Abstract
Soil fertility in smallholder farming areas is known to vary strongly on multiple scales. This study measures the sensitivity of the recorded satellite signal to on-farm soil fertility treatments applied to five crop types, and quantifies this fertilization effect with respect to within-field variation, between-field variation and field position in the catena. Plant growth was assessed in 5–6 plots per field in 48 fields located in the Sudano-Sahelian agro-ecological zone of southeastern Mali. A unique series of Very High Resolution (VHR) satellite and Unmanned Aerial Vehicle (UAV) images were used to calculate the Normalized Difference Vegetation Index (NDVI). In this experiment, for half of the fields at least 50% of the NDVI variance within a field was due to fertilization. Moreover, the sensitivity of NDVI to fertilizer application was crop-dependent and varied through the season, with optima at the end of August for peanut and cotton and early October for sorghum and maize. The influence of fertilizer on NDVI was comparatively small at the landscape scale (up to 35% of total variation), relative to the influence of other components of variation such as field management and catena position. The NDVI response could only partially be benchmarked against a fertilization reference within the field. We conclude that comparisons of the spatial and temporal responses of NDVI, with respect to fertilization and crop management, requires a stratification of soil catena-related crop growth conditions at the landscape scale.
Highlights
The demographic explosion of sub-Saharan Africa presents significant challenges for food production, which cannot be achieved without agricultural intensification [1]
Variograms were developed using the “gstat” add-on package available in the given the non-linear plant response to fertilizer application, we identify the dose increment leading to the strongest reaction through a 2-by-2 comparison of Normalized Difference Vegetation Index (NDVI) means for each available dose increment
Crops clearly responded to fertilization, in terms of both crop height and ground coverage impacting the NDVI that is measured by a satellite sensor
Summary
The demographic explosion of sub-Saharan Africa presents significant challenges for food production, which cannot be achieved without agricultural intensification [1]. Smallholder systems are characterized by enormous yield gaps [2], which may be reduced through integrated soil fertility management, improved crop-livestock integration, multi-purpose crops, and other sustainable intensification practices [3] These options will, be implemented in an evolving socio-economic and climatic context where multiple drivers of change may increase exposure to risk, alter patterns of vulnerability, and open opportunities for agricultural development, especially through changes in urban demand, market access, IT penetration, and social differentiation [4]. Monitoring changes in productivity across scales is a daunting challenge in heterogeneous smallholder systems, for example in Mali’s cotton belt In this area, with an increasing land shortage and reduced options for fallows [7], farmers rotate crops and apply manure, crop residues, household waste and compost to manage soil fertility in increasingly fragmented fields (currently 1.4 ha on average). This seemingly clear-cut relationship of soil characteristics with landscape position is obscured by farm management, the complex superposition of farm-centered management rings [13], by unequal farmer endowment and resource use [14], and by several other abiotic and biotic factors of random variability [15]
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