upwards of an additional 236 million hectares of agro-climatically suitable wetlands is available in Africa. Additionally, they argue that the expansion of rice production need not compete with other food crops, since much of the low-lying wetlands suitable for rice during rainy seasons is inhospitable for other crops.

Still, converting new lands to agriculture seems an unlikely path to expanding crop production. On average, the share of agriculture cropped in Africa stands at 22 percent, leaving large tracts of meadow and pastureland available to convert to crops with fewer up-front investments by farmers.<sup>1</sup> In addition, there are also important advantages to spending public resources on improving the productivity of existing farms rather than investing infrastructure or incentives to carving out new farms. For example, Deininger and Byerlee (2012) estimate that quadrupling maize production through area expansion alone would require 90 million new hectares. Alternatively, closing average maize yield gaps in Sub-Saharan Africa from 80 to 20 percent would bring about an equivalent increase on existing cropped land, leaving land and water sources to sustain natural ecosystems and the services they provide, including carbon sequestration (Satterthwaite et al., 2010; Godfray et al. 2010a, 2010b). Further, closing the yield gap by improving soil nutrient management could halt and possibly reverse the problem of declining soil fertility in Africa (Deugd, Röling, and Smaling, 1998; Place et al., 2003; Zerfu and Larson, 2010).

Even so, while there is widespread agreement that boosting productivity on current lands is the preferred approach to expanding food production, there is less agreement on how to close yield gaps, and even disagreement on the relevance of calculated yield gaps.

First, potential yields are usually based on selected genetic material combined with inputs, especially chemical fertilizer that must be purchased (Lobell et al, 2009). Tautologically, fully closing the gap means inducing farmers to take up the technologies implicit in calculating the gap. As discussed later, seedfertilizer technologies work well for some crops in some parts of Sub-Saharan Africa. In other places, the technologies are simply not adopted. This has led some to question the relevance of conventional yield gap measures and propose alternative measures. For example, Tittonell and Giller (2013) suggest calculating productivity potential based on ecologically intensive farming method. These methods, which depend on ecological processes to minimize eternal purchased inputs, can be indicative where fertilizer and other inputs are expensive relative to output value. Tittonell and Giller (2013) note that common elements of ecological intensification, such as integrated cropping and livestock practices, are already present in many African farming systems; however, they also warn that ecological intensive systems will not suite the needs of all farmers.<sup>2</sup> More generally, calculated yield gaps, regardless of methodology, ultimately depend on a technology choice that may or may not be relevant for place-specific livelihood strategies. For this reason, yield gaps are useful as an indicator of prevailing technology choices, relative to a practical standard, but provide less insight about how yield gaps come about or how they might be closed. In Section 7, we return to the topic and discuss practical aspects of technology choices and the role of policy.

> 1 In contrast, 87 percent of agricultural land in Southeast Asia is cropped (FAOSTAT, 2015).

> 2 Marenya and Barret (2006) make the same point, arguing that poor smallholders in western Kenya are unable to afford integrated soil fertility management techniques.

6