Environmental Engineering Reference
In-Depth Information
for agriculture. In contrast, the lack of many of the agro-aids, inadequate technology, poor
soil management, and unfavorable climate have combined to produce nutrient-depleted
soils in regions of Africa and Asia.
The requirement for suficient food to sustain life has led humans to require substantial
use of grasslands, forests, and freshwater. Factors such as excess or lack of water, drainage,
soil quality and thickness, salinity, annual mean temperatures, and in cold regions, the freez-
ing index and the presence or lack of a permafrost area determine the appropriateness of the
soil for agriculture. Richards (1990) reports that there has been a ive-fold increase in agricul-
tural lands in the last 300 years. This needs to be balanced with the large quantities of land
lost from production due to erosion, salinization from irrigation, desertiication, and conver-
sion to roads and urban uses. The capillary rise of salts (chlorides, sulfates, and carbonates)
due to groundwater extraction and irrigation is threatening the U.S. High Plains, Canadian
prairies, and Australian soils. This rate of loss has increased to 79,000 km 2 /year according
to FAOSTAT (FAO, 2006a). With the exception of a few high value crops, the market value of
the land for nonagricultural purposes is much higher—a factor that appears to drive conver-
sion of these lands to urban uses. The other consideration that is signiicant is the increasing
productivity of agriculture in the last century—a factor that has decreased famine rates sig-
niicantly (Pinstrup-Anderson et al., 1997). Innovations including high-yield crop varieties,
application of fertilizers and pest control, and utilization of mechanized equipment in both
developed and developing countries have substantially increased agricultural productivity.
More than 50% of the land's surface is involved in one way or another with forestry,
agriculture, or animal husbandry. Pastures alone make up 6% to 8% of the land. This does
not include land for grazing. Agriculture, in combination with urban areas occupy up to
10% to 15% of the land (Vitousek and Mooney, 1997). More than 130,000 km 2 /year of forests
were eliminated for the period 1990-2005, and of this, 98,000 km 2 /year were converted
to agricultural crop use (FAO, 2006b). This has substantial implications in global warm-
ing since forest soils and removed trees account for recycling of much larger amounts of
carbon dioxide from the atmosphere than agricultural lands. Deforestation also leads to
increased risk of soil and wind erosion because of exposure to the elements. Although
some forests have been allowed to regrow upon depletion of agricultural lands, reforesta-
tion rates are much smaller than deforestation rates.
Ploughing disrupts (a) soil horizons, (b) chemical weathering, and (c) soil horizon inter-
changes. Ploughing compacts soil and decreases soil permeability. This inhibits evapora-
tion processes and plant germination. Cattle and sheep herds also compact the soil. In arid
climates, the soil is particularly vulnerable to erosion due to mechanization of plough-
ing. Overgrazing and slash-and-burn cultivation also reduces soil cohesion leaving many
areas affected by soil degradation and loss of vegetation. More than 35% of all degraded
land is due to overgrazing (Quendler and Reichert, 2002). Arid and semiarid lands are
particularly susceptible in Oceania and Africa.
From the geoenvironmental perspective, one could ask “how sustainable is agriculture?”
Although most agree that agriculture is not sustainable under present practices, there is
considerable debate concerning the means for measuring sustainability (FAO, 1995, 1996).
Although full consensus is not available, it is agreed that the factors that need consider-
ation are the use of genetically identical plants, irrigation water, fertilizers and pesticides,
and the assortment of wastes produced. Intensive agricultural practices utilizing high lev-
els of technology and mechanization are generally not kind to the soil environment. A
simple summary of the major inputs and outputs of agricultural activities is depicted in
Figure 6.1. The impact of agricultural activities on the land environment will be discussed
in more detail in the next section.
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