11 Table 2: Fertilizer use and fertilizer use expansion do not pay on about half of the maize plots in the cereal‐ root cropping system in Nigeria.

|Maize plots (%) for which net benefit from fertilizer use is positive for a risk neutral farmer (AVCR>=1)<br><br> <br>|
|---|
|_Year_<br>_Full acquisition cost_<br>_Fertilizer available in the village_|
|2010<br>51<br>86|
|2012<br>56<br>88|
|Maize plots (%) for which expanding fertilizer use is profit increasing for a risk neutral farmer (MVCR>=1)|
|_Year_<br>_Full acquisition cost_<br>_Fertilizer available in the village_|
|2010<br>49<br>70|
|2012<br>53<br>86|

Source: Liverpool‐Tasie et al., 2017.

But the limited profitability of fertilizer use in the Nigerian sample is also due to poor marginal yield responses. At an average of about 7.7 kg additional maize per additional kg nitrogen, these are well below the marginal physical products observed in other studies (ranging between 10 and 20 in Kenya) or when research management protocols are followed (rising to 50 in Malawi) (Marenya and Barrett, 2009; Matsumoto and Yamano, 2011; Sheahan, Black and Jayne, 2013; Snapp et al, 2014). In this context, Marenya and Barrett (2009) also point to the importance of good quality soils for inorganic fertilizer to be effective. The efficiency of inorganic fertilizer is for example low on soil with low organic matter content which is needed to prevent run‐off and leaching and for gradual nutrient release. Efficient absorption of nutrients is similarly impeded when soil is too acidic. Both are common problems of African soils (Barrett et al., 2017).

While the relatively high inorganic fertilizer application rates observed in Nigeria are exceptional across the LSMS‐ISA countries (Sheahan and Barrett, 2017), the findings by Liverpool‐Tasie et al (2017), which, in Nigeria, are also confirmed for rice (Liverpool‐Tasie, 2016), underscore the need to better understand the agro‐ecological and market conditions under which inorganic fertilizer use in particular, and other agricultural technologies in general, are profitable. Also, in the absence of adequate ex post coping mechanisms, still higher returns will be needed for fertilizer (and other modern) input use to be profitable or optimal (Dercon and Christiaensen, 2011). The results reported here have abstracted from risk considerations. They also underscore the importance of integrated interventions (access to input use, extension, and markets).

_Women do not provide the bulk of labor in African agriculture._ There is also a gender dimension to low modern input use, with application rates typically lower among female headed households and on female managed plots (Sheahan and Barrett, 2017). This explains an important share of the 20‐25 percent gender gap in agricultural productivity (O’Sullivan et al., 2014). Combined with the widely accepted/quoted notion that women provide the bulk of the labor in agriculture in Africa, regularly quoted to be 60‐80 percent, this has been taken to suggest that closing the gender productivity gap could go a long way in boosting Africa’s food supply. But the basis for this statistic on women’s labor share in Africa’s agriculture is basically unknown and has been questioned (Doss, et al., 2011).