Geoscience Reference
In-Depth Information
conditions. Water availability becomes an issue - the plants suffer from droughts
during extended rainless periods, or they suffer from lack of water during shorter
rainless periods. Or when there are adequate levels of plant available water within
the profi le, owing to the absence of coarse pores in the compressed A horizon and
plow sublayer, the exchange of air between the soil and atmosphere is severely
restricted. And with a lot of oxygen in the soil air being consumed by the decompo-
sition of organic compounds, the soil could even suffer from anaerobic conditions
where the transformation processes produce toxic compounds and the soil increases
its acidity.
Reclamation of the soil at the top of the A horizon is not simple. The compressed
surface without stabile structure forms a crust when soil dries out. When it is wetted
by rain, the surface is transformed into a muddy puddle surface not readily perme-
able to either air or rainwater or to melting snow at the end of a cool winter. All
these changes lead to worsening the condition for planting agricultural crops and to
the reduction of their yields harvested at the end of their growing season.
There is a lot more than just plants that live and grow in soil. Soil-inhabiting
organisms have a variety of types and sizes, e.g., small burrowing mammals, insects,
amphibians, and worms seen with our eyes, and microbiota like nematodes, bacte-
ria, fungi, and actinomycetes. All of them work to decompose, transform, and dis-
perse plant and soil organic residues. They also in many instances help to weather
soil minerals and to enable the dispersion of less soluble compounds. All those
actions cause the release of plant nutrients in mineral forms available for the initial
growth of new plants. Each and every process in soil is a decisive part of nutrient
cycling. Although nutrients from soil organic residues in forms of inorganic com-
pounds are initially not available to plants, they are gradually transformed, stored in
soils, not leached out, and gradually released at times related to the specifi c needs
of cultivated plants. The intensive decomposition of organic compounds and release
of nutrients are realized provided that the soil has an adequate quality.
Another type of soil illness is salinization. Often occurring naturally in arid and
semiarid zones, it was accelerated by irrigation when agriculture started. Without
irrigation, man's greatest revolution could not exist in such zones. Simple irrigation
was merely an imitation of nature, of fl oods across a natural landscape where the
soil is fully saturated by an excess of water. An excess of water means the leaching
of readily soluble salts that were released during the chemical weathering of rocks
or geological sediments. Since the dominant net fl ow of water is in the upward
direction, the solutes are also transported upward as water evaporates from the soil
surface. Each simple irrigation or pulse of soil saturation by water has the conse-
quence of transporting more salt to the topographical surface. Likewise, when the
groundwater level rises owing to the seepage of water from the irrigation ditches
and channels, both water and its dissolved salts migrate upward toward and into the
root zone. The elevated salt concentration in the root zone does not allow economi-
cal farming. As harvests drop because the soil becomes progressively less fertile,
the soil is ill according to our criteria. The best way of curing it is the prevention of
salinization. Local governments should avoid wasting water by seepage from poorly
designed and installed irrigation structures. They should avoid salinization of
