Problem of sodic soils.ppt

Problem of sodic soil

1. PROBLEMS OF SODIC
SOILS

2. Sodic soils
Sodic soilsare characterized by a disproportionately high
concentration of sodium (Na) in their cation exchange
complex.
occureence --withinaridtosemiaridregions.
Innately unstable, exhibiting poor physical and chemical
properties, which impedewater infiltration,water
availabilityand ultimatelyplant growth.

3. Sodic Soils
Sodic Soil
High sodium > 15% CEC
ESP = Exchangeable Sodium Percentage
High pH > 8.5
pH of Na
2CO
3in water is 9.5
“Black alkali” soils –dissolved OM and salt accumulation at
the soil surface.
Main problem is the effect of sodium on flocculation and
aggregation of soil particles.

4. Types of sodic soils
Saline alkali
presence of both saline and alkali.All stages in
transition with varying degree of dominance of high Ph .
Non saline alkali
also knowns as black alkali.
black owing to the effect of the high sodium content which
cause the dispersion of the organic matter
Degraded alkali
Formed through solodization process.

5.  Black alkali / Solonetz :Presence of precipitated CaCO3
 Degraded alkali / Solodi : Absence of precipitated CaCO3
– surface horizon acidic – sub surface horizon pH >8.5
Na
micelle + H2O ⇌
H
micelle + NaOH
 White alkali / Solonchak : Saline soils with white
encrustation on soil surface.
 Brown alkali : Saline soils with high proportion of NO3

6. EC ESP pH
Saline >4.0 <15 <8.5
Sodic <4.0 >15 >8.5
Saline-sodic >4.0 >15 <8.5 or >8.5

7. WHAT IS SODICITY?
Sodicity is a term given to the amount of sodium held
in a soil.
When sodium makes up more than about 5% of all cations
bound to clay particles, structural problems begin to
occur, and the soil is said to be sodic.
The amount of sodium as a proportion of all cations in a
soil is the main measure of sodicity used, and is termed
the exchangeable sodium percentage (ESP).

8. Saline and Sodic Soils
Sodic Soil
Flocculation Van der Waals Attraction
Short range attraction
Ca
2+
Ca
2+
Na
+
Flocculated
Small hydrated
ionic radius
Dispersed
Large hydrated
ionic radius

9. Sodic Soils
Sodic Soil
Sodium causes the soil particles to disperse
No aggregation of soil particles
Individual clay particles clog soil pores
Poor soil physical properties
Very difficult for water and air movement and root penetration
Thus Plants cannot grow in sodic soils.

10. WHY IS SODICITY A PROBLEM?
High sodicity causes clay to swell excessively when wet.
The clay particles move so far apart that they separate
(disperse).
This weakens the aggregates in the soil, causing
structural collapse and closing-off of soil pores.
For this reason water and air movement through sodic
soils is severely restricted.

11. In vegetable crops, sodic layers or horizons in the soil
may prevent adequate water penetration when during
irrigation, making the water storage low.
waterlogging is common in sodic soil, due swelling and
dispersion closes off pores, reducing the internal
drainage of the soil.
Sodicity of the surface soil is likely to cause dispersion of
surface aggregates, resulting in surface crusts.

12. Problems due to Alkali soils
P,Cu,Zn, Mo and Fe become less available in presence of free
carbonate ions as these are ppt. to various insoluble forms.
Poor organic matter content ,contain very low amount of
available K.
EXCESS exchangable sodium and high ph results in dispersion
of clay particals in turn block soil pores.
Hardening and crust formation of surface soils upon drying
cause hinderence to to seedling emergence.

13. Infiltration and permeability of alkali soils is very poor.
Soils are compact and dense and offer mechanical impedance to the
growth of roots..
Permits excessive runoff loss as moisture retention capacity of such
soil is very poor .
Soil erosion and run off loss high due to low moisture retention
capacity.
Impedance of downward root growth and capillary movement of
moisture within the profile.

14. Plant growth on sodic soils
The activity of Ca,Mg,and micro nutrients like Zn, Fe, and Mn
effectively depressed by the relative high percentage of adsorbed
cations.
The activity of the hydroxyl and carbonate ions may be sufficient to be
toxic to the plants.
Excess sodium toxic to plants causing imbalance due to antagonastic
effect on K ,Ca nutrition.
An accumulation of adsorbed Na on the exchange complex may have a
dispersive effect on soil aggregates
Resulting poor water permeability.
For most crops ,calcium becomes unavailable when the exchangeable
Na percentage approaches 50.

15. Problems of sodicityin various
condition.

16. Self-mulching clays
These soils are well structured and non-sodic at the
surface.
There is generally more calcium rather than sodium
attached to the clay particles; this is why self-mulching
clays are well structured.
The deeper subsoil of these soils can be sodic, so
waterlogging is possible.

17. Non-self-mulching clays
These soils are sodic at or near the surface; the sodicity
increases with the depth. Therefore, these soils are
likely to have water storage and waterlogging problems.
Establishment of crops is often difficult due to crusting
and poor tilth.

18. Red brown earths
The topsoils of red brown earths are usually non-sodic,
and relatively low in clay content.
The subsoils are generally sodic and higher in clay
content. This means that water penetration of the
subsoil is low.
Therefore a ‘perched’ water table can form above the
sub soils of red brown earths.
Since water penetration of the topsoil is generally good,
the deeper the topsoil the more water can be stored at
each irrigation.

19. Transitional red brown earths
The top soils of transitional red brown earths sodic.
This will cause crop establishment problems such as
crusting.
The sub soils are generally sodic and will therefore swell
and restrict air and water movement.
If the topsoil is non-sodic, water will move though the
topsoil relatively rapidly, but only very slowly into the
subsoil.
Water storage may be poor if the topsoils are shallow.

20. Sodic Soil -Management
Leach with low Na
+
, salty water to exchange Ca
2+
for Na
+
Gypsum often added to create salty, low sodium water
Na-Soil + CaSO
4→Ca-Soil + Na
+
+ SO
4
2-
Leach with clean water to flush out Na
+
& SO
4
2-
Very difficult because you can’t get water into the soil to do the
leaching.
tolerant or semi tolerant crops, such as rice, sugarbeet, wheat, barley,
amla etc.

21. Saline and Sodic Soils
Sodic Soil -Management
Reduce ESP < 15 % of CEC
Use low sodium irrigation water
Sodium Adsorption Ratio (SAR)
Ion concentrations in irrigation water (meq/L)
SAR = [Na
+
] / √([Ca
2+
] + [Mg
2+
])/2
ESR = 0.015 SAR
SAR > 13 will give ESP > 15%
Figure 3.25 Textbook Classification of Irrigation Water

22. Sodic Soil -Management
Use of Gypsum
Na-Soil + CaSO
4→Ca-Soil + Na
+
+ SO
4
2-
Provides Ca but doesn’t increase pH like limestone
Perception that adding Gypsum will always improve soil
structure
True for sodic and saline/sodic soil but not in most other
cases if Ca is adequate and Na is low
In acid soils we don’t have Na and we get plenty of Ca from
limestone
Amount of Gypsum to provide equivalent Ca
2+
to exchange
with Na
+
.

23. Summary of Sodic Soils
Figure 3.21 in Textbook
Electrical Conductivity (mmhos/cm)

24. References
Biswas, C.R. and A.K. Bandyopathyay. 1987. Agronomy of rice in
coastal saline soils –a review. Journal of Indian Society of Coastal
Agricultural Research.
Biswas, T.D and S.K. Mukherjee, 2005. Textbook of Soil Science: 2nd
edition. Tata McGraw-Hill Publishing Company Limited, New Delhi.
Gupta, R.K. and I.P. Abrol. 1990. Salt–affected soils –Their reclamation
and management for crop production. Advances in soil science.
Sehgal, J, 2005. A textbook of Pedology: Concepts and applications: 2nd
edition. KalyaniPublishers, Ludhiana.