Agriculture Reference
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contrast with average yields of 3 Mg/ha in the developing world as a whole. Many
economic and technological factors have been cited to explain these low crop yields,
including the fact that 80% of the total farm area in SSA is made up of the 33 mil-
lion farms of less than 2 ha. Whereas smallholder farms also dominate agriculture
in most areas of Asia, the high yields there were built on intensification utilizing
the Green Revolution packages of irrigation, modern crop varieties, and external
inputs. Apart from the lack of irrigation in SSA, the most obvious technological fac-
tor contributing directly to the low yields is the inadequate use of external inputs.
The Food and Agriculture Organization (FAO 2009) estimates that fertilizer use
on arable land in SSA in 2002 amounted to only 13 kg/ha compared with 190 kg/
ha in Asia. Continuing low yields will not meet the demand for food as the popula-
tion increases from 770 million in 2005 to between 1.5 and 2 billion in 2050. The
consequent intensification through continuous cropping to produce more food will
exacerbate land degradation, which is already widespread (Vlek et al. 2010).
Human impacts on nutrient cycling and budgets vary widely across the globe. Vlek
et al. (1997) estimate that 230 Tg of plant nutrients are removed yearly from agri-
cultural soils, whereas global fertilizer consumption of N, P
2
O
5
, and K
2
O is 130 Tg.
(This chapter utilizes the International System of Units as follows: Mg = 1000 kg
[1 metric ton]; Gg = 1,000,000 kg [1000 t]; Tg = 1,000,000,000 kg [1 million t]; Pg =
1,000,000,000,000 kg [1 billion t]. Unless specified as the oxide forms P
2
O
5
or K
2
O,
amounts of P and K are expressed as uncombined elements.) Biological nitrogen
fixation (BNF) additionally contributes an estimated 90 Tg. Tan et al. (2005) esti-
mated the global average rates of soil nutrient deficit in 2000 as 19 kg N/ha, 5 kg P/
ha, and 39 kg K/ha. Many of the vast cereal-producing food bowl areas of the United
States, Canada, Australia, Brazil, and Argentina relied for many years on the mining
of native soil fertility before farmers purchased external inputs to maintain yields.
More than half a century ago, Martin and Cox (1956) documented the progressive
loss of up to 40% of the total nitrogen in fertile Vertisols that had been repeatedly
fallowed and cropped without fertilizer inputs in the northern grain belt of Australia.
Continued cropping in these areas was eventually possible only with major invest-
ments in nitrogen fertilizer production and distribution. The global dimensions of
widespread soil fertility decline due to the transformation of land for agriculture
are indicated by the decline in soil organic carbon (SOC) levels, estimated to have
released 55 to 90 Pg to the atmosphere as carbon dioxide and methane (Lal 2006).
Continuing cropping, which exports nutrients from the farm as harvested product,
combined with inadequate fertilizer use, clearly has serious repercussions for the
productivity of the soils as well as the release of greenhouse gases (GHGs) as organic
matter decomposes. Hartemink (2006) differentiates between nutrient depletion or
nutrient decline (larger removal than addition of nutrients) and nutrient mining,
which involves no inputs to replace the removal of large quantities of nutrients.
While this paper focuses on the implications of nutrient mining due to crop inten-
sification, the term is generally used in the ensuing discussion with the knowledge
that it is clearly difficult to differentiate between it and Hartemink's other less severe
categories. The important food security implications in SSA of this problem have
led to extensive literature on the nutrient balance at a range of scales (see Vlek 1990;
Stoorvogel and Smaling 1990; Stoorvogel et al. 1993; Smaling et al. 1993; Van den
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