Conclusions
Many areas of smallholder cultivation in sub-Saharan Africa have been
systematically experiencing reductions in soil organic carbon levels,
particularly where continuous maize cultivation without legume rotations
and/or application of inorganic or organic fertilizer is common
(Shepherd & Soule, 1998; Drechsel et al., 2001; Tittonell et al., 2005;
Lal 2006; Marenya & Barrett 2009). Low soil organic carbon stocks are
known to affect plant uptake of nutrient supplies and figure into
low-responsiveness of yields to nitrogen supply in some areas. Farmers
in such areas are much less likely to find fertilizer investments to be
economically attractive, regardless of how well fertilizer markets are
performing. Because of its role in promoting greater crop biomass
(including roots), inorganic fertilizer is usually a crucial component
of a sustainable agricultural intensification strategy, and its use will
need to rise rapidly to arrest the land degradation processes already
evident in areas where the land frontier has been reached and where
continuous cultivation has become the norm.
This paper has assembled evidence on the ways in which maize yield
responses to inorganic fertilizer are affected by soil organic carbon
and other factors. We have shown that the marginal product of nitrogen
is positively associated with both soil organic carbon stocks (measured
as active carbon), as well as by seasonal rainfall. When farmgate prices
for maize and fertilizer are incorporated into calculations of average
and marginal cost-benefit ratios, we find that economic returns – as
measured by MVCR and AVCR – drop drastically over price-ratio ranges
that are representative of those likely faced by many smallholders in
Tanzania. For example, assuming a price ratio of 0.15, only 22% to
farmers could utilize fertilizer profitably (using AVCR>2).
However, even under more favorable assumptions, the high year-to-year
variability around these expected returns would discourage many farmers
from investing in fertilizer even if it were profitable to do so on
average across all years. Our results indicate that the sensitivity of
fertilizer profitability to such outcome uncertainty will be to be
particularly acute in areas of depleted organic matter.
An important implication of our analysis is the importance of
investments and policy interventions which address the systematic
depletion of soil organic carbon stocks. This study adds to the growing
evidence that cereal crop response to fertilizer requires that attention
to be paid to soil health, as indicated by active soil carbon. There
have been previous calls by soil scientists and economists for
integrated soil fertility management to complement inorganic fertilizer,
yet few previous studies have examined this at scale. Our research,
conducted across a broad range of soil types and market contexts across
Tanzania, reasserts the urgency of this proposition to better inform
discussions of how to stimulate fertilizer investments by African
smallholders. Farmer incentives to make such investments will be
promoted by efforts to raise the agronomic efficiency of fertilizer
sufficiently for fertilizer to be profitable.