4. Definition
•
• Alkaline soils are soils (mostly clay soils) with a
high pH (> 9) and a poor
• soil structure and a low infiltration capacity. The
are not saline, i.e. the
• total amount of soluble soils, especially sodium
chlorides, is not excessive
• (ECe < 4 to 8 dS/m). Often they have a hard
calcareous layer at 0.5 to 1 m.
• depth.
5. Causes
• The causes of soil alkalinity are natural or they
can be man-made. The
• natural development is due to the presence soil
minerals producing sodium
• carbonate upon whethering. The man-made
development is due to the application
• of irrigation water (surface or ground water)
containg a relatively high
• proportion of sodium bicarbonates)
6. Alkaline and Saline Soils
•Saline soils occur in soils with pH>8.5
•Ca2+, Mg2+, K+ and Na+ do not produce acid upon
reacting with water
•The do not produce OH- ions either, but in soils with pH>8.5,
there are higher concentrations of carbonate and bicarbonate
anions (due to dissolution of certain minerals)
CaCO3 Ca2+ + CO3
2- or NaCO3 2Na2+ + CO3
2-
CO3
2- + H2O HCO3
- + OH-
HCO3
- + H2O H2CO3 + OH-
H2CO3 H2O + CO2(gas)
•pH rises more for most soluble minerals (eg. NaCO3)
•pH rise is limited by the common ion effect
7. Nutrient deficiencies in saline soils
•Fe deficiency common because its solubility is
extremely low in alkaline conditions
•Addition of inorganic fertilizer may not improve
this deficiency as they quickly become tied up in
Insoluble forms
•Chelate compounds are often applied to soils (Fe
associated with organic compounds)
• Under high pH, B tightly adsorbs to clays in an
irreversible set of reactions. In sandy soils, B content is
generally low under any pH level (especially acid soils).
8. Effect of soil pH
on nutrient content
and soil
microorganisms
9. Salinization
The process by which salts accumulate in the soil
Soil salinity hinders the growth of crops by lowering the osmotic potential of the soil, thus
limiting the ability of roots to take up water (osmotic effect). Plants must accumulate organic
and inorganic solutes within their cells.
Specific ion effect: Na+ ions compete with K
+
Soil structure breaks down, leading to poor oxygenation and
infiltration & percolation rates
•36% of prairie farmland has 1-15% of its lands affected by
salinization and 2% has more than 15% of its lands affected.
•Most prairie farmland (61% in Manitoba, 59% in Saskatchewan, and 80% in Alberta) has a
low chance of increasing salinity under current farming practices.
10. Conditions promoting salinization:
•the presence of soluble salts in the soil
•a high water table
•ET >> P
These features are commonplace in:
•Prairie depressions and drainage courses
•At the base of hillslopes
•In flat, lowlying areas surrounding sloughs and shallow water bodies.
•In areas receiving regional discharge of groundwater.
16. Where
• The extent of alkaline soils is not precisely
known. Important research on
• alkaline soils has mainly occurred in Central
Europe and North India (above
• the Ganges river), where alkaline soils occur
frequently
17. Problems
• Alkaline soils are difficult to take into
agricultural production. Rainwater
• stagnates on the soil easily and, in dry periods,
irrigation is hardly
• possible. Agriculture is limited to crops
tolerant to surface water logging
• (e.g. rice, grasses) and the productivity is low.
18. The presence of abundant Na+
• ions and the precipitation of Ca++ ions causes
• the clay particles, which have negative electric
charges along their
• surfaces, to adsorb more Na+
• and, in exchange, release Ca++, by which their
• exchangeable sodium percentage (ESP) is
increased as follows:
20. Clay Particles
• Clay particles with considerable ESP (> 16), in non-
saline conditions, and in
• contact with non-saline water occupy a larger volume
than otherwise, because
• the Na+
• ions are quite mobile and have smaller electric charges
than Ca++
• ions, hence they are adsorbed less coherently to the
surface of the clay
• particle and they float farther away: the soil swells
21. • The phenomenon is called sodicity and results in
deterioration of the soil structure, and especially crust
formation and compaction of the top layer.
• Hence the infiltration capacity of the soil and the water
availability
• in the soil is reduced, whereas the surface-water-logging or
runoff is increased. Seedling emergence and crop
production are badly affected.
• Under saline conditions, the many ions in the soil solution
counteract the swelling of the soil. Certain clay minerals
with 100% ESP (i.e. fully sodium saturated) are called
bentonite which is used in civil engineering to place
impermeable curtains in the soil, e.g. below dams, to
prevent seepage of water.
22. • The problems of alkalinity and sodicity go hand in
hand. Alkaline/sodic soils are not necessarily
saline, and the alkalinity problem is worse as
thesalinity is less. Alkalinity problems are more
pronounced in clay soils than in loamy, silty or
sandy soils. The clay soils containing
montmorrillonitic or smectitic minerals (swelling
clays) are more subject to alkalinity problems
than illitic or kaolinitic clay soils. The reason is
that the former types of clay have larger specific
surface areas (i.e. the surface area of the soil
particles divided by their volume).
23. Solutions
• Alkaline/sodic soils with solid CaCO3 can be
reclaimed with grass cultures, ensuring the
incorporation of much acidifying organic material
into the soil, and leaching of the excess sodium.
Deep ploughing and incorporating the calcareous
subsoil into the topsoil also helps.
• It is also possible to reclaim alkaline soils by
adding acidifying minerals like pyrite (see next
section on acid sulphate soils).
• If necessary, gypsum (calcium sulphate, CaSO4)
can also be applied as a source of Ca++ ions.
24. • To reclaim the soils properly one needs
prohibitively high of doses of amendments.
Most efforts are therefore directed to
improving the top layer only (say the first 10
cm of the soils), as the top layer is most
sensitive to structure deterioration. The
treatments, however, need to be repeated in
afew (say 5) years time It will be important to
refrain from irrigation with poor quality water.