The following nine main cropping systems were evaluated: maize/maize in rotation without N-fertilizer (control); maize/cowpea in rotation where maize received N-fertilizer; maize/pigeonpea intercropping without N-fertilizer; maize/cowpea/pigeonpea in rotation where maize received N-fertilizer; maize/maize in rotation with N-fertilizer; maize/pigeonpea fallow in rotation where maize received N-fertilizer; maize/pigeonpea intercropping where maize received N-fertilizer; pigeonpea/maize/cowpea in rotation where maize received N-fertilizer; pigeonpea/maize/maize where maize received N-fertilizer. The rate of N-fertilizer applied was 74 kg N as urea (28 kg N/ha 2 weeks after planting maize and 46 kg N/ha 6 weeks after planting maize). Compared to the control, the increase in maize grain yield ranged from –1.55 to 215 % for all cropping systems receiving N-fertilizer and intercropping without N-fertilizer, maize grain yield in intercropping without N-fertilizer increased by 9 % in first year and decreased by 1.55 % in second year compared to the control. The maize stover yield with N-fertilizer application increased by -24 up to 203 %. Pigeonpea grain yield (in intercropping) without N-fertilizer was 56 % higher than with N-fertilizer. Pigeonpea as fallow produced 73-97 % more fresh wood than intercropped pigeonpea without fertilization and 91-118 % more fresh wood than intercropped pigeonpea with N-fertilizer. The intercropped Cajanus cajan with N-fertilizer produced 11 % less fresh wood than intercropped without N-fertilizer. The total N amount in the maize grain yield increased by 5-245 % for the first season of year 1 compared to the control and increased by 241-260 % for the second season of year 1 compared to the control. The total N amount in the maize grain of the intercropped pigeonpea/maize with N-fertilizer was 190 % higher than the intercropped pigeonpea/maize without N-fertilizer. The total amount of P follows the same trend as Nfertilizer in intercropped systems. The yield efficiency was low in year 1 compared to year 2.

Low soil fertility is one of the main constraints to crop production in the West African savanna. However, the response of major cereals to fertilizer applications is often far below the potential yields. Low fertilizer efficiency, inadequacy of current fertilizer recommendations, and the ignorance of nutrients other than N, P, and K may limit crop production. Nutrient limitations to maize production were identified in on-farm trials in Togo and in several long-term experiments in Nigeria and Benin. Maize ear leaf samples were analyzed for macro and micro-nutrients, and the Diagnosis and Recommendation Integrated Systems (DRIS) was applied to rank nutrients according to their degree of limitation to maize. In the on-farm trials, both yield and DRIS results indicated that, when N is supplied, P limited maize production in all fields, reducing yields by 31% on average. Sulfur was limiting in 81% of the fields and was responsible for an average yield reduction of 20%. In the long-term experiments where N, P, and K had been annually applied, Ca and Mg indices were strongly negative, indicative of deficiency. Zn indices were negative in all trials. Despite N-fertilizer additions, N indices remained negative in some of the long-term experiments, pointing to low efficiency of applied fertilizers. There was a direct link between DRIS indices and the management imposed in the different experiments, indicating that DRIS is a useful approach to reveal nutrient deficiencies or imbalances in maize in the region.

One of the options to alleviate soil fertility constraints for sustainable agriculture in West African’s savanna is to develop soil nutrient management technologies from an adequate supply and feasible share of organic and inorganic fertilizers. The impact of combined application of organic input (fresh Cajanus cajan pruning residues) and inorganic fertilizers (Togo phosphate rock) on maize performance was investigated on 2 sites (Zouzouvou and Eglime) in the derived savanna benchmark of Benin, where 2 main geological units can be distinguished, giving rise to distinct soil associations. One (Zouzouvou) is a Rhodic Ferralsol and one (Eglime) a complex pattern of Acrisols, Lixisols, Luvisols and Leptosols with inclusions of Vertisols and Cambisols. The application of rock phosphate increased the shoot dry weight of Cajanus cajan by 29 to 145 % in 5 out of 12 farmers’ fields at Zouzouvou and 17 to 53 % in 3 out of 12 farmers’ fields at Eglime. The dry matter of the leaves increased by 13 to 227 % in 6 out of 12 farmers’ fields at Zouzouvou, while at Eglime, in 6 out of 11 fields, it increased by 7 to 31 %. A significant increase was obtained with symbiotic parameters of Cajanus cajan such as % AMF, nodule number and nodule fresh weight with rock phosphate application, at both places. Only 40 % of farmers’ fields at Zouzouvou had a subsequent maize grain increase (by 17 to 90 %) as a result of the residual effect of combining organic (Cajanus cajan ) and inorganic input. There was no effect at Eglime.

The functioning of trees as a safety-net for capturing nutrients leached beyond the reach of crop roots was evaluated by investigating changes in exchangeable cations (Ca, Mg, and K) and pH in a wide range of medium to long term alley cropping trials in the derived savanna of West Africa, compared to no-tree control plots. Topsoil Ca content, effective cation exchange capacity, and pH were substantially higher under Sennasiamea than under Leucaena leucocephala, Gliricidia sepium, or the no-tree control plots in sites with a Bt horizon rich in exchangeable Ca. This was shown to be largely related to the recovery of Ca from the subsoil under Senna trees. The increase of the Ca content of the topsoil under Senna relative to the no-tree control treatment was related to the total amount of dry matter applied since trial establishment. The lack of increase in Ca accumulation under the other species was related to potential recovery of Ca from the topsoil itself and/or substantial Ca leaching. The accumulation of Ca in the topsoil under Senna had a marked effect on the topsoil pH, the latter increasing significantly compared with the Leucaena, Gliridia, and no-tree control treatments. In conclusion, the current work shows that the functioning of the often hypothesized ‘safety-net’ of trees in a cropping system depends on (i) the tree species and on (ii) the presence of a subsoil of suitable quality, i.e., clay enriched and with high Ca saturation.

Although the West-African moist savanna zone has a high potential for crop production, yields on farmers' fields are, on average, far below this potential, mainly due to the low use of external sources of nutrients. Since the mid-1990s, it has become clear that in order to upgrade crop production to levels needed to sustain the growing population without further degrading the soil resource base, inorganic fertilizers are required. Due to the physico-chemical nature of these soils and the relatively high cost of inorganic fertilizers, a general consensus exists in the research and development community that these inorganic inputs need to be complemented with organic matter. Here, we explore options to produce organic matter in-situ and evaluate the impact of combining inorganic and organic sources of N on maize yields, focusing on the densely populated derived savanna (DS) benchmark of Benin Republic. Although most of the farmers (93%) in this benchmark use inorganic fertilizer, applications rates are low (on average, 27 kg N ha−1). A significant response to N was observed for 96% of the studied farmers' fields.

Although the fertility status of soils in the West African moist savanna is generally believed to be low, crop yields on farmers' fields vary widely from virtually nil to values near the potential production. The soil fertility status was evaluated for a number of farmers' fields selected at random in 2 villages (Zouzouvou and Eglimé) representative for the derived savanna (DS) benchmark area and in 2 villages (Danayamaka and Kayawa) representative for the Northern Guinea savanna (NGS) benchmark area. The relation between soil fertility status and soil type characteristics and fertilizer use was explored. In an accompanying missing nutrient greenhouse trial, the most limiting nutrients for maize growth were determined. While soils in the DS villages were formed on different geological units, soils in the NGS villages could be differentiated according to their position on the landscape. Generally, soils in the DS contained a smaller amount of silt (104 vs. 288 g kg −1 ), a larger amount of sand (785 vs. 584 g kg −1 ), C (9.3 vs. 6.3 g kg −1 ), N (0.7 vs. 0.5 g kg −1 ), Olsen-P (10.7 vs. 5.4 mg kg −1 ), and had a higher CEC (7.0 vs. 4.8 cmolc kg −1 ) than soils in the NGS villages. The large silt content of the soils in the NGS is a reflection of the aeolian origin of the parent material. Within the benchmark areas, general soil fertility characteristics were similar in the villages in the NGS, except for a larger amount of particulate organic matter in Kayawa than in Danayamaka. This may also have led to a significantly larger amount of ammonium-N content in the 0–20 and 20–40 cm soil layers in Kayawa compared to Danayamaka (42 vs 24 kg N ha −1 in the 0–20 cm soil layer). Differences in topsoil soil characteristics between the DS villages were a reflection of differences in clay quality (kaolinitic vs. 2:1 clay minerals) of the parent material and past fertilizer use. The Olsen-P and exchangeable K contents were observed to increase with increased fertilizer application rate in both benchmarks, while fertilizer application rate had no significant effect on the organic C or total N content of the soil nor on its ECEC. The response of maize shoot biomass production to applied N was similar for both benchmarks (biomass accumulation in the treatment without N was, on average, 55% of the biomass production in the treatment which received all nutrients), while soils in the NGS responded more strongly to applied P than soils in the DS (37% vs 66% of biomass production in the treatment which received all nutrients). The more favourable P status of soils in Eglimé (DS) was attributed to the more intense use of P fertilizers, as a result of government-supported cotton production schemes. Response to cations, S or micronutrients were neglegible. A significant linear relationship was found between the soil Olsen-P content and the response to applied P up to levels of 12 mg kg −1 in the topsoil. Above this level, a plateau was reached.

Nutrient depletion is a major constraint to crop production for moist savanna soils, and inputs of nutrients are required to overcome this constraint. The impact of sole and combined applications of organic inputs (OIs) [fresh tree prunings, Pueraria phaseoloides (Roxb.) Benth. residues, and manure] and urea [(NH 2 ) 2 CO] on maize ( Zea mays L.) performance was investigated at four sites in West Africa. Interactions between OIs and urea resulting in added benefits from their mixed rather than sole application were quantified, and likely causes were evaluated. Maize in the mixed treatments, receiving 45 kg ha −1 urea N and 45 kg ha −1 N as OIs, produced 1.6 and 3.7 Mg ha −1 grain in Sékou and Glidji, respectively. Based on the yields from sole application of either OIs or urea, added benefits from the mixture were 0.49 Mg ha −1 grain ( P < 0.001) in Sékou and 0.58 Mg ha −1 ( P < 0.15) in Glidji. These benefits were generated during grain filling, which was characterized by drought, and they were likely caused by improved soil water conditions with mixed applications compared with sole applications. Nitrogen recovery from urea was higher in the combined treatments (44% in Sékou and 32% in Glidji) relative to the sole urea treatments (22% in Sékou and 15% in Glidji). Positive interactions between OIs and urea occurred at two of four sites and were likely caused by improved soil water conditions after applying OIs. Organic inputs can alleviate constraints to crop growth other than N depletion and, as such, improve the use efficiency of N fertilizer.

Prunings in hedgerow intercropping systems are a potential source of organic matter and their quality is an important characteristic driving decomposition and nutrient release. To determine the potential range of residue characteristics and the impact of canopy age on those, selected characteristics of Leucaena leucocephala, Gliricidia sepium, and Senna siamea hedgerow leaf residues were determined for ‘young’ (5–10 weeks) and ‘old’ (39–47 weeks) hedgerow canopies in six sites, representative in terms of soil for the derived savanna zone of West-Africa. The N content of the Leucaena and Senna residues decreased with age (from 4.8 to 3.9% and from 4.1 to 3.0%, respectively, while the N content of the Gliricidia residues remained constant (3.9%). The P content of all species decreased with age (from 0.30 to 0.20%, on average). The lignin content of the Leucaena residues increased, and their polyphenol content decreased with age, while for the Senna residues only an increase in polyphenol content with canopy age was evident. Neither the lignin nor the polyphenol content of the Gliricidia leaf residues was markedly affected by canopy age. For all species, the P and ADF content were positively correlated with the ash and lignin content, respectively. The N content of the Leucaena and Senna residues was positively correlated with their P content while the lignin content of the same species was negatively correlated with the polyphenol content. Because of the low variation in certain residue characteristics between sites, the presented data and equations could potentially be used to reasonably estimate those characteristics if no other information is available. For other characteristics, such as polyphenol contents, measurements are preferable in view of the relatively high variation encountered.

The study was conducted in research villages in the derived savannah (DS) and northern Guinea savannah (NGS) benchmark areas of the Ecoregional Programme for the humid and sub-humid tropics in Sub-Saharan Africa to investigate the possible benefits of combining organic matter with urea under a range of on-farm conditions. Biophysical and socioeconomic characteristics of the area are discussed. The maize response to combined applications of organic matter and urea in on-station field trials at four locations (Bouaké, central Côte d'Ivoire; Glidji, southern Togo; Sékou, southern Benin and Zaria, northern Nigeria) and the maize grain yield in a researcher-managed on-farm trial in two villages (Eglimé and Zouzouvou) in the DS benchmark area, southern Benin, and in two other villages (Kayawa and Danayamaka) in the NGS benchmark area, northern Nigeria, are presented and discussed. In the DS, a 2-year farmer-managed trial was initiated in 1999 by the Research and Development branch of INRAB in southern Benin, wherein cowpea was used as an in-situ source of organic matter. In the NGS, demonstration trials were initiated at seven states of the northwestern and northeastern Nigeria. Preliminary yield data support observations made earlier under on-station and researcher-managed on-farm conditions: maize grain yields in the sole fertilizer treatment were similar to yields in the mixture treatment in which approximately 40% of the fertilizer N was substituted by manure.

The question of how increased soil organic carbon (SOC) levels, through increased buffering for water, cations, and against pH-changes, translate into maize yields and increased efficiency of inorganic fertilizers is addressed. The study investigates how much organic matter needs to be applied to the soil to achieve a certain SOC increase for West African conditions. This will give a more realistic view on the magnitude of the achievable SOC increase, and facilitate the discussion on potential benefits of the SOC increase. This study is mainly limited to the savanna zone in West Africa, and considers the situation where the land is cultivated continuously or almost continuously.

The impact of land use (unfertilized continuous maize cropping, unfertilized and fertilized alley cropping with maize, Gliricidia sepium tree fallow, natural fallow) on the soil organic matter (SOM) status and general soil fertility characteristics were investigated for a series of soils representative for the West African moist savanna zone. Three soils from the humid forest zone were also included. In an associated pot experiment, relationships between maize N and P uptake and SOM and general soil characteristics were developed. Soils under natural fallow contained the highest amount of organic C (1.72%), total N (0.158%), and had the highest effective cation exchange capacity (ECEC) [8.9 mEq 100 g–1 dry soil], while the Olsen P content was highest in the fertilized alley cropping plots (13.7 mg kg–1) and lowest under natural fallow (6.3 mg kg–1). The N concentration of the particulate organic matter (POM) was highest in the unfertilized alley cropping plots (2.4%), while the total POM N content was highest under natural fallow (370 mg N kg–1) and lowest in continuously cropped plots (107 mg N kg–1). After addition of all nutrients except N, a highly significant linear relationship (R2=0.91) was observed between the total N uptake in the shoots and roots of 7-week-old maize and the POM N content for the savanna soils. POM in the humid forest soils was presumably protected from decomposition due to its higher silt and clay content. After addition of all nutrients except P, the total maize P uptake was linearly related to the Olsen P content. R2 increased from 0.56 to 0.67 in a multiple linear regression analysis including the Olsen P content and clay content (which explained 11% of the variation in P uptake). Both the SOM status and N availability were shown to be improved in land-use systems with organic matter additions, while only the addition of P fertilizer could improve P availability.

The potential of alley cropping systems supplied with a limited amount of fertilizer to restore crop productivity on a degraded site and to maintain crop productivity on a recently cleared, non-degraded site on ‘terre de barre‘ soils in Southern Bénin was investigated from 1994 to 1996. Leucaena leucocephala, Senna siamea and Gliricidia sepium were used as hedgerow species. Maize yields of the no-tree control plots dropped from the initial (1990) 401 kg ha−1 and 2181 kg ha−1 on the degraded and non-degraded sites, respectively, to 109 kg ha−1 and 1346 kg ha−1 in 1996, even with application of a minimal amount of mineral fertilizer. The alley cropping systems produced on average (mean of three treatments and three years) 107% more grain than the initial 1990 values on the degraded site and 11% less grain than the initial 1990 values on the non-degraded site. Especially the Senna and to a lesser degree the Leucaena treatment yielded consistently more grain than the control. The Senna trees contained a larger amount of N and produced more wood during the first pruning on the degraded site (155 kg N ha−1 and 14.0 ton fresh wood ha−1) than on the non-degraded site (49 kg N ha−1 and 6.6 ton fresh wood ha−1) most likely because of differences in subsoil fertility, as indicated by the higher clay, exchangeable bases, and N content between 60 and 125 cm cm. N accumulation and wood production by the Leucaena and Gliricidia trees was similar in both sites (82 and 36 kg N ha−1 and 4.6 and 9.3 ton fresh wood ha−1, respectively). When a limited amount of fertilizer is available, Senna appears to be the best choice as hedgerow species on sites with a relatively fertile subsoil. For other soils, a N2-fixing species may be a better choice.