Soil Erosion: The greatest challenge for sustainable soil management.

 

Despite almost a century of research and extension efforts, soil erosion by
water, wind and tillage continues to be the greatest threat to soil health and
soil ecosystem services in many regions of the world. Our understanding of
the physical processes of erosion and the controls on those processes has
been firmly established. Nevertheless, some elements remain controversial.
It is often these controversial questions that hamper efforts to implement
sound erosion control measures in many areas of the world.
Regional and global estimates of soil loss rates due to erosion differ
substantially depending on the method used to derive them. Generally,
estimates of mean annual soil loss from field plots are substantially higher
(8 to almost 50 t ha-1 yr-1) than those from regional and global models (2 to
4 t ha-1 yr-1). Any estimate of erosion must also be placed in the context of
the acceptable or tolerable rate of loss. Rates of tolerable soil loss calculated
using soil production rates range from 0.2 to 2.2 t ha-1 yr-1 and tolerable rates
based on maintenance of crop production range from approximately 1 to 11
t ha-1 yr-1. The ranges for both soil loss and tolerable soil loss demonstrate
the need for site-specific estimates to reflect the different sensitivity of soils
to removal of surface soil through erosion.
According to the definition of Sustainable Soil Management adopted by
the Food and Agriculture Organization of the United Nations (FAO) in 2015,
the definition of tolerable soil loss should also consider the impact of soil
erosion on ecosystem services provided by soil, such as regulation of water
and air quality.
The impact of erosion on crop production has been estimated at a 0.4
percent reduction in global crop yields per year due to erosion. Modelling
of the impact of this yield loss on the overall agricultural economy (using
general equilibrium models) suggests a lower overall impact as land prices
and the labour force adjust to the changes in soil productivity. A recent study
from Malawi suggests that the negative impact of soil and nutrient loss falls
most heavily on the poorest members of society and on households headed
by women, and this result is supported by qualitative information from many
other studies.
Recently, regional and global models for water erosion have begun to
published, along with initial efforts at global wind erosion modelling. The
results of these models can be compared with site-specific studies and with
anecdotal information in order to identify global hotspots of erosion. In
many cases, the level of agreement across studies is strong. These hotspots
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should be the priority for targeted soil control measures. It is also essential
that the modelling results be validated through structured field assessments
of erosion.
There are many examples of successful implementation of soil erosion
control measures. The widespread adoption of reduced tillage and no-till
practices has significantly reduced wind and water erosion in many drier
regions, but significant impediments exist to its adoption in more humid
regions. In general, measures using vegetation cover in order to reduce
erosion (through enhanced residue cover, cross-slope plantings of erosion
resistant grasses, or planting of shrubs or trees to reduce wind erosion)
appear to be more readily adopted than are engineered, structural measures
such as terraces.
Issues related to soil governance are the most significant impediments to
the adoption of erosion control measures. Two overarching issues have been
identified. In the first place, many of the impacts of erosion occur off-site,
and there is no direct benefit for the soil user to implement control measures
that minimize these off-site impacts. Second, the long time period required
for many erosion control measures to have a clear beneficial effect limits
their adoption, especially for soil users who do not have secure tenure rights
to their land. The successful implementation of erosion control measures
shows that these impediments can be overcome. For this to happen, the
decision making factors that lead to successful adoption by soil users needs
to be better understood and adapted to diverse conditions.