Agriculture Reference
In-Depth Information
cycling. Conservation and sustainable management of soil resources involve adap-
tation of no-till farming, using crop residue mulch, growing cover crops, creating
positive nutrient balance, using soil-specific or precision farming, conserving and
recycling water, and using drip or subirrigation. Biofuel must be produced from lin-
gocellulosic biomass grown on energy plantations of short-rotation woody perenni-
als or warm-season grasses.
IntRoductIon
Of the six prehistoric natural global ecosystems, 36.0% of Earth's land area was
occupied by forests, 26.3% by grassland, 12.5% by shrubland, 12.2% by desert, 7.5%
by woodland, and 5.7% by tundra (Table 18.1). The land area under different ecosys-
tems progressively changed since about 10 to 12 thousand years ago when the settled
agriculture and domestication of plants and animals began. Increase in food supply
with expansion of agriculture led to increase in human population, from about 4 mil-
lion in 10,000 B.C. to 170 million in 0 B.C., 190 million in 500 A.D. to 6.5 billion
in 2006 A.D. (Table 18.2). The world population is presently increasing at the rate
of 1.3% per year and is projected to be 7.5 billion by 2020, 9.4 billion by 2050, and
10 billion by 2100 (Fischer and Helig 1997; Cohen 2003). Almost the entire increase
in future population will occur in developing countries, where soil resources are in
short supply and are prone to degradation because of the harsh climate and extractive
farming practices.
Increase in agricultural land area was caused by conversion of natural to managed
ecosystems. The latter involved reduction in area by 750 million hectare (Mha) of the
forests, 660 Mha of grasslands, 180 Mha of woodland, 140 Mha of shrubland, and 90
Mha of deserts. Of these changes, 1550 Mha were used for cultivation of food crops
(Table 18.1). Furthermore, anthropogenic transformation of ecosystems had a strong
impact on soil processes and properties, energy balance, water balance, and elemen-
tal cycling. The soil organic carbon (SOC) pool declined by 66-90 Pg (Petagram,
one billion metric tons) (Lal 1999), with the attendant adverse impact on soil qual-
ity and reduction in ecosystem services. Decline in soil quality was exacerbated by
other degradative processes (Lal 2004), such as erosion by water and wind, nutri-
ent depletion, elemental imbalance, acidification, salinization, compaction, crusting,
and decline in soil structures. The objective of this chapter is to deliberate processes,
factors, and causes of soil degradation; the attendant impact on the environment
and ecosystem services; and the conservation and management options to restore
degraded soils and achieve sustainable management.
soIl AReA undeR nAtuRAl And cultIvAted ecosystems
Of the world cropland area of 1550 Mha, 19.4% is on Oxisols, 18.7% on each of
Alfisols and Mollisols, 8.4% on Ultisols, and 5.2% on Entisols (Table 18.3).
Growing crops, especially by the use of extractive farming practices, leads to
decline in soil quality by accelerated erosion and other degradation processes.
The problem is more severe in developing countries of the tropics and subtropics
(Table 18.4) because the climate is harsh, soils are fragile, and farmers are resource
