Agriculture Reference
In-Depth Information
agriculture is carried out along the Nile River, which is used as a source of irrigation
water. In other cases water for irrigation may be produced by desalination, that is,
removal of salt from seawater, or obtained from wells.
As seen in Table 9.2, there is little indication of a relationship between percentages
of arable land and the well-being of the inhabitants of a country. For some countries this
is because they can go from food surplus to deficit without any change in the agriculture
situation, arable land, rainfall, and so forth. In other cases it may be due to agricultural
practices, government policies, or land tenure systems. However, the amount of arable
land does put an upper limit on the potential agriculture productivity of a country.
9.4
SOIL TYPES
Different soils are grouped into similar observable characteristics. There are many ways
of grouping soils. Many countries, France, the former Soviet Union, and the United
States, are just a few examples of countries that have developed different classifications
of soils. FAO, under the auspices of the United Nations, also classifies soils into 26
different groups (http://www.fao.org/AG/agl/agll/key2soil.stm). In the United
States soils are classified as being in one of 12 soil groups, given in Table 9.3.
Figure 9.4 shows the extent of each soil type in the world. Of the 12 soil orders
defined by the USDA (U.S. Department of Agriculture), the Entisols, Inceptisols, Ari-
disols, Mollisols, and Alfisols are most widely distributed and used for agricultural pro-
duction. Of the soil orders, the Gelesols are probably the least likely to be use for food
production because they are frozen much of the time.
Table 9.4 gives an indication of soil types designated in the FAO nomenclature.
Although the FAO system contains many more primary soil designations, they can
be related back to the USDA system and to the soil's suitability for agricultural pro-
duction by reference to Table 9.4.
Each type of soil occupies a certain extent of the landscape but may also be found
in many different localities, as shown in Figure 9.4. On a medium scale the observable
characteristics of individual soils and soil components such as horizons, structure, and
color are used to differentiate one soil from another. These same characteristics, such as
the thickness and condition of the A horizon, which is the surface horizon of a soil, are
very important in determining the management of a soil for maximum food
productivity. A deep dark uncompacted A horizon will be more conducive to crop
production than will other types of A horizons.
The geographic and other large features of soil are easy to observe but give little
hint as to the complexity of soil. When looking at soil on a small to molecular level,
its complexity becomes very evident. On the small scale we can observe soil com-
ponents, that is, sand, silt, clay, organic matter, air, and water. Each of these components
is complex in its own way and contributes to the overall molecular complexity of soil.
The complexity occurs even within soil texture, which is the relative proportions of
sand, silt, and clay in a soil. Sand size particles in soil can be divided into many differ-
ent subsizes from very fine to very coarse. Clay components may be very well defined
crystalline structures or they may be amorphous, that is, without definite form. In
