Principles of Soil Management - Principles of Sustainable Soil Management in Agroecosystems - page 6

TABLE 1.5
Temporal Changes in Primary Energy Source from 1850 to 2008

Primary Source (EJ)

Other
Renewables

Year Biomass Coal Oil Gas Hydropower Nuclear Total

1850 27.13 1.43 - - - - - 28.56

1875 28.56 7.14 - - - - - 35.70

1900 30.35 19.63 1.07 - - - - 51.05

1925 34.27 29.99 10.71 3.57 1.79 - - 80.33

1950 39.27 41.06 23.20 7.14 3.57 - - 114.24

1975 30.35 69.61 117.81 39.27 17.85 3.57 - 278.46

2000 46.41 96.18 158.86 89.25 28.56 28.56 3.15 450.97

2008 49.98 142.37 173.15 116.02 32.13 24.99 4.14 542.78

Source: Johansson, T.B. et al., Global Energy Assessment: Towards a Sustainable Future. Cambridge
University Press, Cambridge, UK, 2012.

Increasing energy use and enhancing its efficiency in agroecosystems are high priorities
in countries with low relative agronomic yields of food crop staples ( Table 1.4 ).

1.3 YIELD GAP
While there is some potential to expand the arable land area, such as in Latin America
and Africa (Bruinsma 2003; Rosegrant et al. 2001), the focus must be on increasing
productivity from the existing cropland by abridging the yield gap. The latter is defined
as the difference between the technical yield potential (at the national or regional
level) and the average farmers' yield for a specific crop (Lobell et al. 2009). The tech-
nical yield potential refers to the agronomic productivity of the modern variety grown
under the best management practices (BMPs). With these criteria, the yield gap of
major crops in India is 5%-60% for wheat and 33%-70% for rice. Similarly, the yield
gap for maize is 54%-84% in Sub-Saharan Africa and 60%-70% in Latin America.
In comparison, the yield gap for rice in China is only 17%-20% (Lobell et al. 2009).
In general, the lower the national/regional farmers' yield and the more degraded the
soil, the larger the yield gap. High yield gaps have been reported for crop produc-
tion under semiarid conditions (Fischer et al. 2009; Rockström and Faulkenmark
2012), for wheat in northeastern Spain (Abeledo et al. 2008), rice in Southeast Asia
(Laborte et al. 2012), and major food crops in West and Central Africa (Nin-Pratt et
al. 2011). Witt et al. (2009) used an agronomic model based on “yield gap” analysis,
fertilization for attainable yield, and the area growth in Indonesia. There are several
options of closing the yield gap, which are crop specific (Tran 2010). Furthermore,
there is a wide range of controls that affect the yield gap ( Figure 1.2 ). Important
among these are the following: (1) natural resources including climate, terrain, soil,
water, and so forth; (2) technology adoption comprising management of soil, water,
nutrients, climate, and human capital; (3) institutional support such as research and