N Sanginga

Long-term integrated soil fertility management in South-western Nigeria: Crop performance and impact on the soil fertility status

Crop response, tree biomass production and changes in soil fertility characteristics were monitored in a long-term (1986–2002) alley-cropping trial in Ibadan, Nigeria. The systems included two alley cropping systems with Leucaena leucocephala and Senna siamea on the one hand and a control (no-trees) system on the other hand, all cropped annually with a maize–cowpea rotation. All systems had a plus and minus fertilizer treatment. Over the years, the annual biomass return through tree prunings declined steadily, but more drastically for Leucaena than for Senna. In 2002, the nitrogen contribution from Leucaena residues stabilized at about 200 kg N/ha/year, while the corresponding value for Senna was about 160 kg N/ha/year. On average, the four Leucaena prunings were more equal in biomass as well as in amounts of N, P and cations, while the first Sennapruning was always contributing up to 60% of the annual biomass or nutrient return. Maize crop yields declined steadily in all treatments, but the least so in the Senna + fertilizer treatment where in 2002 still 2.2 tonnes/ha of maize were obtained. Nitrogen fertilizer use efficiency was usually higher in the Senna treatment compared to the control or the Leucaena treatment. Added benefits due to the combined use of fertilizer N and organic matter additions were observed only for the Sennatreatment and only in the last 6 years. At all other times, they remained absent or were even negative in the Leucaenatreatments for the first 3 years. Most chemical soil fertility parameters decreased in all the treatments, but less so in the alley cropping systems. The presence of trees had a positive effect on remaining carbon stocks, while they were reduced compared to the 1986 data. Trees had a positive effect on the maintenance of exchangeable cations in the top soil. Exchangeable Ca, Mg and K – and hence ECEC – were only slightly reduced after 16 years of cropping in the tree-based systems, and even increased in the Senna treatments. In the control treatments, values for all these parameters reduced to 50% or less of the original values after 16 years. All the above points to the Senna-based alley system with fertilizers as the more resilient one. This is reflected in all soil fertility parameters, in added benefits due to the combined use of fertilizer nitrogen and organic residue application and in a more stable maize yield over the years, averaging 2.8 tonnes/ha with maximal deviations from the average not exceeding 21%.

Grain legume rotation benefits to maize in the northern Guinea savanna of Nigeria: fixed-nitrogen versus other rotation effects

The yield increases often recorded in maize following grain legumes have been attributed to fixed-N and ‘other rotation’ effects, but these effects have rarely been separated. Field trials were conducted between 2003 and 2005 to measure these effects on maize following grain legumes in the northern Guinea savanna of Nigeria. Maize was grown on plots previously cultivated to two genotypes each of soybean (TGx 1448-2E and SAMSOY-2) and cowpea (IT 96D-724 and SAMPEA-7), maize, and natural fallow. The plots were split into four N fertilizer rates (0, 30, 60 and 90xa0kgxa0Nxa0ha−1) in a split plot design. The total effect was calculated as the yield of maize following a legume minus the yield following maize, both without added N and the rotation effect was calculated as the difference between rotations at the highest N fertilizer rate. The legume genotypes fixed between 14 and 51xa0kgxa0Nxa0ha−1 of their total N and had an estimated net N balance ranging from −29.8 to 9.5xa0kgxa0Nxa0ha−1. Positive N balance was obtained only when the nitrogen harvest index was greater than the proportion of N derived from atmosphere. The results also indicated that the magnitude of the fixed-N and other rotation effects varied widely and were influenced by the contributions of the grain legumes to the soil N-balance. In general, fixed-N effects ranged from 124 to 279xa0kgxa0ha−1 while rotation effects ranged between 193 and 513xa0kgxa0ha−1. On average, maize following legumes had higher grain yield of 1.2 and 1.3-fold compared with maize after fallow or maize after maize, respectively.

Genetic diversity of indigenous Bradyrhizobium nodulating promiscuous soybean [Glycine max (L) Merr.] varieties in Kenya: Impact of phosphorus and lime fertilization in two contrasting sites

While soybean is an exotic crop introduced in Kenya early last century, promiscuous (TGx) varieties which nodulate with indigenous rhizobia have only recently been introduced. Since farmers in Kenya generally cannot afford or access fertilizer or inoculants, the identification of effective indigenous Bradyrhizobium strains which nodulate promiscuous soybean could be useful in the development of inoculant strains. Genetic diversity and phylogeny of indigenous Bradyrhizobium strains nodulating seven introduced promiscuous soybean varieties grown in two different sites in Kenya was assayed using the Polymerase Chain Reaction-Restriction Fragment Length Polymorphism (PCR-RFLP) of the 16S-23S rDNA intergenic spacer region and 16S rRNA gene sequencing. PCR-RFLP analysis directly applied on 289 nodules using Msp I distinguished 18 intergenic spacer groups (IGS) I–XVIII. Predominant IGS groups were I, III, II, IV and VI which constituted 43.9%, 24.6%, 8.3% 7.6% and 6.9% respectively of all the analyzed nodules from the two sites while IGS group VII, IX, X, XI, XII, XIV, XVI, XVII, XVIII each constituted 1% or less. The IGS groups were specific to sites and treatments but not varieties. Phylogenetic analysis of the 16S rRNA gene sequences showed that all indigenous strains belong to the genus Bradyrhizobium. Bradyrhizobium elkanii, Bradyrhizobium spp and Bradyrhizobium japonicum related strains were the most predominant and accounted for 37.9%, 34.5%, and 20.7% respectively while B. yuanmigense related accounted for 6.9% of all strains identified in the two combined sites. The diversity identified in Bradyrhizobium populations in the two sites represent a valuable genetic resource that has potential utility for the selection of more competitive and effective strains to improve biological nitrogen fixation and thus increase soybean yields at low cost.

Impact of residue characteristics on phosphorus availability in West African moist savanna soils

The role of residue characteristics in enhancing the availability of P was investigated in a greenhouse study using two soils from the northern Guinea savanna (NGS) and four from the derived savanna (DS) zones of the West African moist savanna. Eight organic residues of varying C-to-P ratio were used and maize ( Zea mays) was grown for 7xa0weeks. The effect of the organic residues on P availability (measured as resin P and maize P accumulation) differed among the soils. On average, the increase in resin P, calculated as {[(soil+residue)−control]/(control)×100}, was between 8% (Davié, DS) and 355% (Danayamaka, NGS). Maize P accumulation was increased by ca. 11% in Davié and Niaouli (DS) soils and 600% in Danayamaka soil. The increase in maize total dry matter yield (DMY) ranged from 2% to 649%. Residues with C-to-P ratio >200 produced lower DMY than those with lower ratios. Residue organic P (Po) extractable with 0.2xa0N H2SO4 (acid-Po) accounted for 92% ( P =0.0001) of the variation in DMY in a step-wise regression with residue parameters as independent variables and mean DMY as the dependent variable. The residue Po extractable with 0.5xa0M NaHCO3 (HCO3-Po) correlated significantly with DMY in Danayamaka and Davié soils, and with P accumulation in Danayamaka soil. The relationships between the residue Po and DMY might imply that Po fractions in decomposing residues contribute to P availability. However, the suitability of using the Po content of organic residues to predict their agronomic value with respect to P nutrition needs further evaluation.

Inter and Intra-Specific Variation of Legumes and Mechanisms to Access and Adapt to Less Available Soil Phosphorus and Rock Phosphate

Legume integration in African agricultural production systems is among the options with demonstrated potential for sustainable land management. Best bet legumes for promotion in cropping systems should play multiple roles in soil fertility, food and fodder provision. The main constraint to growth and productivity of legumes i.e., phosphorus deficiency, is variably addressed through use of inorganic P fertilizers and slow reactive phosphate rock, genotype adaptation to low P soils and role of phosphate solubilizing micro-organisms (PSMs). Research findings using grain legumes show greater yields for rotated cereal and higher land equivalent ratios for intercropping than cereal monocropping systems across different environments. The current research priority is to enhance recovery of soil P by legumes to increase productivity of cropping systems in low-nutrient environments. A number of research priorities are outlined among them being the integration of physiological research with genetic improvement.

Plant age and rock phosphate effects on the organic resource quality of herbaceous legume residues and their N and P release dynamics

Many tropical agro-ecosystems rely on organic resources for nutrient provision. Various factors affect the nutrient release dynamics of organic resources, including application of external nutrients during their growth and their age or physiological stage at the time of harvest. Understanding relationships between organic resource management factors and the resulting nutrient release dynamics is important for optimizing nutrient recovery by crops and minimizing nutrient losses to the environment. Here, Mucuna pruriens and Lablab purpureus legumes were grown on a ‘slope’ and ‘plateau’ field, treated or not with rock phosphate, and sampled from 12 to 30 weeks after planting. These plant residues were then incubated in a sandy soil under laboratory conditions and mineral N and Olsen-extractable P dynamics were measured for 28 days. Our results showed that plant age had an impact on N, P and polyphenol contents. Application of rock phosphate only altered the P content of the legumes on the ‘plateau’ field, which had a lower soil-available P content. We also observed that mineral N dynamics of the organic resources varied between −62% of the added N released after 28 days, indicating net immobilization, and +23%, indicating net mineralization, with most organic resources showing an initial N immobilization phase. N release was negatively related to the lignin-to-N and (lignin+polyphenol)-to-N ratios of the organic resources. Lastly, we observed that the net release of Olsen P relative to the control soil varied between 0.4 and 4.9 mg kg−1 and that net P release was positively related to the P content of the organic resources. In summary, legume age was the parameter that most strongly influenced the quality of the legume residues, and consequently its N and P release dynamics, with potentially significant consequences for N and P uptake recovery and losses and, ultimately, cropping system sustainability.

Balanced Nutrient Management System Technologies In The Northern Guinea Savanna Of Nigeria: Validation And Perspective

Based on experimental evidence that combining mineral fertilizers with organic matter may address poor soil fertility status and result in added benefits, farmer-managed demonstration trials were initiated in 9 villages in the northern Guinea savanna (NGS) of northern Nigeria. The trials had four treatments: (i) a farmers control in which the farmer grows maize according to his usual practice, (ii) the maize technology being promoted by the NGO Sasakawa-Global2000 (SG2000), involving hybrid seeds, proper plant density and fertilizer application practice, and fertilizer application rates that are relatively high for the region (136 kg N, 20 kg P, and 37 kg K ha-1), (iii) the Balanced Nutrient Management Systems (BNMS) manure technology that follows the SG2000 package for maize, except that part of the fertilizer quantity is replaced by animal manure; and (iv) a soybean-maize rotation, again with reduced fertilizer rate to the maize.

The Resources-to-Consumption System: A Framework for Linking Soil Fertility Management Innovations to Market Opportunities

Recent paradigms in soil fertility management research have evolved from the initial reductionist approaches of nutrient replenishment to embrace a more holistic integrated soil fertility management (ISFM) approach that goes beyond soils to address the full chain of interactions, from resources to production systems, to markets and policies. It is now recognized that the adoption of ISFM technologies critically depends on market opportunities. It is argued that without well functioning markets, the adoption of ISFM innovations will remain limited. This paper examines this “market-led hypothesis” that linking farmers to better market opportunities will provide incentives for adoption and re-investment in ISFM innovations. This hypothesis is tested with empirical data from cross-sectional household surveys and action research on linking farmers to markets in selected sites in Malawi, Uganda and Tanzania. Analysis revealed mixed results, with significant differences based on gender, wealth categories, crops and areas. On one hand, there is evidence that better access to markets and increased income led to positive investments in agricultural inputs, increased fertilizer use and soil conservation measures. On the other hand, for the majority of women and poor farmers, particularly in Uganda, re-investing in ISFM was not even among the first three priorities, compared to other livelihood needs (buying or renting more farmland, livestock, paying school fees and buying clothes). The paper outlines a novel approach for demand-driven and market-led ISFM research for development. This approach termed the Resource-to-Consumption offers a practical framework to link ISFM research to market opportunities identification in a way that empowers farmers to better manage their resources and offers them incentives to invest in soil fertility improvement. The success of this approach is highly dependent on the development of effective quality partnerships with research and extensions systems, government and non-governmental organizations, business support services, farmer communities and the private sector; and building multi-institutional and trans-disciplinary research for development teams, with complementary skills and expertise. The paper suggests four key areas that need concerted efforts by a variety of stakeholders (i) improving output and input market access; (ii) participatory technology development; (iii) strengthening farmers’ institutions, and (iv) influencing policy change

Innovations as key to the green revolution in Africa: exploring the scientific facts

Africa remains the only continent that has not been able to fully benefit from the effects of the Green Revolution. The first Green Revolution failed in Africa because it did not take account social and ecological variability, culture, institutional bottlenecks and a host of other issues. This Revolution failed to recognize that although agricultural technology is an important factor, it is only one aspect of a complex socio-economic-ecological system. Various local, regional and international forums have been held to discuss how Africa’s Green Revolution can be achieved. The African Heads of State and governments have developed the Comprehensive African Agricultural Development Program (CAADP) as a framework for agricultural growth, food security, and rural development. CAADP has set a goal of 6% annual growth rate in agricultural production to reach the UN’s Millennium Development Goal of halving poverty and hunger by 2015. The African Heads of State Fertilizer Summit held in Abuja Nigeria in June 2006 led to the Abuja Declaration on Fertilizer for the African Green Revolution. The Summit identified three most critical issues that need to be addressed if millions of African farmers are to increase utilization of fertilizer. These are access, affordability and the use of incentives. The Summit recognized that given the strategic importance of fertilizers in achieving the African Green Revolution, there is need to increase the level of use of fertilizer from the current average of 8 kg ha-1 to an average of at least 50 kg ha-1 by 2015. Similar sentiments were echoed at the African Green Revolution Conference in Oslo where it was resolved to take concrete and concerted action towards the development of self-sustaining changes in African agricultural growth through the use of enhanced approaches to public–private partnerships. Achievement of the desired growth in agricultural production calls for deliberate effort to increase access and affordability of inorganic fertilizers, seed, pesticides and profitable soil, water, and nutrient management technologies by the smallholder farmers in Africa. For many years now, agricultural research has generated numerous technologies that can launch the African Green Revolution but these have not been widely adopted and benefited the smallholder farmers in the continent. There is need for a shift in paradigm from the linear model of research-to-development to the systems perspective. This calls for agricultural innovation, which is the application of new and existing scientific and technological (S&T) knowledge to achieve the desired growth in agricultural production and overall economic development in Africa. N. Sanginga (&) Tropical Soil Biology and Fertility (TSBF), Institute of International Centre for Tropical Agriculture (CIAT), Croydon, UK e-mail: N.Sanginga@cgiar.org

Soybean Varieties, Developed in Lowland West Africa, Retain Their Promiscuity and Dual-Purpose Nature Under Highland Conditions in Western Kenya

Entry points that give farmers immediate benefits are required to reverse the ever-declining soil fertility status of a substantial area in sub-Saharan Africa. In West Africa, dual-purpose, promiscuous soybeans (Glycine max (L.) Merr.) that produce a substantial amount of grains and leafy biomass and do not require inoculation with specific Rhizobium spp. strains were developed and have increased resilience of farming while providing income to farmers. These crops could be a potential entry point for soil fertility improvement in western Kenya, provided they retain their promiscuity and dual-purpose character in this new environment. The major objective of this work was to quantify nodulation, biomass production and grain yield characteristics of a set of best-bet, dual-purpose varieties relative to a locally available variety at two sites (Vihiga and Siaya districts) in western Kenya. In the presence of P, most promiscuous soybean varieties showed substantial improvements in nodulation (19–165 nodules per 0.5 m of soybean) than did the local variety (3–13 nodules per 0.5 m of soybean). While grain yield was for all but one variety as good as the local control (845 kg ha–1, on average), nearly half of the varieties produced significantly higher amounts of biomass at 50% podding than did the local variety (865 kg ha–1 in Siaya and 1877 kg ha–1 in Vihiga). Increases in nodulation, biomass production and grain yield were mainly observed after application of P fertilizer; in the absence of P, almost none of the varieties performed better than the local control for any of the measured characteristics. To fully exploit the potential soil fertility-improving characteristics of these varieties, it will be necessary to facilitate availability of P fertilizer and to foster demand for soybean at the farm, community and national levels.