By Moritz Laub, Marc Corbeels, Samuel Mathu Ndungu, Monicah Wanjiku Mucheru-Muna, Daniel Mugendi, Rebecca Yegon, Wycliffe Waswa, Bernard Vanlauwe, and Johan Six
Maize productivity in sub-Saharan Africa often falls below its potential due to soil fertility challenges. Researchers assessed the potential to counteract soil organic carbon (SOC) losses and yield declines using different organic resource treatments, with and without mineral nitrogen (N) fertilizer. While all treatments that received mineral N had similar yields in the first year, the application of farmyard manure (FYM) combined with mineral N was the only treatment that did not lose yields over the two decades, eventually outperforming all others. The results suggest that combining FYM with mineral N is the most effective of all tested strategies for simultaneously sustaining maize yields and SOC levels in similar tropical soils.
Maize is the main staple crop in sub-Saharan Africa. Yet, due to persistent soil fertility challenges and nutrient deficiencies, its productivity remains significantly below its potential in many countries. For example, current average maize yields in Kenya are around 1.65 t ha-1. (World-Bank, 2021), starkly contrasting with the potential yields of up to 10 t ha-1. achievable under optimal conditions (Ittersum et al., 2016). To bridge this yield gap, effective nutrient and soil management practices are essential.
Integrated Soil Fertility Management (ISFM) has emerged as a promising strategy to enhance maize productivity by combining the use of mineral fertilizers, organic resources, and improved crop varieties adapted to local conditions (Vanlauwe et al., 2010). ISFM addresses both the short-term nutrient needs of crops and the long-term maintenance of soil health. It aims to leverage the synergistic effects of mineral and organic inputs to optimize nutrient use efficiency and mitigate environmental impact (Vanlauwe et al., 2010).
Recent research has highlighted the potential of ISFM to improve maize yields and soil health, although results can vary based on local conditions. For example, while short-term studies in central Kenya have demonstrated significant yield increases with ISFM practices (Chivenge et al., 2009; Mutuku et al., 2020), long-term experiments have yielded mixed outcomes. In Burkina Faso, ISFM led to increased sorghum yields over 50 years (Adams et al., 2020), whereas similar practices in Nigeria resulted in declining maize yields over time (Vanlauwe et al., 2005). These discrepancies emphasize the need for context-specific recommendations based on soil and climatic conditions.
Here, the authors summarize the results of two recent studies (Laub et al., 2023b, 2023a) that analyzed the impact of different qualities of organic resources combined with or without mineral N fertilizers on the maintenance of maize yields and SOC as an indicator of soil health.
Study description and methodology
The four long-term field experiments were all conducted in Kenya. The specific sites were strategically chosen to represent a range of agroecological conditions for maize cultivation in both central and western Kenya (Fig. 1). Each site offered distinct environmental and soil characteristics that influence maize productivity. In central Kenya, the Embu site is at an altitude of approximately 1,400 meters, has a mean annual temperature of 20.1 C, and receives 1,175 mm of rainfall annually. The soil at Embu is classified as a Humic Nitisol with a high clay content of 60%. The Machanga site is characterized by a drier climate with only 795 mm of annual rainfall and a mean annual temperature of 23.7 C. The soil at Machanga is a Ferric Alisol and has a coarse texture with less than 15% clay, which impacts its capacity to hold moisture and nutrients. In western Kenya, Sidada experiences the highest annual rainfall of 1,730 mm and a mean annual temperature of 22.6 C. The soil here is a Humic Ferralsol with a clay content of 56%, contributing to its high fertility and water retention capacity. Also in western Kenya, Aludeka receives 1,660 mm of rainfall annually and has a mean annual temperature of 24.4 C. The soil at Aludeka is a Haplic Acrisol with less than 15% clay and thus low nutrient retention. Each site supports two maize growing seasons per year due to a bimodal rainfall pattern: the long rainy season from March to August/September and the short rainy season from September/October to January/February.
Experimental Design and Treatments
The experiments used a split-plot design with three replicates per site. Plot sizes were approximately 12 m x 6 m (subplots 6 m x 6 m). The main treatments involved applying different organic resources at two rates: 1.2 and 4.0 t C ha-1 yr-1. Here, the authors only focus on the lower rate of 1.2 t C ha-1 yr-1, which is considered the more feasible rate for most farmers. Subplot treatments included the addition of either 120 kg mineral N ha-. season-. (+N treatment) or no additional mineral N (-N treatment). To ensure that phosphorus (P) and potassium (K) were not limiting, all plots received a blanket 60 kg P ha-1 and 60 kg K ha-1 each season. The organic resources used in the experiments were selected based on their quality and turnover rates, as outlined by Palm et al. (2001). Tithonia diversifolia is known for its rapid decomposition and high nutrient release. Calliandra calothyrsus is characterized by its slower decomposition rate and high nutrient content. Maize stover, with a rapid turnover but lower nutrient content. Sawdust from Grevillea robusta, which decomposes slowly and has a very low nutrient content. Farmyard manure, which has a high nutrient content that releases steadily over a long period of slower decomposition. A control treatment with no addition of organic resources was also present (Fig. 2). The organic resources were applied annually before planting during the long rainy season and incorporated into the soil to a depth of 15 cm by hand hoe. The mineral N fertilizer was applied in two splits: 40 kg N ha-1 at planting and 80 kg N ha-1 about six weeks later. At harvest, maize yield was sampled in a representative fraction of each plot; all maize residues were removed to ensure that only the applied organic resources contributed to SOC dynamics. The soil sampling was carried out every few years, with the latest campaign in 2021. Samples were collected from the top 15 cm. The air-dried samples were analyzed for SOC by an elemental analyzer. Researchers used mixed linear effect models, to estimate the trends of yields and SOC across the four sites, adhering to the split-plot design by nested random effects, and treating sites as random effects as well (Laub et al., 2023a, 2023b).
Results
Though our results revealed a significant site-specificity of yields and trends in yields over time in response to the treatments (see Laub et al., 2023b), some general trends across sites were evident (Fig. 3).
While initially, the yields of all treatments receiving mineral N were similar, the FYM treatment with mineral N application was the only treatment that maintained yield over time, leading to significantly higher yields than all other treatments after two decades. In the treatments without mineral N, FYM was the only treatment that out-yielded the control treatment with no inputs from the beginning to the end of two decades of experimentation, whereas Tithonia and Calliandra treatments without mineral N application only out-yielded the control without mineral N application at the end of the two decades. Both maize stover and sawdust did not provide any significant benefit over the control treatment, neither with nor without the application of mineral N. It is noteworthy that yields differed significantly between the four sites. For example, the initial yields of the control and the different organic resource treatments that received mineral N were between 3.7 and 4.1 t ha-. season-. at Aludeka, 2.7 and 3.4 t ha-1 season-1 at Embu, 1.8 and 2.3 t ha-1 season-1 at Machanga, and 4.1 and 4.8 t ha-1 season-1 at Sidada. However, the general superiority of FYM with mineral N addition was present at all sites, and further site-specific details can be obtained from Laub et al. (2023b).
With the lower, but more realistic, rate of 1.2 t C ha-1 yr-1 of the organic resources added, SOC losses were generally unavoidable at all sites (Fig. 4) with the only exception being the Calliandra treatment at Aludeka (see Laub et al. 2023a). However, FYM was the organic soil amendment that best reduced SOC losses across sites, with the only other treatment that would lead to significantly lower SOC losses than the control being the Calliandra treatment (Fig. 4). Also here, there were some site specific differences, for example, the reduction of SOC losses was more pronounced in the clay-rich sites, Embu and Sidada, compared to the others (for more details on the site-specificity see Laub et al. 2023a). While not presented here, due to the unrealistically high rates for most farmers, the advantage of FYM was more visible in the treatments that added 4 t C ha-1 yr-1, where the FYM treatment consistently had the lowest SOC losses of all organic resource treatments at all sites, even leading to gains at Aludeka (Laub et al., 2023a). In contrast to the yields, the addition of mineral N had generally no effect on the temporal evolution of SOC across the sites.
Summary
Our results showed clearly that the combination of mineral N addition and FYM was the best treatment to maintain or even increase maize yields over time, significantly outperforming the treatments that either added only mineral N or only FYM in the second decade of the experiment. Calliandra and Tithonia addition also showed a tendency of positive effects on yield maintenance, but it was weaker than that of FYM and only significantly higher than the control after two decades and in the treatments without mineral N.
In addition, FYM was also the treatment most efficient in reducing SOC losses, with Calliandra being the only other treatment that, compared to the control, also reduced losses at the low rate of 1.2 t C ha-1 yr-1. The loss trajectory of SOC at all sites was likely an effect of rather high initial SOC contents and because none of the sites had been under cultivation for longer time prior to experiment establishment.
Overall, one should note that the input rates of mineral N of the sites were rather high, and have been reduced in a recent redesign of the experiments (Laub et al., 2024). Nonetheless, the responsiveness of yields to treatments solely adding organic resources at several sites suggests that positive effects should also be observable at lower rates of mineral N. If anything, the efficiency of applied mineral N is usually higher at low rates of N input (Vanlauwe et al., 2011).
In conclusion, the study suggests that moderate inputs of farmyard manure combined with the addition of mineral N are the best solution to maintain or increase maize yields and to maintain soil health as much as possible.
Dr. Laub (e-mail: moritz.laub@usys.ethz.ch) is with the Department of Environmental Systems Science, ETH Zurich (Swiss Federal Institute of Technology), Zürich, Switzerland. Dr. Corbeels is with CIRAD, Montpellier, France and the International Institute of Tropical Agriculture (IITA), Nairobi, Kenya. Dr. Mathu Ndungu is with IITA, Nairobi, Kenya. Dr. Wanjiku Mucheru-Muna is with the Department of Environmental Sciences and Education, Kenyatta University, Nairobi, Kenya. Dr. Mugendi and Dr. Yegon are with the Department of Water and Agricultural Resource Management, University of Embu, Embu, Kenya. Dr. Waswa and Dr. Vanlauwe are with IITA, Nairobi, Kenya. Dr. Six is with the Department of Environmental Systems Science, ETH Zurich (Swiss Federal Institute of Technology), Zürich, Switzerland.
Cite this article
Laub, M., Corbeels, M., Ndungu, S.M., Mucheru-Muna, M.W., Mugendi, D., Yegon, R., Waswa, W., Vanlauwe, B., Six, J. 2024. Combining Farmyard Manure with Mineral Nitrogen Fertilizer Boosts Maize Yields and Maintains Soil Health: Insights from Four Long-Term Experiments in Kenya, Growing Africa 3(1), 16-20. https://doi.org/10.55693/ga31.ZUZS1184
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