57 cmol kg(-1)). The application of 10 Mg ha(-1) PM with L and/or NPK reduced the possibility of Mn toxicity, with soil levels ranging from 108 to 136 mg kg(-1). The combined use of the three amendments gave the highest leaf nutrient concentrations. The highest maize grain yield (4.62 Mg ha-1) was obtained with L + 10 Mg ha-1 PM; with no amendment the grain yield was 1.9 Mg ha-1. The application of PM enhanced the effects of L and NPK in improving soil productivity. However, separate applications of 5 and 10 Mg ha-l PM similarly affected soil productivity; the sandy nature of the soil at depths of 0-20 cm seems to have prevented differences between the two rates from becoming manifested.
The Food and Agriculture Organisation (FAO) irrigation and drainage Paper 56 recommends the use of Penman-Monteith ( PM) method for calculating reference evapotranspiration ( ET 0 ). This method has been widely accepted. Alternative methods recommended where data requirements for the PM cannot be met are Hargreaves ( HG) and pan methods. Therefore, this study was carried out to evaluate ET0 with the PM method and develop its relationship with HG and pan methods for Onne (humid), Ibadan (sub-humid) and Kano (semi-arid), Nigeria using 1990-2005 daily climatic data. The data were resolved to daily means of each week of the year, and monthly and annual totals. Deviations of the data from long-term means were determined and the ET0 methods were compared using root mean square error ( RMSE) and mean bias error ( MBE). Autocorrelation coefficients and regression analysis were also carried out. The daily means for each week with PM ET0 ranged from 2.39-3.82 mm in Onne, 2.45-4.48 mm in Ibadan and 3.62-7.92 mm in Kano. Mean annual PM ET0 was 1130 mm vs. 2450 mm of rainfall in Onne; 1249 mm in Ibadan vs. 1286 mm of rainfall and 2007 mm in Kano vs. 786 mm of rainfall. The HG method over-predicted PM ET0 in Onne and Ibadan and under-predicted it in Kano. The pan method under-predicted it in Onne and Ibadan. Nonetheless, the HG method was a better estimator of PM ET0 in Kano than Onne and Ibadan, although daily means in the dry season were more variable. Daily means of PM ET0 were significantly related to means HG ET0 ( P
A decreasing trend of rainfall has been observed in West Africa, where rainfall erosivity is also considered to be high. Therefore, this study was carried out to evaluate the variability of rainfall and its erosivity in two contrasting zones in southern Nigeria between 1977 and 1999 to understand the implications of climate variability on rainfall erosivity. The study sites were Ibadan, a sub-humid zone, and Port-Harcourt, a humid zone. Time of occurrence of rainfall, rainfall amount (A), intensity (I-15 and I-30), kinetic energy (E) and rainfall erosivity factor (R), were evaluated. Kinetic energy was estimated with Brown Foster (BF) equation, making the rainfall erosivity (product of kinetic energy and intensity) to be designated as EI30-BF and EI15-BF. The frequency of rainfall during daylight (06:00-18:00 h) was 48% for Ibadan and 69% for Port-Harcourt. There were time-specific differences in daily rainfall occurrence between the zones, suggesting a strong influence of local effects on rainfall generation, such as, relief in Ibadan and proximity to the sea in Port-Harcourt. Annual E was 213 MJ ha(-1)for Ibadan and 361 MJ ha(-1) for Port-Harcourt. Ibadan had a significantly higher daily E than Port-Harcourt because of higher intensity while Port-Harcourt had significantly higher annual E than Ibadan because of higher annual rainfall amount. Annual erosivity at Ibadan using the EI30-BF was 9,742 MJ mm ha(-1) h(-1) whereas it was 15,752 MJ mm ha(-1) h(-1) at Port-Harcourt. Using the EI15-BF, Ibadan had an annual value of 14,806 MJ mm ha(-1) h(-1) while Port-Harcourt had 20,583 MJ mm ha(-1) h(-1). Thus, annual rainfall erosivity was significantly higher in the humid than the sub-humid zone because of higher amount of rainfall but the reverse was the case with daily erosivity because of higher intensities in the sub-humid zone. Rainfall intensity was, therefore, a key measure of erosivity. There was a strong positive relationship between rainfall erosivity and rainfall amount. Between 1977 and 1988, 50-88% of the 12 years had rainfall erosivity which exceeded the long-term average but rainfall erosivity was less than the long-term average between 1989 and 1999. This suggested a decreasing trend in erosivity due to the decreasing trend in rainfall amount in West Africa. However, the trend did not imply lesser soil erosion and environmental degradation risks.
A factorial experiment with a randomised complete block design (three replicates) was performed to determine the effects of poultry manure (PM), lime (L) and NPK 15-15-15 fertilizer on soil chemical properties, and to determine the effects of their combinations on soil productivity and maize yield. The factors were PM (0, 5 and 10 Mg ha(-1)), L as CaCO(3) (0 and 250 kg ha-1) and NPK 15-15-15 (0 and 100 kg ha-1). The soil had a loamy sand texture. The application of L and PM increased the surface soil pH in a similar fashion. In both years of the experiment, the effective cation exchange capacity (ECEC) of the soil after the combined application of 10 Mg ha-1 PM, L and NPK was significantly higher than after the individual application of L or NPK or their combination (5.75-7.65 cmol kg(-1) compared to 3.36-4.57 cmol kg(-1)). The application of 10 Mg ha(-1) PM with L and/or NPK reduced the possibility of Mn toxicity, with soil levels ranging from 108 to 136 mg kg(-1). The combined use of the three amendments gave the highest leaf nutrient concentrations. The highest maize grain yield (4.62 Mg ha-1) was obtained with L + 10 Mg ha-1 PM; with no amendment the grain yield was 1.9 Mg ha-1. The application of PM enhanced the effects of L and NPK in improving soil productivity. However, separate applications of 5 and 10 Mg ha-l PM similarly affected soil productivity; the sandy nature of the soil at depths of 0-20 cm seems to have prevented differences between the two rates from becoming manifested.
This study was carried out in Ibadan, southwestern Nigeria between 1997 and 1999 to determine changes in soil chemical properties and yields on a previously eroded Alfisol. A factorial experiment in which legumes (Vigna unguiculata (cowpea), Mucuna pruriens and Pueraria phaseoloides) and residue management (burned and mulched residues) were factors was set up in two replicates of runoff plots. Yam was planted in 1997 and 1999 whereas maize + legume intercrops were planted in 1997. Soil properties measured were particle size distribution, pH, organic C, total N, Ca, Mg, Mn, exchangeable acidity and effective cation exchange capacity. There was increased acidity at the site. Soil chemical properties were significantly improved by burning of cowpea and Pueraria residues and mulching with Mucuna. Yam tuber yields of 14-18 t ha-1 in 1997 was
This study was carried out at Abeokuta, south-western Nigeria, to understand the variation in soil strength, gravel distribution, and bulk density along a toposequence. In 2003, a 120-m transect on a fallowed land was sampled at every 1 m for topsoil bulk density measurement by excavation (3278 cm(3) pits), while soil strength was measured at every soil depth increment of 25 mm to 0.50 m depth. Total dry (rho(t)) and fine earth (16 mm. In 2006, four 100-m transects were considered; two each on adjacent fallowed and cultivated lands. Soil strength and water content were measured. The fine earth fraction of topsoil ranged from 62 to 90.6%. Gravel in the 2-4 mm class was dominant with a range of 0.8-35.7%. Thus, cores >= 50 mm could be used in the topsoil to obtain reliable estimates of bulk density. Total bulk density (rho(t)) was reduced by 4-19% when corrected for gravel to obtain rho(f). Soil strength of the lower slope was highest in 2003 (1981-4482 kPa) and lowest in 2006 (1546 kPa). In spite of the apparent signigicant influence of water content on soil strength, the relationship was weakly expressed by regression analysis, as only 35% of variation in soil strength was explained by water content at 0.10-0.15 m soil depth in 2003. No relationship was found in 2006; the cultivated segment had higher soil strength (2045 kPa) than the fallowed segment (1970 kPa) even though the water contents were similar. Also, only the 2-4 mm gravel significantly influenced rho(t). Land use, soil depth, and slope position significantly affected soil strength. Root-limiting soil strength (>2000 kPa) would certainly be encountered below 0.20 m soil depth in the wet season irrespective of land use. Management of this gravelly landscape must be based on the heterogeneous nature of soil physical properties along the toposequence, and this could be made effective by grouping the soils according to slope position and taking interest in the few portions of the landscape with extreme values of gravel distribution and high soil strength
Vast areas of degraded soils exist in southwestern Nigeria due to topsoil removal by soil erosion and gravel/stone mining operators. The restoration of such soils has become imperative to sustain food production in most rural communities. Therefore, a factorial field experiment in three replicates of a randomized complete block design was carried out in 2003 and 2004 with the factors being slope positions (upper and lower slopes), topsoil removal (0, 15 and 25 cm depths) and nutrient amendments (0, 10 t ha(-1) poultry manure and 60:30:30 N:P2O5:K2O as NPK + urea). This was complemented with a laboratory study to determine the effects of soil water, gravel concentration and gravel size on soil strength. Maize (Zea mat's L.) was planted as a test crop to determine the effects of treatments on yield and the effect of root growth on soil strength. Soil strength was measured with a self-recording penetrometer at soil depth interval of 2.5-50 cm depth. Soil bulk density, water content, maize root and shoot biomass and grain yield were measured. In the laboratory, soil strength decreased from 483 to 314 kPa as water content increased from 0.05 to 0.62 cm(3) cm(-3) while it increased from 294 to 469 kPa as gravel concentration increased from 100 to 500 g kg(-1). Soil strength was affected more by water content and gravel concentration than gravel size. Under various moist conditions in the field, soil strength increased with soil depth from 1 177 to 5000 kPa at the upper slope and from 526 to 5000 kPa at the lower slope. Thus, the lower slope had significantly lower soil strength than the upper slope. Soil strength increased with increasing soil depth removal and was significantly reduced by poultry manure. For the 2 years of study, high grain yields were sustained with poultry manure/no topsoil removal (1784-3571 kg ha(-1)) and NPK + urea/no topsoil removal (2371-2600 kPa) at the lower slope. However, soil at the upper slope was more resistant to degradation as 16-67% loss in yield was observed compared to 65-75% for lower slope when no nutrients were applied. Nonetheless, both the upper and lower slope positions were productive with the application of poultry manure irrespective of topsoil removal, compared to NPK + urea, which was only effective without topsoil removal. Therefore, poultry manure was a better soil ameliorant than NPK + urea after topsoil removal.
Resource-poor farmers in developing nations cultivate marginal lands, thereby exacerbating the problem of soil degradation through poor plant growth and ground coverage. An assessment of ground cover under such a practice will provide a guideline for soil conservation. Ground cover by leguminous cover crops (e.g., Mucuna pruriens, Pueraria phaseoloides and Vigna unguiculata), associated with yam, maize and rice was measured in three different experiments in southwestern Nigeria using beaded-string method while leaf area was measured using a flat-bed scanner. The leaf area was used in obtaining equivalent of ground cover fraction from the leaf area index. Ground cover by yam was
Resource-poor farmers in developing nations cultivate marginal lands, thereby exacerbating the problem of soil degradation through poor plant growth and ground coverage. An assessment of ground cover under such a practice will provide a guideline for soil conservation. Ground cover by leguminous cover crops (e.g., Mucuna pruriens, Pueraria phaseoloides and Vigna unguiculata), associated with yam, maize and rice was measured in three different experiments in southwestern Nigeria using beaded-string method while leaf area was measured using a flat-bed scanner. The leaf area was used in obtaining equivalent of ground cover fraction from the leaf area index. Ground cover by yam was
Field litterbag studies were conducted in the 2000 rainy season and the 2000/2001 dry season along the transect of West African major agroecological zones (agroeco-zones) to measure the decomposition of, and N and P release from 5 plant residues (leaves of woody species) with increasing quality: Dactyladenia barteri, Pterocarpus santalinoides, Alchornea cordifolia, Senna siamea and Gliricidia sepium. The decomposition rate constant (wk(-1)) ranged from 0.034 (Dactyladenia, subhumid zone) to 0.49 (Gliricidia, humid zone) in the rainy season, and from 0.01 (Dactyladenia, subhumid zone) to 0.235 (Pterocarpus, arid zone) in the dry season. The direct correlation between the decomposition rate of plant residues and their quality was only valid in agroeco-zones where there is not moisture stress. Similarly, the direct correlation between the decomposition rate of plant residues and moisture availability was only valid for plant residues with high quality. The decomposition rate of the low quality plant residue could increase from humid to arid zone in West Africa. In the arid zone, the low quality plant residue could also decompose faster than high quality plant residue. The climate-residue quality interactive effects on plant residue decomposition in West Africa were attributed to the feedback of low quality plant residue's mulching effect, soil fauna and appreciable photodegradation in dry regions. A decomposition equation that could be used to predict the decomposition rate of plant residues with various qualities across agroeco-zones in West Africa was obtained from this study. The equation was expressed as follow: k = 0.122 -0.000747*PRQI(2)-0.0233*PRQI*CI + 0.00337*CI* PRQI(2), in which k is the decomposition rate constant (wk(-1)), PRQI the plant residue quality index, and CI the climate index (ratio of rainfall to sunshine hours cumulative during the entire decomposition). The response of N and P release from plant residues to residue quality and climate was similar to that of residue decomposition. At the late stage of the dry season decomposition, the high C/N and C/P ratio plant residue (Dactyladenia leaves) that immobilized N and P in wet zones showed a release of N and P in the dry zone.
Soil erosion by water is a major cause of land degradation in the tropics. Its quantitative assessment requires evaluation of factors such as rainfall variability and erosivity. Therefore, pluviograph data from 1977 to 1999 for the subhumid (Ibadan) and humid forest (Port-Harcourt) zones of southern Nigeria were analysed to determine the temporal and spatial variability of rainfall erosivity. Analyses were carried out generally at 15-minute intervals to obtain rainfall amount, duration, intensity and kinetic energy. Descriptive statistics and t-tests were used for data analyses. Rainfall intensities between 100 and 150 nun h(-1) occurred every year. Rainfall intensities were higher in the subhumid (SH) than humid forest (HF) zone whereas the kinetic energy of rainfall was higher in the HF than the SH zone. Thus, the high erosivity in the SH was mainly due to high intensities whereas it was mainly due to high amounts in the HF zone. In both zones, 42 percent of the years exceeded long-term annual average of erosivity. The mean annual EI30 was 11107 MJ mm ha(-1)h(-1) in the SH zone and 17 988 MJ nun ha(-1) h(-1) for the HF zone. Rainfall erosivity indices (E and EI30) correlated significantly with rainfall amount, with 86-99 per cent of variations in daily erosivity explained, and 55-99 percent of variations in annual erosivity explained by rainfall amount. Spatial and temporal variations were not only influenced by rainfall characteristics but also by inherent differences in rainfall erosivity indices. The data presented are required in the prevention or control of land degradation as well as in the development of watersheds, particularly in the tropics.
The vast area of savanna ecology in Africa plays a significant role in food production, making a study of soils in this zone very important. Therefore, soil physical and chemical properties of 14 soil profiles were studied in a derived savanna zone of southwestern Nigeria on 2 toposequences at 2 locations (Ibadan and Alabata), which were 20 km apart. Six soil profiles were studied at Ibadan while 8 were studied at Alabata. Morphological descriptions of profiles were carried out. Data collected included particle size distribution, bulk density, clay dispersion, water retention characteristics, pH, organic carbon, exchangeable K, Ca, Mg and available P. Soil profiles along the toposequences were well developed with depths exceeding 180 cm, except for one profile at the lower slope position where an indurated plinthic layer was found at 68 cm depth. The horizons were easily distinguished with color, texture and consistency. Total sand, with the main component being coarse sand, decreased with depth from 813 to 502 g kg-1 at Ibadan and from 824 to 635 g kg-1 at Alabata. The clay content increased with depth from 54 to 356 g kg-1 at Ibadan and from 63 to 279 g kg-1 at Alabata. Gravel concentration was highest for soil horizons found between 20 and 102 cm depth. Also, bulk density increased with soil depth from 1.35 to 1.51 g cm-3 at Ibadan and from 1.38 to 1.64 g cm-3 at Alabata, indicating that subsoil horizons were more compact due to higher clay and gravel contents, and sticky consistency. The A horizon had a significantly higher water content at water potentials N2 kPa while the subsoils had higher water content at b2 kPa. Soil organic C and total N decreased with soil depth at both locations with the A horizon having significantly higher organic C (7.1012.69 g kg-1) and total N (0.841.2 g kg-1) than deeper horizons (1.94.47 g kg-1 for organic C and 0.120.58 g kg-1 for N). Particle size distribution was significantly different among the slope positions at both locations. Also, soil water retention, soil pH, total N and exchangeable K were distinguishing parameters among slope positions. The interaction of soil depth and slope position was, however, not significant suggesting that processes influencing soil horizon development acted independently in the vertical and lateral directions. Soil pH was the only attribute that distinguished the toposequences between the two locations.
A study was carried out on a previously eroded Oxic Paleustalf in Ibadan, southwestern Nigeria to determine the extent of soil degradation under mound tillage with some herbaceous legumes and residue management methods. A series of factorial experiments was carried out on 12 existing runoff plots. The study commenced in 1996 after a 5-year natural fallow. Mound tillage was introduced in 1997 till 1999. The legumes Vigna unguiculata (cowpea), Mucuna pruriens and Pueraria phaseoloides were intercropped with maize in 1996 and 1998 while yam was planted alone in 1997 and 1999. This paper covers 19971999. At the end of each year, residues were either burned or mulched on respective plots. Soil loss, runoff, variations in mound height, bulk density, soil water retention and sorptivity were measured. Cumulative runoff was similar among interactions of legume and residue management in 1997 (57151 mm) and 1999 (206397 mm). However, in 1998, cumulative runoff of 95 mm observed for Mucuna-burned residue was significantly greater than the 46 mm observed for cowpea-burned residue and the 3951 mm observed for mulched residues of cowpea, Mucuna and Pueraria. Cumulative soil loss of 7.6 Mg ha 1 observed for Mucuna-burned residue in 1997 was significantly greater than those for Pueraria-mulched (0.9 Mg ha-1) and Mucuna-mulched (1.4 Mg ha-1) residues whereas in 1999 it was similar to soil loss from cowpea treatments and Pueraria-burned residue (2.35.3 Mg ha 1). There were no significant differences in soil loss in 1998 (13.2 Mg ha 1) whereas Mucuna-burned residue had a greater soil loss (28.6 Mg ha 1) than mulched cowpea (6.9 Mg ha 1) and Pueraria (5.4 Mg ha 1). Mound heights (23 cm average) decreased non-linearly with cumulative rainfall. A cumulative rainfall of 500 mm removed 0.32.3 cm of soil from mounds in 1997, 3.56.9 cm in 1998 and 2.34.6 cm in 1999, indicating that (detached but less transported) soil from mounds was far higher than observed soil loss in each year. Soil water retention was improved at potentials ranging from 1 to 1500 kPa by Mucuna-mulched residue compared to the various burned-residue treatments. Also, mound sorptivity at 1 cm water head (14.3 cm h 1/2) was higher than furrow sorptivity (8.5 cm h 1/2), indicating differences in hydraulic characteristics between mound and furrow. Pueraria-mulched residues for mounds had the highest sorptivity of 17.24 cm h 1/2, whereas the least value of 6.96 cm h 1/2 was observed in furrow of Mucuna-burned residue. Pueraria phaseoloides was considered the best option for soil conservation on the previously eroded soil, cultivated with mound tillage.
The vast area of savanna ecology in Afiica plays a significant role in food production, making a study of soils in this zone very important. Therefore, soil physical and chemical properties of 14 soil profiles were studied in a derived savanna zone of southwestern Nigeria on 2 toposequences at 2 locations (Ibadan and Alabata), which were 20 kin apart. Six soil profiles were studied at Ibadan while 8 were studied at Alabata. Morphological descriptions of profiles were carried out. Data collected included particle size distribution, bulk density, clay dispersion, water retention characteristics, pH, organic carbon, exchangeable K, Ca, Mg and available P. Soil profiles along the toposequences were well-developed with depths exceeding 180 cm, except for one profile at the lower slope position where an indurated plinthic layer was found at 68 cm depth. The horizons were easily distinguished with color, texture and consistency. Total sand, with the main component being coarse sand, decreased with depth from 813 to 502 g kg(-1) at Ibadan and from 824 to 635 g kg(-1) at Alabata. The clay content increased with depth from 54 to 3 5 6 g kg(-1) at Ibadan and from 63 to 279 g kg(-1) at Alabata. Gravel concentration was highest for soil horizons found between 20 and 102 cm depth. Also, bulk density increased with soil depth from 1.35 to 1.51 g cm(-3) at Ibadan and from 1.38 to 1.64 g cm(-3) at Alabata, indicating that subsoil horizons were more compact due to higher clay and gravel contents, and sticky consistency. The A horizon had a significantly higher water content at water potentials > 2 kPa while the subsoils had higher water content at
A study was carried out on a previously eroded Oxic Paleustalf in Ibadan, southwestern Nigeria to determine the extent of soil degradation under mound tillage with some herbaceous legumes and residue management methods. A series of factorial experiments was carried out on 12 existing runoff plots. The study commenced in 1996 after a 5-year natural fallow. Mound tillage was introduced in 1997 till 1999. The legumes - Vigna unguiculata (cowpea), Mucuna pruriens and Pueraria phaseoloides - were intercropped with maize in 1996 and 1998 while yam was planted alone in 1997 and 1999. This paper covers 1997-1999. At the end of each year, residues were either burned or mulched on respective plots. Soil loss, runoff, variations in mound height, bulk density, soil water retention and sorptivity were measured. Cumulative runoff was similar among interactions of legume and residue management in 1997 (57-151 mm) and 1999 (206-397 mm). However, in 1998, cumulative runoff of 95 mm observed for Mucuna-burned residue was significantly greater than the 46 mm observed for cowpea-burned residue and the 39-51 mm observed for mulched residues of cowpea, Mucuna and Pueraria. Cumulative soil loss of 7.6 Mg ha(-1) observed for Mucuna-burned residue in 1997 was significantly greater than those for Pueraria-mulched (0.9 Mg ha(-1)) and Mucuna-mulched (1.4 Mg ha(-1)) residues whereas in 1999 it was similar to soil loss from cowpea treatments and Pueraria-burned residue (2.3-5.3 Mg ha(-1)). There were no significant differences in soil loss in 1998 (1-3.2 Mg ha(-1)) whereas Mucuna-burned residue had a greater soil loss (28.6 Mg ha(-1)) than mulched cowpea (6.9 Mg ha(-1)) and Pueraria (5.4 Ms ha(-1)). Mound heights (23 cm average) decreased non-linearly with cumulative rainfall. A cumulative rainfall of 500 mm removed 0.3-2.3 cm of soil from mounds in 1997, 3.5-6.9 cm in 1998 and 2.3-4.6 cm in 1999, indicating that (detached but less transported) soil from mounds was far higher than observed soil loss in each year. Soil water retention was improved at potentials ranging from -1 to -1500 kPa by Mucuna-mulched residue compared to the various burned-residue treatments. Also, mound sorptivity at -1 cm water head (14.3 cm h(-1/2)) was higher than furrow sorptivity (8.5 cm h(-1/2)), indicating differences in hydraulic characteristics between mound and furrow. Pueraria-mulched residues for mounds had the highest sorptivity of 17.24 cm h(-1/2), whereas the least value of 6.96 cm h(-1/2) was observed in furrow of Mucuna-burned residue. Pueraria phas eoloides was considered the best option for soil conservation on the previously eroded soil, cultivated with mound tillage.
Tree species in agroforestry systems contribute to soil improvement through the litter produced on the soil surface and below-ground modification of soil structure by tree roots. Therefore, litter production and soil characteristics under I I tree species were evaluated in the derived savanna (Ibadan) and humid tropical forest (Onne) zones of southern Nigeria between 1998 and 1999 in 7-year old arboreta. In Ibadan, the soil was a gravelly Alfisol, while in Onne it was an Ultisol. The trees were Alchornea cord folia. Baphia pubescens, Calliandra calothyrsus, Dactyladenia barteri, Dialium guineense, Grewia pubescens, Inga edulis, Irvingia gabonensis, Nauclea diderrichii, Ptero-carpus santalinoides and Treculia africana. Forest and natural regrowth (no-tree) plots were included as control. Litter-fall under the trees ranged from 5 to 14 t ha(-1) y(-1) compared to 15 t ha(-1) y(-1) for the forest at Ibadan, while it ranged from 7 to 16 t ha(-1) y(-1) compared to 20 t ha(-1) y(-1) for the forest at Onne. Litter production was high under D. barteri and I. edulis and relatively low under A. cord folia, G. pubescens, I. gabonensis and A santalinoides. There was a more drastic change in soil chemical properties in the Alfisol than the Ultisol, particularly with available P. Soil bulk density and penetrometer resistance were improved by A. cordifolia, C. calothyrsus, D. guineense, G. pubescens and T africana. This study showed that ecological differences resulted in differences in annual litter production for only 33% of the species considered. However, the effects of the trees on soil properties were strongly linked to inherent soil properties.
Trees play a significant role in improving soils for crop production through root activities. This study was carried out on four farmers' fields in Mokwa, central Nigeria in May 1999 to quantify root distributions of 2-year-old Gliricidia sepium (G. sepium) and Acacia leptocarpa (A. leptocarpa). Root samples were taken up to 300-cm soil depth. Root length, surface area, and volume densities were evaluated by using Delta-T Scan device. Mass density was also evaluated. Root length densities of G. sepium ranged from 0.56-11mm mL(-1) while those of A. leptocarpa ranged from 0.135.93mmmL(-1), declining with soil depth. Potential contribution of roots to soil total porosity was less than 3%. Soil bulk density ranged from 1.28-1.51 g cm(-3) under G. sepium and from 1.29-1.61 g cm(-3) under A. leptocarpa. Although G. sepium had relatively more roots deeper in the soil than A. leptocarpa, both trees demonstrated deep rooting, suggesting that the trees would improve soil productivity if sufficient fallow length were allowed.
The effects of various fallow management systems and cropping intensities on water infiltration were measured on an Alfisol at Ibadan in southwestern Nigeria. The objective was to determine the influence of the land use systems (a combination of crop-fallow sequences and intercropping types) on soil hydraulic properties obtained by disc permeameter and double-ring infiltration measurements. The experiment was established in 1989 as a split-plot design with four replications. The main plots were natural fallow, planted Pueraria phaseoloides and planted Leucaena leucocephala. The subplots were 1 year of maize/cassava intercrop followed by 3-year fallow (25% cropping intensity), or 2-year fallow (33% cropping intensity), or 1-year fallow (50% cropping intensity), or no fallow period (100% cropping intensity). Water infiltration rates and sorptivities were measured under saturated and unsaturated flow. Irrespective of land use, infiltration rates at the soil surface (121-324 cm h(-1)) were greater than those measured at 30 cm depth (55-144 cm h(-1)). This indicated that fewer large pores were present below 30 cm depth compared with 0-30 cm, depth. Despite some temporal variation, sorptivities with the highest mean value of 93.5 cm h(-1/2) increased as the cropping intensity decreased, suggesting a more continuous macropore system under less intensive land use systems. This was most likely due to continuous biopores created by perennial vegetation under long fallow systems. Intercropped maize and cassava yields also increased as cropping intensity decreased. The weak relationship between crop yields and hydraulic conductivity/infiltration rates suggests that the rates were not limiting.
The effects of various fallow management systems and cropping intensities on water infltration were measured on an Alfisol at Ibadan in southwestern Nigeria. The objective was to determine the influence of the land use systems (a combination of crop±fallow sequences and intercropping types) on soil hydraulic properties obtained by disc permeameter and double-ring infiltration measurements. The experiment was established in 1989 as a split-plot design with four replications. The main plots were natural fallow, planted Pueraria phaseoloides and planted Leucaena leucocephala. The subplots were 1 year of maize/cassava intercrop followed by 3-year fallow (25% cropping intensity), or 2-year fallow (33% cropping intensity), or 1-year fallow (50% cropping intensity), or no fallow period (100% cropping intensity). Water infiltration rates and sorptivities were measured under saturated and unsaturated flow. Irrespective of land use, infiltration rates at the soil surface (121±324 cm h±1) were greater than those measured at 30 cm depth (55±144 cm h±1). This indicated that fewer large pores were present below 30 cm depth compared with 0±30 cm depth. Despite some temporal variation, sorptivities with the highest mean value of 93.5 cm h-1 increased as the cropping intensity decreased, suggesting a more continuous macropore system under less intensive land use systems. This was most likely due to continuous biopores created by perennial vegetation under long fallow systems. Intercropped maize and cassava yields also increased as cropping intensity decreased. The weak relationship between crop yields and hydraulic conductivity/infiltration rates suggests that the rates were not limiting.
In 1989, 15 fallow species were planted on a degraded Alfisol in southwestern Nigeria, but only Pueraria phaseoloides, Senna siamea, Leucaena leucocephala, Acacia leptocarpa and Acacia auriculiformis survived beyond 1993. After clearing different subplots of the fallowed plots in 1993 and 1995, intercropping of cassava + maize was practiced with level (minimum) and mound tillage. Nitrogen fertilizer, recommended while in the third year, mound tillage with 60 kg N ha(-1) was recommended
Leguminous cover crops have the potential of making cropping systems in the tropics sustainable if they would not deplete resources such as soil water and nutrients to the detriment of companion crops. Therefore, a study was carried out at Alabata, Ibadan, southwestern Nigeria, to evaluate the effects of leguminous cover crops on soil water suctions in 1993 and 1994 in order to assess the possibility of integrating them into the farming systems of the savanna zone of West Africa. In 1993, 13 leguminous cover crops (Aeschynomene histrix, Centrosema brasilianum, Centrosema pascuorum, Chamaecrista rotundifolia, Cajanus cajan, Crotalaria verrucosa, Crotalaria ochroleuca, Lablab purpureus, Mucuna pruriens, Psophocarpus palustris, Pseudovigna argentea, Pueraria phaseoloides and Stylosanthes hamata) were planted in a randomized complete block design with four replications. Maize and natural fallow (mainly Chromolaena odorata and Imperata cylindrica) were included as comparisons. Only six of the legumes (A. histrix, C. pascuorum, C. cajan, C. ochroleuca, M. pruriens, and P phaseoloides) were included in the measurements in the 1994 new plots. Soil water suctions at various stages of legume growth were measured at daily or weekly intervals (depending on the frequency of rainfall events) using tensiometers installed at 0-15 and 15-30 cm soil depths. Soil water suctions exceeding 10 kPa (theoretical field capacity) were observed mainly between 6 and 12 weeks after planting (WAP), and by 20 WAP when cover crops had matured and rainfall frequency was very low. Soil water suctions were significantly related (r(2) > 0.80) to dry matter between 8 and 10 WAR The studied cover crops were classified in three groups which can be used as a guide for choosing the legumes in tropical farming systems. Soil water depletion was markedly influenced by growth characteristics of legumes and distribution of rainfall during the rainy season. Leguminous cover crops conserved soil water after their growth needs were satisfied.
This study was carried Out in 1994 in the derived (DS) and northern Guinean savanna (NGS) zones of Nigeria at about 17-18 weeks after planting forage legumes. The forage legumes, which included Cajanus cajan, Crotalaria ochroleuca, Stylosanthes hamata, Centrosema pascuorum, Lablab purpureus, Mucuna pruriens (black and white) and Pueraria phaseoloides, were treatments in the randomised complete block design. Root morphology and length were evaluated for the 2 zones, but light received under canopies, soil temperature and soil water potential were measured only in the DS. Roots on soil profile walls were traced, and in the laboratory, computer software Was used to evaluate root length after scanning. Canopy coverage was evaluated using a light meter. In the DS, the presence of gravel in-the subsoil modified the morphology and distribution of roots even for the same species compared with the root architecture in the NGS zone. A hardpan in the shallow soil layers in the NGS caused a proliferation of roots in the topsoil, and a tendency for increased diameter (visual observation) of the primary roots. Total root length ranged from 834-1861 turn in the DS, and 939-1277 mm in the NGS, and was similar in both locations. Legume canopies intercepted 81-99% of sunlight, suggesting that they had the potential to maximise sunlight for their growth under adequate soil moisture and nutrient in the moist savanna. Legumes such as C. cajan, L. purpureus, M, pruriens and P phaseoloides with potential to grow roots beyond 60 cm depth and which intercepted greater than or equal to95% of incident Sunlight were recommended for sustained growth and rapid regeneration after cutting or browsing. Furthermore, the legumes can be combined with shallow-rooted crops for enhanced crop production in a crop-livestock farming system or in pasture, based oil the differences in their root and canopy growth.
The effects of different fallow management systems on aggregate stability were studied on an Ultisol and an Alfisol in southern Nigeria. Aggregate stability was measured in natural regrowth, and planted fallows of Pueraria phaseoloides Benth. and Leucaena leucocephala Lam de Wit in a trial established in 1989 on an Alfisol at Ibadan (7 degrees 30'N, 3 degrees 54'E), southwestern Nigeria. Soil samples (0-30 cm depth at this instance) were wetted by immersion for 2 and 10 minutes before wet-sieving at 30 rpm for 5 to 35 minutes at 5 minute increments. Mean-weight diameter (MWD), geometric mean diameter (GMD) and proportions of water-stable aggregates (WSA) were calculated. GMD and WSA were not affected by the length of pre-wetting or by the length of the wet-sieving period. Thus, wet-sieving for more than 5 minutes at 30 rpm was not necessary for these sandy soils. The trends observed for soil aggregate stability differed between the Alfisol and Ultisol, and for the Alfisol, it differed between a degraded and a managed fallow site. Thus, soil aggregate stability was influenced by soil type and soil management. The GMDs for the natural regrowth at 0-10 cm soil depth (0-15 cm soil depth sampling at 5 cm increment in this case) were between 1.12 and 1.42 mm, 1.14 and 1.46 mm for the Pueraria system, and 1.12 and 1.33 mm for the Leucaena system. An adjacent forest soil (0-10 cm) had GMDs between 1.24 and 1.54 mm. On a continuously cropped Alfisol, aggregate stability was significantly higher in the Pueraria live mulch system than in Leucaena alley cropping and natural regrowth. Fallowing for 2 or 3 years after 1 year cropping was essential under any of the systems to keep aggregate stability within the range of the forest soil. The fallow management practices enhanced surface soil aggregate stability.
The effects of legume-based soil management on soil dispersion were studied on an Alfisol between 1994 and 1995 in an experiment which was established in 1989 in southwestern Nigeria. The fallow systems, which constituted the main plots in the split-plot experiment, included natural fallow, Pueraria phaseoloides (Roxb.) Benth, and Leucaena leucocephala Lam de Wit. The subplots were 25, 33, 50 and 100 per cent cropping intensities. Apart from indices of soil dispersion such as water-dispersible clay and dispersion ratio, the fractal theory was applied to describe the fragmentation of soil aggregates less than 4 mm. under the systems and cropping intensities. Although water dispersible clay was less than 60 g kg(-1) in the 0-15 cm. soil depth because it was inherently low in clay content, the soil dispersion ratio was generally above 50 per cent. Also, the fractal dimensions, which ranged between 2.75 and 2.89, were similar among the fallow systems with cropping intensities for the surface soil. However, the interaction of slope position with season caused significant differences in fractal parameters, suggesting that the processes of soil degradation were different for the upper and lower slopes even with similar microaggregate distribution. The soil was inherently vulnerable to soil dispersion, although, the fallow management systems with less than 100 per cent cropping intensity would maintain soil structure at similar level as the forest
The capacity of vegetation fallow to replenish carbon and nutrients in degraded soil is related largely to the nature of the fallow vegetation, particularly the chemical composition. Therefore, a study was conducted at Ibadan, southwestern Nigeria (humid tropics), to look into these relationships using, fallow species with varying chemical compositions. The treatments include three woody species, Senna siamea, Acacia leptocarpa and Leucaena leucocephala, planted in 1989, and the natural shrub Chromolaena odorata. A continuous cropping of maize/cassava was maintained as a control (no fallow). Composite surface soil (0-15 cm) at three distances from a tree hedgerow (0.5, 2.0, and 3.5 m) was sampled in 1996 for the determination of soil C and nutrient stocks. Maize and cassava were planted as a test crop of soil productivity after fallowing. Litterbags were placed to determine the N immobilization during the decomposition of fallow leaves. Soil. organic C (SOC) within 0-15 cm ranged from 19,100 (continuous cropping) to 26,400 kg ha(-1) (Leucaena fallow), whereas total N ranged from 1820 (continuous cropping) to 3110 kg ha(-1) (Leucaena fallow). High polyphenols in fallow leaves favored SOC and N accumulation. The amount of potentially mineralizable N was increased from 41 kg ha(-1) in continuous cropping to 159 under Leucaena fallow and 176 under der Acacia fallow. The percentage increase in mineralizable N stock under fallow was, on average, 5.5 times greater than that in total N, implying that fallow causes change in soil organic matter quality.. The available P (Olsen) was 8.6 kg ha(-1) in the continuous cropping and ranged from 14.1, kg ha(-1). (natural fallow) to 29.2 kg ha(-1) (Leucaena fallow). Greater maize and cassava yields were obtained in the fallow plots than in the control during, the subsequent cropping. The maize grain yield after fallow could be predicted by the potentially mineralizable N in surface soil (0-15 cm). Leucaena and Acacia are promising species for planted fallows for soil regeneration in the humid tropics. The binding of protein by polyphenols during leaf decomposition, as confirmed by higher N immobilization with the increase in leaf polyphenols, could be the main mechanism in the contribution of polyphenols to SOC and N replenish, ment in the degraded soil. The study suggests the possibility of stabilizing C and N in tropical ecosystems by manipulating polyphenols in vegetation.
To rehabilitate a degraded Alfisol at Ibadan, southwestern Nigeria, Senna siamea (non-N-fixing legume tree), Leucaena leucocephala, and Acacia leptocarpa (N-fixing legume trees) were planted in 1989, and Acacia auriculiformis (N-fixing legume tree) in 1990. Pueraria phaseoloides (a cover crop) and natural fallow were included as treatments. Litterfall and climatic variables were measured in 1992/1993 and 1996/1997 while biomass production and nutrient concentrations were measured in 1993 and 1995. Total litter production from the natural and planted fallows was similar, with means ranging from 10.0 (L. leucocephala) to 13.6 t ha(-1) y(-1) (natural fallow) during the 1996/1997 collection. Leaves constituted 73% (L. leucocephala) to 96% (A. auriculiformis) of total litterfall. Acacia auriculiformis grew most quickly but S. siamea produced the highest aboveground biomass which was 127 t ha(-1) accumulated over four years, and 156 t ha(-1) accumulated over six years of establishment. The aboveground biomass of P. phaseoloides and natural fallow was only 6 to 9 t ha(-1) at six years after planting. Nitrogen concentration in the leaves/twigs of was 2.5% for L. leucocephala, and 2% for other planted species and natural fallow. Pueraria phaseoloides had concentrations of P, K, Ca and Mg comparable to levels in the leaves/twigs of the tree species. Through PATH analysis, it was found that maximum temperature and minimum relative humidity had pronounced direct and indirect effects on litterfall. The effects of these climatic variables in triggering litterfall were enhanced by other variables, such as evaporation, wind, radiation, and minimum temperature. Improvement in chemical properties by fallows was observed in the degraded soil
Topsoil (0-15 cm) hulk density, aggregate stability soil dispersibility, water retention and infiltration were measured between 1989 and 1996 on an Alfisol under rehabilitation in southwestern Nigeria. The planted leguminous species were Pueraria phaseoloides, Senna siilnrrir, Leucaena leucocephala,, Acacia leptocarpa and A. auriculiformis. plots with natural fallow and maize/cassava intercropping were included. Level (minimum) and mound tillage with hoes was adopted for the cultivated areas under study after 4 and 6 year fallow periods. Under fallow, the soil bulk density decreased from 1.56 to 1.11 t m(-3). The continuously cropped treatment (level tillage) had significantly higher bulk density than the fallowed subplots after 6 years. Mean soil penetrometer resistance ranged from 75 to 157 kPa for fallowed plots and from 192 to 295 kPa for the continuously cropped (level tillage) subplot. Surface soil water contents were similar for all the treatments during the soil strength measurements. Although soil aggregates were generally of low stability and not well formed, they were improved by fallowing. Soil structural improvement by planted fallows was similar to that by natural fallow, but the trees were more promising for long-term fallow (>6 years) than the herbaceous P. phaseoloides. However, the improvement in soil structure after 4 or 6 rear falloff could not be maintained in subsequent cropping. Furthermore, the significant improvement in soil bulk density caused by A. auriculiformis and natural fallow was more rapidly lost on the cultivated subplots compared with other fallow treatments. Thus, soil structure recovery under a fallow does not imply a sustained improvement when stress is applied to this soil. Post-fallow soil management options such as residue incorporation and tillage to ameliorate compaction or soil strength mill be necessary to enhance the improvements by fallow species.
After six years of bush-fallow, residual effects on soil productivity of tillage practices prior to the fallow were investigated on an Alfisol in southwestern Nigeria. In 1996 fallow was followed by maize intercropped with cover crops of Pueraria phaseoloides, Mucuna pruriens or cowpea (Vigna unguiculata) and no intercrop. Parameters measured included soil properties, ground cover, crop growth and yield, rainfall erosivity, runoff and soil loss. In spite of six-years of bush-fallow and establishment of cover crops, soil erosion was significantly greater on plots that had been conventionally cultivated previously using disc ploughs, harrows and mechanical rotovators (1.78 t ha71season71) compared to previously no-till plots (1.34 t ha-1season-1). Crop growth and yields were least and soil loss greatest (2.83 t ha-1season-1) on the previous bare plot. Maize grain yield was highest using Pueraria phaseoloides as an intercrop (2.15 t ha-1) followed by a cowpea inter-crop (1.92 t ha-1), maize without intercrop (1.87 t ha-1) and Mucuna pruriens intercrop (1.71 t ha-1). The maize grain yields reflected levels of competition from the cover crops. Cowpea-maize intercrop may be most suitable for farmers because maize yields were satisfactory and cowpea grain serves as additional subsistence. Cowpea yields were 390 kg ha-1. Soil erosion was also moderate using cowpea as an intercrop (1.71 t ha-1season-1). However, Pueraria phaseoloides gave the best erosion control with a soil loss of 1.34 t ha-1 season-1
Legume cover crops are a potential means for overcoming N depletion in the derived savanna of West Africa. A 3-year trial was, therefore, conducted near Ibadan, southwestern Nigeria to measure the N contribution of 13 legume cover crops as compared to urea N, using a N fertilizer replacement index for a maize test crop. Two series of trials involved the following legume cover crop species: Aeschynomene histrix, Centrosema brasilianum, Centrosema pascuorum, Chamaecrista rotundifolia, Cajanus cajan, Crotalaria verrucosa, Crotalaria ochroleuca, Lablab purpureus, Mucuna pruriens, Psophocarpus palustris, Pseudovigna argentea, Pueraria phaseoloides and Stylosanthes hamata. Trials were undertaken using a complete block design. Cover crops were planted in 1994 (Series 1) and 1995 (Series 2) in separate sites and each series was subsequently slashed and planted for one season with maize (Zea mays) in 1995 and 1996. At the 50% flowering stage, N concentration of above-ground vegetation of cover crops ranged from 21 to 38 g N kg??1. Nitrogen accumulated by 4.5-month old cover crops ranged from 14 to 240 kg N ha??1, depending on species and year. Cover crops increased grain yield of the subsequent maize crop by 25136% over the control without N application. Nitrogen uptake by the maize crop was higher following cover crops than after maize or natural grass. The N fertilizer replacement index of cover crops for maize ranged from 11 (A. histrix) to 96 kg N ha??1 (C. cajan) in Series 2. Perennial (C. brasilianum, S. hamata, C. cajan, P. phaseoloides and C. verrucosa) and annual (C. rotundifolia, M. pruriens, C. ochroleuca and L. purpureus) species could potentially save 50 to 100 kg N ha??1 for maize crops. The cover crops accumulated more N in the wetter than in the drier year. However, the N fertilizer replacement index was higher for subsequent maize grown in the drier year. The cover crop-N recovery in maize was also higher than the urea-N uptake in the drier year. The N fertilizer replacement indexes can be predicted using the above-ground biomass amount of cover crops at 20 weeks after planting (drier year) or the N concentration at that stage (wetter year).
Soil degradation is a major problem in west African land use environments. Although burning of crop residue is known to enhance soil degradation, it is still the most commonly used practice for land preparation. Intercropping with herbaceous legumes is a promising method to combat fertility decline and minimise resource degradation. A study was conducted from 1996 to 1999 in southwestern Nigeria to investigate the interaction of residue burning and legume intercropping on soil degradation as an improved and practical approach toward rehabilitating and conserving an Alfisol. Leguminous cover crops, [Mucuna pruriens (mucuna), and Pueraria phaseoloides (pueraria) with grain legumes [Vigna unguiculata (cowpea)] as control were intercropped with maize in 1996 and 1998; yam was cultivated in 1997 and 1999. Following local practices, mound tillage with hoe was adopted since 1997 for yam production. Subtreatments were burning and mulching of residues from the cover crops, cowpea and maize. Soil erosion and yam mound stability were used as sustainability indicators. The weighted annual mean erosivity was 6.5 103 MJ mm ha-1h-1 (EI30 index), 422 cm2 h-1 for the AIm index. Mean runoff amounts ranged from 20 to 32 mm in 1996, from 94 to 158 mm in 1997, and from 37 to 103 mm in 1998. Mean soil erosion ranged from 0.9 to 2.1 Mg ha-1 in 1996, from 3.8 to 7.8 Mg ha-1 in 1997, and from 1.8 to 3.7 Mg ha-1 in 1998. The burned residue plots had higher runoff and soil erosion than the unburned residue plots. Soil particles were eroded in proportions similar to plot particle size distribution. Mounding in 1997 accentuated soil erosion although transportation of detached soil by runoff was impeded by the mounds. Variations in mound heights were due to accretion as well as removal of soil from the furrows. There was no significant difference in soil erosion between the cover crops and cowpea. Yam mound erodibility (slumping) was largely determined by the length of exposure to rain in the 1998 cropping season. Mounds on the burned residue plot of each legume, except cowpea, slumped more than mounds on the unburned residue plot between April and August 1998 . The combined yields (1996-98) of maize, grain and yam tuber were higher in the unburned residue treatment compared to the burned residue treatment. Cover crop intercropping resulted in higher crop yield than the cowpea. We concluded that the Alfisol is best managed for sustainable crop production by no tillage, residue mulching and cover cropping. Residue burning should be avoided even if cover crops are used.
Evaluation of soil aggregate stability under managed fallow systems is very relevant in the assessment of their agricultural sustainability. It is also important to identify the parameters that are most responsive to the effects of these fallow systems on aggregate stability. This study was carried out in southwestern Nigeria to quantify the effects of various fallow management systems on the macroaggregate stability of surface soil (0-15-cm depth) using a long-term fallow management trial established in 1989 on an Alfisol toposequence. Data were collected between 1994 and 1995. Three fallow systems (bush fallow, Pueraria phaseoloides, and Leucaena leucocephala) and three fallow periods (1-3 years after 1-year cropping) were evaluated. Cropping consisted of maize + cassava intercropping. A split-plot design with fellow systems as main plot and cropping intensities as subplot in four replications was used in the trial. Soil aggregate samples were wet-sieved to evaluate their mean-weight diameter (MWD) and fractal dimension (D). The means of MWD for the fallow systems and cropping intensities ranged from 2.4 to 6.4 mm. The mean D values ranged from 2.29 to 2.72 while the mean intercepts (log k) of the regression ranged from 2.01 to 2.28. Low D values (cohesive and stable aggregates) were associated with fallowing whereas high D values (fragmented aggregates) were associated with cultivation. Fallowing enhanced soil aggregate stability. The Pueraria system also enhanced soil aggregate stability more than the bush fallow and Leucaena systems when continuous cropping was practised. The subplots, which were fallowed for 2 or 3 years after 1 year of cropping, were usually similar in aggregate stability to the secondary forest soil. Fractal analysis showed further that soil aggregates in the dry season and on the upper slope of the toposequence were more cohesive than in the wet season and on the lower slope. Observed D and log k values were also significantly influenced by laboratory method
The potential of planted leguminous cover crop fallow as an alternative to the natural regrowth fallow for sustaining the productivity of low activity clay (LAC) soils in the tropics as the fallow period shortens was tested at Ibadan in the forest-savanna zone of southwestern Nigeria. The study, initiated in 1990, compared an improved system (cover crop-fallow) and a traditional system (natural fallow), with the aim of developing an improved cover crop-fallow system for sustainable management of LAC soils of the tropics. Pueuraria phaseoloides, an herbaceous, N-fixing, perennial legume with a slow initial growth habit, was selected as a test cover crop. Pueraria was sown with a maize-cassava intercrop at the same season. In a 2-year cycle, 1 year of cropping was followed by 1 year fallow with pueraria fdr the cover crop-fallow system or with natural regrowth (mainly Chromolaena odorata) for the natural fallow system. Maize-cassava intercropping without a fallow period (continuous cropping) was included as a control. No fertilizer was applied throughout the experimental period. Pueraria produced 253 kg N ha(-1) after the fallow period, compared with 109 kg N ha(-1) for chromolaena; pueraria was also better in recycling P (11 kg P ha(-1)) than was chromolaena (9 kg P ha(-1)), Natural fallow for 1 year increased the maize yield from 75 to 350% from 1992 to 1996, and the cassava yield from 9 to 130% compared with continuous cropping. Cover crop-fallow resulted in a 22 to 72% higher maize yield than the natural fallow from 1992 to 1996. Although in 1992 and 1994 cassava tuber yield was lower with the cover crop-fallow than with the natural fallow, with better pueraria husbandry in 1996, cover crop-fallow increased the cassava tuber yield by 41%. With residue burning, the N balance was 27 kg N ha(-1) yr(-1) for the pueraria cover crop-fallow system and -15 kg N ha(-1) yr(-1) for the chromolaena natural fallow system. Cover crop-fallow maintained soil organic carbon status better than the natural fallow. Integration of a pueraria legume cover crop did not affect soil pH. Results indicate cover crop-fallow with pueraria could be a better alternative to traditional natural fallow under shortened fallow periods for raising or maintaining productivity of LAC soils of the humid tropics
The effects of pre-fallow tillage practices on the productivity of a 6 year fallowed tropical Alfisol was investigated in South Western Nigeria. Post-fallow land use in 1996 was maize production intercropped with legume. The parameters evaluated included soil properties, ground cover, crop growth and yield, rainfall erosivity, runoff and soil loss. In spite of the six-year bush fallow and establishment of cover crops, soil erosion was significantly higher on plots previously under conventional tillage compared to no - tillage plots. Crop growth and yields were low on the previous bare plot which had maximum erosion compared to the previously cropped plots in spite of bush fallowing. Maize grain yield was highest under Pueraria phaseoloides intercrop (2.15 t/ha) followed by Cowpea intercrop (1.92 t/ha), sole maize (1.87 t/ha) and Mucuna pruriens intercrop (1.7 t/ha). Soil erosion was moderate under this system. However, Pueraria performed best as an intercrop for soil conservation in this first year of intercropping.
The restorative ability of herbaceous (Psophocarpus palustris, Pueraria phaseoloides) and woody (Leucaena leucocephala, Senna siamea, Acacia leptocarpa, Acacia auriculiformis) legume species and of natural regrow-th was studied on an eroded and compacted Oxic Paleustalf in southwestern Nigeria. Compared to the control treatment that was continuously cropped for 15 years, four years of fallowing significantly improved test crop yields. However fallowing with the above species did not substantially improve soil properties, particularly soil bulk density. A longer fallow period may be needed to amend soil physical conditions of this degraded Alfisol. Soil chemical properties were greatly improved following land clearing and plant biomass burning in 1993. However, the residual effect of burning on soil fertility was insignificant in the second cropping year. Among the fallow species, P palustris and natural fallow-showed the best residual effect on test crop performance. Despite the high biomass and nutrient yields of S. siamea and A. auriculiformis, test crop yields on these plots were low due to the border effects from the uncleared and fallowed subplots.
Rainfall data collected from 1986 to 1990 at Okomu (6 degrees 25'N; 5 degrees 12'E; 76 m above mean sea level), a humid forest area near Benin City, southern Nigeria, were used to evaluate erosivity characteristics, The average annual rainfall was 2048 mm and the distribution pattern was unimodal. 49% of rainfall amount fell at 7.5-minute intensifies exceeding 25 mm/h. A maximum 7.5-minute intensity of 240 mm/h was observed at the site. The median rain drop size (D-50) was 2.3 mm. The mean monthly kinetic energy, using the Wischmeier and Smith (1978) recommended procedure, ranged from 1.5 to 87 MJ/ha; whereas the values ranged from 1.5 to 140 MJ/ha using the Kowal and Kassam (1976) equation, These results suggest that equations developed in the tropics would estimate kinetic energy higher than those developed in the temperate regions. The annual value of erosivity was 18510 MJ . mm/ha . h by the EI(30) index, 216 MJ/ha by the KE greater than or equal to 25 mm/h index and 1329 cm(2)/h by the AI(m) index. The study indicates that rainfall of small and large amounts are capable of causing soil erosion at the site because they often fall at erosive intensities and contain big drops. The complementary role of the small- and large-amount storms made the cumulative erosivity of the rains at the study site very high. The information provided in this study shall be useful in estimating the erosive nature of rain in similar environments and will augment other available information in drawing an iso-erodent map for Nigeria, Furthermore, the erosivity factor for soil loss estimation on monthly basis in the area of study can be quantified by selecting the appropriate index values for soil loss equations.
Rainfall erosivity was characterized for the Guinea Savanna, Forest and Coastal belts of southeastern Nigeria (4 degrees and 7 degrees N; 6 degrees 30/ and 9 degrees 30/E). The highest maximum rainfall amounts ranged from 117 to 183 mm per rain event whereas the maximum 6-minute intensities ranged from 191 mm h(-1) to 254 mm h(-1). Advanced storms were dominant in the region. The values of the Kowal and Kassam kinetic energy equation (designated E(k)) were 1.6 times higher than the values obtained using Wischmeier and Smith's equation (designated E). Compound rainfall erosivity used were the EI(30), KE greater than or equal to 25 mm h(-1), AI(m), E(k)I(30) and E(k)I(m). The mean annual erosivity values using the EI(30) index ranged from 12,814 to 18,611 MJ . mm/ha . h. The KE greater than or equal to 25 mm h(-1) ranged from 141 to 249 MJ ha(-1), the AI(m) from 849 to 1421 cm(2) h(-1), the E(k)I(30) from 16,697 to 29,610 MJ mm/ha h and E(k)I(m) from 32,752 to 62,238 MJ bullet> mm/ha . h. Rainfall erosivity approximations from rainfall amounts using indices from Roose and Arnoldus may be converted to SI units (Foster et al.) by multiplying by a factor of 17. Rainfall erosivity differences were more pronounced between the Guinea Savanna and Forest or Coastal belts than between the Forest and Coastal belts. Erosivity is higher in the Forest or Coastal belts than the Guinea Savanna belt. The high erosivity of rains in southeastern Nigeria can be attributed to heavy storms of comparatively high intensities and, often, long duration. The magnitude of rainfall erosivity provides a useful insight into the causes of the catastrophic erosion problems in southeastern Nigeria.