Land degradation is a threat to human security from two aspects: freedom from want (famine) and
freedom from fear (environmental destruction) as pointed out by the UN Secretary General Kofi
Annan in 2000 as the twin goals for the world community to advance. It is induced by natural hazards
as well as human activities which include not only agriculture, grazing and logging but the
construction of roads, water ways, mining operation, etc. Land degradation is the integrated
deterioration of land cover, which is detected as the reduction in vegetation coverage of the land
surface, the change of the type of plant communities, soil erosion and the decline in soil fertility.
This paper focuses on degradation from soils point of view, i.e. soil degradation, which is one of the
major topics for the soil science community to work for public awareness in the International Year of
Soils 2015. Some examples of soil degradation observed in different environmental conditions are
described and compared in terms of its type, causes, impact on human welfare and possible
countermeasures.
Lal and Stewart (1990) classified the processes of soil degradation into the following three;
1) physical processes such as the deterioration of soil structure (slaking and the formation of surface
crust), the increase in bulk density (compaction), the changes of soil water and temperature regime,
2) chemical processes such as leaching, acidification and elemental imbalances (salinization,
deficiency or toxicity of a specific element, and laterite formation), and
3) biological processes such as the depletion of soil organic matter, the decrease in bio-diversity and
the increase in soil-borne pathogens.
These processes are usually not independent but work together and make the problems more serious
and complicated (Lal 1990). These processes are affected by natural factors as well as
socio-economic, even political, factors. They can be initiated by natural causes as well as
anthropogenic causes. Natural causes are soil characteristics such as an effective soil depth, clay
mineralogy type and texture. Anthropogenic causes include improper management in farming
systems and socio-political problems. In most of the cases of soil degradation in the world, the
anthropogenic causes play more important roles than the natural ones. Followed are some examples
of the analysis on the type, causes, an impact of soil degradation and possible countermeasures
observed in the world.
Soil degradation associated with modernization in agricultural production system
Summer fallowing has been commonly practiced in wheat-based agricultural production systems
in drylands including some parts of Russia and Central Asia in order to store water, to control weed
hazard, and to accumulate mineral N through mineralization of soil organic matter. However, since
summer fallowing prevents any vegetative growth by shallow tillage, it has often been reported that
this practice on Chernozem soils or Ustolls had accelerated organic matter decomposition, causing the
risk in lowering the sustainability of agricultural production. We found that an extensive and uniform
application of summer fallowing is not beneficial to sustainable grain production in Central Asia, and
thus, an alternative soil and land management technology must be developed with taking account of
snow-collection-based water harvest management and its site-specific application in accordance with
soils and topographical conditions (Funakawa et al. 2007).
Can traditional farming become a savior in degraded land in the 21
st
Century?
Although shifting cultivation is often claimed as a cause of soil degradation, it is still practiced for
staple food production in many of the developing countries, particularly in the tropics. It is well known
that the main profit of slash-and-burn is the supply of bases, P and N and that of fallowing is the
recharge of soil organic carbon and nutrients. The fallowing stage has, however, been given limited
information, and thus it is hardly to answer the question: How long is fallowing necessary to
sustainably maintain soil productivity? Based on the monitoring and modeling in Northern Thailand in
terms of carbon and nitrogen dynamics such as emission of CO2 due to soil organic matter
decomposition, litter fall, nitrogen leaching through soil profiles from farms under shifting cultivation,
the followings were found. 1) Soil productivity is restored in the fallowing stage of 6 to 7 years by not
only the addition of C and nutrients but also the suppression of N loss through leaching by the
succession of the soil microbial community from rapid consumers of resources to stable and slow
utilizers, resulting in an increase in the amount of N storage in the microbial biomass and a decrease in
the rate of soil organic matter decomposition. And thus, 2) shifting cultivation system can be seen to be well adapted to soil-ecological conditions and sustainable in northern Thailand provided the traditional
style of fallowing for about 10 years is maintained (Funakawa et al. 2011).
Wind erosion is a major contributor to desertification, one of the soil degradation phenomena,
particularly on the over-grazed/cultivated land in the Sahel region of West Africa. Topsoil, which
typically contains more nutrients than subsoil, is detached and removed from arable land by wind.
Thus, wind erosion affects the soil nutrient level and soil productivity. Moreover, abrasion and burial
by the wind-blown sand can damage pearl millet stands, a staple cereal crop in the Sahel. Effective
measures for wind erosion control have been proposed and their effects have been demonstrated.
However, none of these measures have been adopted by Sahelian farmers. The main reason for
non-adoption may be that developers and/or extension workers of these techniques did not give
sufficient attention to their practicality for farmers. We proposed a new land management practice,
called the “Fallow Band System (FBS),” which is useful for both wind erosion control and
improvement of soil fertility and crop production in the Sahel. The FSB may be considered as a
shifting herbaceous windbreak. This method does not impose an additional expenses and labor
requirements on Sahelian farmers who are economically challenged and have limited manpower. We
conducted field experiments and showed that 1) a fallow band can capture a great deal of wind-blown
soil particles and coarse organic matter, leading to an effective control of wind erosion, 2) the amount
of soil nutrients available for crops in a former fallow band was increased by the decomposition of
trapped soil materials, and 3) the amount of soil water available for crops in a former fallow band was
increased by the trapped wind-blown soil materials through improvement of rainwater infiltration into
surface soil (Ikazaki et al. 2011).
The examples of soil degradation described above are induced by human activities, i.e. improper
management of land use. Although some long term changes in local and/or global climate may affect
the type and extent of soil degradation, human activities often contribute more directly and drastically
to soil degradation. The types, causes and impact of soil degradation vary from one place to another,
thus the possible countermeasures against soil degradation should be tested and evaluated with taking
account of physical as well as socio-economic conditions in an individual situation. The application
of a given set of countermeasures without detail analysis of the site in question may result in disastrous
and irreversible changes in the environment and finally jeopardize human security.