Environmental Engineering Reference
In-Depth Information
surface (Az horizon) or salt crust. The enrichment of salts is common wherever ground
water comes close to the soil surface, such as in river flood plains and low-lying
depressions. Also, salts will accumulate where waters from inland drainage accumulate
or where lakes exist or existed in the past. Coastal areas can also accumulate salt from
aerial sea spray ('cyclic salt'), and from the incursion of salt water into the coastal
aquifer. However, faulty irrigation schemes have been a major reason for the spread of
salt-affected soils in the twentieth century. Misuse by agriculture has resulted from the
over-application of irrigation water or the failure to provide efficient drainage to remove
surplus water from the soil. As the water table rises, capillary forces are able to move
water containing ions, and water vapour, to the soil surface, where, under the intense
prevailing evaporation, salts are deposited in pores and on the surface. This is the so-
called 'wick' effect. Where salt crusts are formed, they usually consist of a finely
comminuted salt dust which can be blown up into the atmosphere, eventually to come
down by gravity or in rain to influence soil formation in surrounding areas. Salinity is
also added to the soil surface by the addition of fertilizers to the irrigation water, much of
which will not be taken up by plants and will increase the salt content of the surface
horizon. Salty soils will also occur where saline ground water results from the presence
of salt deposits in the geological column.
Saline soils described above are classified as
solonchaks
, though they have also been
called
white alkali soils
in the United States. They contain sulphates and chlorides of
sodium and potassium, though magnesium and nitrate ions may also occur. They show
white salt efflorescences at the surface but usually no change in structure down the
profile (Figure 19.14). They are usually low in humus, on account of the low productivity
of natural vegetation on such soils, and consequently the low input of dead plant residues.
The pH values are in the range 8·0 to 8·5 but go no higher because of the high
concentration of neutral soluble salts. Soil-forming processes are inhibited and profile
development is minimal (see Colour Plate 21 between pp. 400 and 401).
If a situation arises whereby salts no longer accumulate at the surface, there will be
far-reaching changes in the soil profile. For example, if the water table falls or rainfall
increases, rainfall may wash the salts through the profile. If the salts are mainly calcium
salts, soil formation will go in the direction of xerosols or chernozems. However, if a
significant proportion of the exchangeable cations are sodium (perhaps over 15 per cent),
sodium carbonate will be formed, owing to the carbonate (CO
3
−
) and bicarbonate
(HCO
3
−
) anions continually being produced by plant roots and organisms. This gives the
soil a pH of 9·0 or over, a structure which becomes unstable and deflocculated, and a soil
surface which is dark-coloured, often black, owing to dispersed humic particles.
Dispersed clay particles are washed down in the profile and form a clay pan which dries
into hard columnar units. These soils are called
solonetz
and are also termed
black alkali
soils
in the United States.
As the leaching of salt proceeds and more clay and organic matter moves into the clay
pan, a distinct pale horizon (E) forms above a spectacular columnar structure, with the
top of the columns having a white amorphous silica coating. Soils with this striking
profile are known as
solodized solonetz
. As even more leaching takes place, significant
amounts of sodium are removed from the exchange complex and the B horizon structure
is lost. The resulting soil has a loose, coarse-textured, acidic A horizon over a hard,
compact B horizon with a pH neutral to acidic. This soil profile is a
solodic planosol
,
