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Abstract: 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 >2 kPa while the subsoils had higher water content at <2 kPa. Soil organic C and total N decreased with soil depth at both locations with the A horizon having significantly higher organic C (7.10–12.69 g kg−1) and total N (0.84–1.2 g kg−1) than deeper horizons (1.9–4.47 g kg−1 for organic C and 0.12–0.58 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. [Copyright &y& Elsevier]

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 (r2>0.80) to dry matter between 8 and 10 WAP. 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. [Copyright &y& Elsevier]

A study was conducted in 1998 and 1999 on a long-term fallow management trial, established in 1989 at the International Institute of Tropical Agriculture (IITA), Ibadan, in the derived savanna of southwestern Nigeria, to quantify P fractions under natural fallow (NF) and a Pueraria cover crop fallow. Plots with previous 1:1, 1:2, and 1:3 crop/fallow ratios before reverting to fallow in either 1998 or 1999 were considered. Biomass accumulation under the two fallow types was comparable and increased linearly with fallow age, reaching slightly above 7 t dry matter ha-1 after 8 months of fallow. Phosphorus accumulation in the fallow vegetation ranged from 2.1 – 9.1 kg ha-1 for natural fallow (NF) and from 1.5 – 6.6 kg ha-1 for Pueraria. Magnesium was also higher under NF (9.1 – 21 kg ha-1) than under Pueraria (4.4 – 13 kg ha-1), whereas N, Ca, and K contents were higher in Pueraria biomass than under NF at 1 year after fallow. Pueraria fallow tended to lower soil pH compared with NF. However, plots with less frequent cropping (1:3 crop/fallow ratio) did not have significantly different pH irrespective of the fallow vegetation type. Olsen extractable soil P increased as fallow length increased irrespective of the fallow system and previous crop/fallow ratio. For example, under NF (0–5 cm depth, 1:1 crop/fallow ratio in1998) Olsen P increased from 12 mg kg-1 to 17 mg kg-1 after 1 year of fallow and under Pueraria, it increased from 8 mg kg-1 to 15 mg kg-1. Fallow type and previous crop/fallow ratio had no significant and consistent effects on soil P fractions. However, NaOH- and concentrated HCl- extractable organic P fractions increased with fallow length. In 1998, under NF, NaOH- extractable organic P increased from 12 to 21 mg kg-1 (1:1 crop/fallow ratio) and from 10 to 19 mg kg-1 for both 1:2 and 1:3 crop/fallow ratio. HCl- extractable organic P increased from 11 to 30 mg kg-1 (1:1 crop/fallow ratio), from 13 to 27 mg kg-1 (1:2 crop/fallow ratio) and from 18 to 35 mg kg-1 (1:3 crop/fallow ratio). Similar trend was observed under Pueraria fallow. These results suggest that P was reallocated to non-readily available organic P fractions irrespective of fallow type and previous land use. These organic P fractions, which are usually more stable, reflect the overall change in soil organic P levels when the soil was stressed by cultivation and then reverted to fallow. These pools may thus represent an active reservoir (source and sink) of P in shifting cultivation under tropical conditions without inorganic fertilizer application. [ABSTRACT FROM AUTHOR]

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 followed plots in 1993 and 1995, intercropping of cassava + maize was practiced with level (minimum) and mound tillage. Nitrogen fertilizer, between 0 and 60 kg ha[sup -1], was also applied to subplots of natural regrowth where planted fallows did not survive. Although a maize grain yield of 2.3 t ha[sup -1] and a cassava root yield of 18.3 t ha[sup -1] were obtained with level tillage in 1995, grain yield was increased to 2.6 t ha[sup -1] and cassava yields to about 22 t ha[sup -1] by mounding and 60 kg N ha[sup -1] application. Effects of mounding and N application were pronounced in the second and third consecutive years of cropping. Planted tree fallows like S. siamea, A. leptocarpa and L. leucocephala were highly suitable for soil rehabilitation. For sustainable crop production in the second consecutive year of cropping after a 6-year fallow period, mound tillage was recommended while in the third year, mound tillage with 60 kg N ha[sup -1] was recommended. [ABSTRACT FROM AUTHOR]

A study was conducted in long‐term alley cropping plots established in 1989 at International Institute of Tropical Agriculture (IITA), Ibadan, southwestern Nigeria to determine the seasonal and yearly dynamics of dry matter and nutrient concentrations of hedgerow pruning under continuous cropping and response to fallow of 1–3 years. At one year after Leucaena leucocephala transplanting, pruning was started at the 75 cm height. The pruning frequency increased from one to five times per year as trees grew. In continuous alley cropping without fertilizer application, pruning dry matter decreased gradually from a peak of 5 t ha−1 at 3 years after Leucaena transplanting to about 2.5 t ha−1 at 11 years after transplanting, while the ratio of dry season to wet season biomass increased. Within 5 years after establishment, nitrogen (N) and phosphorus (P) concentrations in pruning decreased and Mg increased with the continuous cropping. Fallow clearly increased pruning biomass and nutrient concentrations. At the completion of the first cycle (1993), the pruning N yield was 170 kg ha−1 for the continuous cropping plot, and 269 for plots with 1 yr fallow, 344 for plots with 2 yr fallow and 241 kg ha−1 for plots with 3 yr fallow. In 1993 pruning P yield increased from 6.5 to 10.9 in the continuous alley cropping and in 1995, it increased from 4.5 to 7.0 kg ha−1 in one‐year fallow plots. Fallow effect on potassium (K), calcium (Ca), and magnesium (Mg) yield in pruning was also evident. These results suggest alley cropping with one‐year fallow could be an effective rotation system for nutrient cycling in Alfisols of West Africa. #The work was conducted when the senior author was employed by International Institute of Tropical Agriculture (IITA), PMB 5320, Ibadan, Nigeria [ABSTRACT FROM AUTHOR]

Provides information regarding biomass and litter production from planted and natural fallows on a degraded soil in southwestern Nigeria. Similarity between total litter production from natural and planted fallows; Data on litterfall constituted by leaves; Production of highest aboveground biomass by Senna siamea.

Examines the soil fertility of a degraded Alfisol under humid tropical conditions in Nigeria. Decrease of soil bulk density; Low stability of soil aggregates; Loss of natural fallow in cultivated subplots.