## 1 Introduction  
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, however, 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].  
The huge variability in farming and cropping practices [5] and returns on investments already observed today [6] is therefore likely to exacerbate in the future. Higher dependency on information may entail additional risk for smallholders and other agricultural stakeholders, notably in the presence of information asymmetries.  
Monitoring changes in productivity across scales is a daunting challenge indeed 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). Inorganic NPK fertilizer is commonly applied to cotton and maize in moderate quantities, but overall low organic matter content and nutrient deficiencies prevail. Catenary variation in soil properties is well known to farmers [8], and hence determines in part land management practices and observed spatial cropping patterns across the landscape. Field position on the topo-sequence influences soil fertility management [9], crop types and rotations [10], agro-biodiversity management [11] and agro-forestry practices [12]. However, 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].  
Fortunately, modern information technologies including remote sensing can help to monitor crop performance at levels of granularity increasingly compatible with smallholder farming [16]. This may open practical support applications for precision agriculture, allowing the exploitation, rather than the mitigation, of spatial heterogeneity, and the demonstration that increased productivity and enhanced livelihoods are not necessarily antithetical with complex cropping systems, and may actually thrive on the latter.  
This study focuses on the analysis of remote sensing's potential to retrieve crop status information at sub-field scales in heterogeneous smallholder rainfed production systems. The objective of the paper is to measure the sensitivity of the NDVI signal to on-farm fertility treatments applied to five locally important crops: cotton (_Gossypium_ sp.), pearl millet (_Pennisetum glaucum_ (L) R. [PERSON]), sorghum (_Sorghum bicolor_ (L.[PERSON]), maize (_Zea mays_ L. ssp.), and peanut (_Arachis hypogaea_ L.). These five crops represent over 84% of the harvested area in Mali's old cotton basin [17]. NDVI sensitivity to fertilizer applications was first analyzed at the field scale. For each crop, the amplitude and period of strongest plant response to fertilization is discussed. The spatial variance components were quantified with respect to the effects of fertilization and to within and between field variability associated with management, soil characteristics and catena position in the landscape.