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In-depth Analyses |
Does China have a soil degradation problem? |
Introduction |
Tables & Charts |
Soils are of
vital importance to China's food production capacity. Soil is a natural resource that is
not renewable in the short term once it is eroded by water or wind, physically degraded,
or chemically depleted. If reclamation is possible at all, it is very costly.
In the 1980s and early 1990s various publications painted a grim outlook for world soil
resources. Dramatic statements were made predicting that soil erosion would undermine the
future prosperity of mankind. China was among the areas where the problems were considered
to be the most serious. However, the empirical basis for these predictions was rather
weak. While soil-degradation problems could certainly be identified in specific
regions and case-study areas, the overall agricultural impact of the problem and its
specific geographical distribution on a global or even on a country basis were highly
speculative. |

Map 1
Map 2

Map 3

Map 4

Map 5

Map 6

Map 7

Map 8

Map 9
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The GLASOD Project
Under these circumstances, UNEP commissioned a major study by the International
Soil Reference and Information Centre (ISRIC) in the Netherlands. Its objective was a
global assessment of the status of human-induced soil degradation (GLASOD). The project
began in 1988. The results were published as a world map at an average scale of 1:10
million. There were also additional statistics on the global and continental extent of
various types of degradation, including water and wind erosion, and chemical and physical
degradation.
The results of this study came as a shock to many experts. According to GLASOD, the soils
of some 1,964 million ha worldwide were classified as degraded in one way or the other.
This was more then 22% of the combined area of agricultural land, pastures, forests, and
woodland. Estimates based on agricultural land alone, were, of course, even higher. In
Central America, some 75% of the agricultural land was classified as more or less
degraded; in Asia the estimate was about 38%. The World Resources Institute compiled many
of the GLASOD results in its 1992-1993 World Resources Report. |
A number of scientists have
strongly criticized the study. Crosson (1994), for instance, pointed out that, according
to GLASOD, most areas in six states of the USA were classified as moderately degraded,
which according to the definition should result in greatly reduced agricultural
productivity. In reality, however, crop yields in those areas have been rising steadily
for the past 40 years (Crosson, 1994). According to the GLASOD map for China, there is
almost no cultivated land that is not classified as degraded in one way or the
other. For instance, almost all the areas of rice cultivation in the South and Southeast
were classified as being affected by high or very high water erosion; large wheat-growing
areas southeast of Beijing were classified as suffering from medium levels of chemical
deterioration (salinization). In the Northeast, the GLASOD map indicated medium, high, or
even very high wind erosion. According to the GLASOD classification, a medium level of
soil degradation indicates that "the terrain has greatly reduced agricultural
productivity." A high degree of degradation means that "the terrain is
nonreclaimable at farm level" and that "original biotic functions are largely
destroyed" (Oldeman et al., 1990, p. 15). Yet, under these conditions China was able
to increase its wheat production between 1961 and 1996 from about 16 to 110 million tons,
its rice production from less than 40 to 130 million tons, and its maize production from
under 20 to over 120 million tons.
It should be mentioned that the GLASOD results were not based on quantitative data,
but on "expert opinions." Also, the project's main objective was to identify the
geographical distribution and the type of human-induced soil degradation, and not to
assess the impact of these soil problems on agricultural productivity. It was also the
first global data set on soil degradation. |
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The ASSOD Project
To improve on the methodology in the GLASOD approach, and to get a more detailed
assessment of Asia, a second major project was started in 1993: the assessment of the
status of human-induced soil degradation in South and Southeast Asia (ASSOD). This project
was also commission by UNEP from ISRIC. Based on a modified and refined GLASOD
methodology, the ASSOD project developed a 1:5 million map of soil degradation in South
and Southeast Asia. The map was based on the Soils and Terrain Digital Database, SOTER
(see van Lynden and Oldeman, 1997). The information in the ASSOD project was compiled in
the form of spatial databases linked to digital GIS maps, which allows detailed
statistical analysis. Six major categories were used to classify soil degradation:1. Types of soil degradation (such as water erosion,
chemical deterioration, etc.)
2. Degree of degradation, expressed as the impact on productivity
(five classes from negligible to extreme)
3. Extent of soil degradation (as a percentage of the mapping area
affected)
4. Rate of soil degradation [from +3 (rapidly increasing) to -3 (rapidly
decreasing)]
5. Causative factors (five classes, such as improper management of
cultivated land, overgrazing, etc.)
6. Rehabilitation or protective measures (four classes, such as plant
management, land management, etc.)
The most obvious difference between the GLASOD and ASSOD maps on
China is the much greater detail of the ASSOD results. A further improvement was the
better differentiation of degradation types. For instance, whereas for South and Southeast
Asia GLASOD had indicated that 73% of the degradation was due to water erosion, 20% due to
wind, and 7% due to chemical deterioration, ASSOD found only 47% water erosion, but 20%
wind erosion, 24% chemical deterioration, and 9% physical deterioration. The most
important difference between the two studies, however, was a more realistic assessment of
the severity of degradation in ASSOD. While the ASSOD results indicated an even larger
area of soil degradation than the GLASOD estimates, the degree of degradation was
estimated to be much lower. For instance, whereas for the whole study region of South and
Southeast Asia, GLASOD estimated 36.2 million ha of strong and extreme water erosion, the
ASSOD project found only 15.9 million ha. On the other hand, GLASOD reported only 62.8
million ha of light water erosion, whereas ASSOD found 175.8 million ha of light
water erosion and another 58.7 million ha of negligible water erosion. There were,
however, a few exceptions to this trend: GLASOD found much more "terrain
deformation," which was defined as an "irregular displacement of soil material
by wind action." |
Both the GLASOD
and ASSOD assessments paint a serious picture of soil degradation. The ASSOD estimates,
however, seem to be more realistic in that the degree of degradation is more
closely related to its impact on soil productivity. In other words, the estimates
of ASSOD take into account that a given amount of soil erosion is a more serious problem
on poor, shallow soil than on deep, fertile soil. In the case of China, this leads to a great
reduction in extent of strong or extreme soil degradation, a result we believe is
more compatible with the history of spectacular crop production increases in
China. |
ASSOD Estimates of Soil Degradation in China |
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IIASA's LUC
Project analyzed the ASSOD database specifically for China. While the ASSOD assessment is
certainly more detailed and seems to be more realistic than the GLASOD maps, we still find
that more than 466 million ha (or 50% of the land) in China are affected by one type of
soil degradation or another (see Table 1). This is certainly a dramatic figure. However,
on closer inspection, we find that the situation is much less serious than it seems at
first glance.
First, the various degradation types can overlay each other in one mapping unit. The
actual size of affected land is therefore certainly smaller than the aggregated total in
Table 1.
Second, we have to take into account the degree of degradation. According to the
ASSOD assessment, there are only two types of soil degradation in China that have an extreme
impact on soil productivity: (a) loss of topsoil due to water erosion on some 200,000 ha
and (b) loss of topsoil due to wind erosion on between 10,000 and100,000 ha. In other
words, extreme soil degradation affects just 5.9% of the total land area of
China. On the other hand, 306.9 million ha (or almost 66%) of the degraded land, have only
a negligible or light degree of degradation.
Third, we must also take into account that the ASSOD estimates are based on total
land area; it is not known (at least not from the ASSOD assessment) how much of the
degraded land is in cultivated areas. For instance, according to ASSOD estimates,
water erosion affects 180 million ha in China to some extent. This is certainly a
staggering figure compared with China's total cultivated land, which we have estimated at
132 million ha. However, the number also shows that much of the water erosion in China
must be occurring outside cultivated areas. Unfortunately, the ASSOD database
does not include information on actual land use. The degradation information is based on
SOTER polygons, which include all land areas. We can be quite sure that significantly less
then 5% of the cultivated land in China is affected by extreme forms of
soil degradation.
On the other hand, there can be no doubt that, according to ASSOD, soil degradation is a
serious problem in China: 73 million ha of the total land area are affected by moderate
degradation and 86 million ha by strong soil degradation. If only 50% of this land is in
cultivated areas (which is probably a low estimate), then some 80 million ha (or over 60%)
of the cultivated land are affected by some kind of moderate or strong soil degradation. |

Table 1 |
Our Evaluation of the Situation |
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There is little
doubt that China is facing serious soil degradation problems. From studies on specific
areas (such as on the Loess Plateau, or on the desertification in Northern China), as well
as from global or regional assessments (GLASOD, ASSOD), we have evidence of
significant water and wind erosion and chemical deterioration. However, the situation
should not be over-dramatized. There are at least three factors that must be taken into
account for a balanced assessment of the situation: |
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All national or regional
assessments of soil degradation in China are based on expert opinion. Quantitative
data are available for small local areas and experimental sites. It is extremely difficult
to generalize information from specific problem areas in order to characterize the
situation in a much larger region. We have to expect that national estimates on
soil degradation in China in particular are highly uncertain. |
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It is also not clear to what
extent the degradation actually affects agriculture. As we have emphasized, the ASSOD
estimates are based on total land area. Clearly, additional research is needed to estimate
the extent of degradation in cultivated areas. |
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We also
believe that the methodology for assessing the degree of degradation in the ASSOD
assessment is extremely difficult and open to subjective interpretation. In the ASSOD
project, local experts assessed the relative impact of a given amount of a
certain type of degradation on the productivity of the soil. This approach should address
the problem that the impact of degradation is quite different for different soils. For
instance, the productivity impact of water erosion depends very much on the
thickness of the topsoil layer. When a layer of a few centimeters is washed away from a
deep fertile soil (such as in China's Loess Plateau), the impact on productivity is
certainly lower than when the same amount of water erosion occors on a poor,
shallow soil. This partly explains why China's farmers have been able to cultivate the
soils of the Loess Plateau for thousands of years, despite ongoing erosion. In the ASSOD
project, experts estimated the extent to which the soil productivity of a particular area
was reduced. Obviously, this is an extremely difficult, error-prone task. |
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Finally, soil
degradation has to be related to the protection potential and to possible rehabilitation
measures. These measures greatly depend on (future) technology, the amount of investment
capital, the know-how of farmers, and the efficiency of political and administrative
measures. |
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In conclusion,
various types of soil degradation are certainly causing serious problems in China. The
ongoing measures to stop or diminish loss of topsoil due to water erosion must therefore
be intensified, not only to prevent productivity loss in agriculture, but also to minimize
hydrological problems in areas downstream. These include, for instance, the serious
problems of sedimentation in the middle and lower reaches of the Yellow River, where the
rising ground level increases the danger of flooding due to broken dykes. There is also
the problem of eutrification, which threatens the ecological balance of rivers and lakes.
Wind erosion is a major problem in the arid regions of Northern and Central China, and
further reforestation is necessary. This degradation problem primarily affects marginal
agricultural areas and grasslands, which currently contribute little to China's overall
food supply. The core agricultural regions that produce the bulk of China's crops
are largely unaffected by this problem. However, if China wants to increase grassland
productivity for livestock, wind erosion problems are of critical relevance.
Chemical and physical forms of degradation (such as aridification, sealing, or crusting),
on the other hand, are more serious in the major crop areas (see Map 9). However, various
forms of vegetation conservation methods and adequate land management practices are
available. These practices can obviously reduce the degradation impact considerably,
otherwise it would be difficult to explain how China has been able to increase crop
production over the past few decades. |
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Revision 2.0 (First revision published in 1999)
- Copyright © 2011 by Gerhard K. Heilig. All rights reserved. (First revision: Copyright © 1999 by IIASA.) |
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