Managing Soil Fertility and Nutrient Cycles Through Fertilizer Trees in Southern Africa

19 Managing Soil Fertility and Nutrient Cycles through Fertilizer Trees in Southern Africa Paramu L. Mafongoya, Elias Kuntashula, and Gudeta Sileshi Q1 World Agroforestry Centre (ICRAF), Zambia CONTENTS 19.1 Fertilizer Trees and a Typology of Fallows ................................................................ 274 19.1.1 Use of Non-coppicing Fertilizer Trees.......................................................... 274 19.1.2 Use of Coppicing Fertilizer Trees.................................................................. 275 19.1.3 Mixed-Species Fallows .................................................................................... 276 19.1.4 Biomass Transfer Using Fertilizer-Tree Biomass......................................... 276 19.2 Mechanisms for Improved Soil Fertility and Health................................................ 279 19.2.1 Biomass Quantity and Quality ...................................................................... 279 19.2.2 Biological Nitrogen Fixation and N Cycles ................................................. 279 19.2.3 Deep Capture of Soil Nutrients ..................................................................... 280 19.2.4 Soil Acidity and Phosphorus ......................................................................... 280 19.2.5 Soil Physical Properties................................................................................... 281 19.3 Effects on Soil Biota........................................................................................................ 282 19.4 Sustainability of Fertilizer Tree-Based Land Use Systems ...................................... 285 19.5 Discussion........................................................................................................................ 286 Acknowledgments ..................................................................................................................... 287 References ................................................................................................................................... 287 Low soil fertility is increasingly recognized as a fundamental biophysical cause for declining food security among small-farm households in sub-Saharan Africa (SSA) (Sanchez et al., 1997). Because maize is the staple food crop in most of southern Africa, it will be our focus in this chapter. In 1993, SSA produced 26 million metric tons of maize on approximately 20 m ha; approximately 54 million metric tons is expected to be needed by 2020. Meeting this maize production goal will depend on sustaining and improving soil fertility levels that have been declining in recent years. Soil fertility is not the only significant constraint; lack of appropriate, high-quality germplasm, unsupportive policies, and inadequate rural infrastructure also limit maize production. However, protecting and enhancing soil fertility is the most basic requirement for achieving production goals. As discussed in Chapters 40 and 41, even controlling the parasitic weed Striga hinges on this fundamental factor. In most cases, nitrogen is the main nutrient that limits maize productivity, with phosphorus and potassium being occasional constraints. Although inorganic fertilizers 273 274 Biological Approaches to Sustainable Soil Systems are used throughout the region, the amounts applied are seldom sufficient to meet crop demands due to their high costs and uncertain availability. Most countries in southern Africa have formulated fertilizer recommendations for all their major crops, sometimes with regionally specific adaptations. However, the amount of fertilizer used in southern Africa is very small in comparison to other parts of the world. For most smallholders, fertilizer use averages as low as 5 kg ha21 year21 (Gerner and Harris, 1993). While the need for increasing the availability of soil nutrients in southern Africa is quite apparent, increasing their supply is very challenging. A high-external-input strategy cannot rely on standard fertilizer-seeds-credit packages without addressing other requirements for successful uptake of Green Revolution technologies, including reliable irrigation, credit systems, infrastructure, fertilizer manufacture and supply, and access to markets. Most African conditions differ starkly from those in the prime agricultural regions of Asia. Approaches that produced successes in Asia are not readily transferable to the African continent. Considering the acute poverty and the limited access to mineral fertilizers in SSA, therefore, an ecologically robust approach of promoting “fertilizer trees” is discussed here. This is a product of many years of agroforestry research and develop- ment by the International Center for Research on Agroforestry (ICRAF), now called the World Agroforestry Center, working with various partners in eastern and southern Africa. 19.1 Fertilizer Trees and a Typology of Fallows Improved fallows involve the deliberate planting of fast-growing species, usually woody tree legumes, referred to here as fertilizer trees, for the rapid replenishment of soil fertility. Improved fallows were not a major area for research during the Green Revolu- tion due to its focus on eliminating soil constraints by use of mineral fertilizers. Biological approaches to soil fertility improvement began to receive attention in connection with the articulation of a second soil-fertility paradigm based on adaptability and sustainability considerations (Sanchez, 1994). Research on fertilizer trees had begun increasing from the mid-1980s, so by the mid-1990s they had growing justification in research (e.g., Kwesiga and Coe, 1994; Drechsel et al., 1996; Rao et al., 1998; Snapp et al., 1998). Large-scale adoption of fertilizer trees by farmers is now taking place across southern and eastern Africa. A more general consideration of fallows is presented in Chapter 29. 19.1.1 Use of Non-coppicing Fertilizer Trees Non-coppicing species do not resprout and regrow when cut at the end of the fallow period, typically after 2 years of growth. Non-coppicing species include Sesbania sesban, Tephrosia vogelii, Tephrosia candida, Cajanus cajan, and Crotalaria spp. Since the work of Kwesiga and Coe (1994) on Sesbania fallows, much has been learned about the performance of improved fallows using tree species that do not coppice. There has been extensive testing of various species and fallow length on-farm to determine their impact on maize productivity and to assess the processes that influence fallow performance. The performance of Sesbania and Tephrosia under a wide range of biophysical conditions is shown in Table 19.1. Trials at Msekera Research Station, Zambia, have shown that natural regeneration of Sesbania fallows is possible through self-reseeding, but it is highly erratic. Improved fallows of 2-year duration using either Tephrosia or Sesbania significantly increased maize yields well above those of unfertilized maize, the most common farmer practice in the region. While it was true that fertilized maize usually performed better than improved Managing Soil Fertility and Nutrient Cycles through Fertilizer Trees in Southern Africa 275 fallows in most cases, this required a greater cash outlay, so improved fallows could be more profitable. The problem demonstrated in these trials was that the residual effects of these improved fallows on maize yield declined after the second year of cropping (Table 19.1). In a third year of cropping, maize yields following fallow were similar to those of unfertilized maize. The marked decline of maize yields two or three seasons after a non-coppicing fallow is probably related to depletion of soil nutrients and/or to deterioration in soil chemical and physical properties. 19.1.2 Use of Coppicing Fertilizer Trees Coppicing species include Gliricidia sepium, Leucaena leucocephala, Calliandra calothyrsus, Senna siamea, and Flemingia macrophylla. Fallowing with a coppicing species, in contrast to a non-coppicing species, shows increases in residual soil fertility beyond 2–3 years because of the additional organic inputs that are derived each year from coppice regrowth that is cut and applied to the soil. An experiment was established in the early 1990s at Msekera Research Station to examine these relationships. These plots have now been cropped for 9 years during which time both maize yields and coppice growth were monitored. The species evaluated showed significant differences in their coppicing ability and biomass production, with Leucaena, Gliricidia, and Senna siamea having the greatest coppicing ability and biomass production, while Calliandra and Flemingia performed poorly. The trends in maize yields have been tracked over nine seasons. In the plots with Sesbania fallow, while maize yields were high for the first three seasons, they then declined to the same level as control plots. Flemingia and Calliandra showed low maize yields over all years. There were no significant differences in maize grain between Gliricidia and Leucaena fallows over the seasons. The effects of different fallow species on maize yield can be explained partly by the different amounts of biomass added and the quality of the biomass and coppice regrowth. Species such as Leucaena and Gliricidia, which have good coppicing ability, produce large amounts of high-quality biomass with high nitrogen content and low contents of lignin and polyphenols, thereby contributing to higher maize yields (Mafongoya and Nair, 1997; Mafongoya et al., 1998). While Sesbania produces high quality biomass, its lack of coppice

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