Saline and Sodic Soils

Chapter 10

Mostly a problem in arid and semiaridregions but not always

Effects on Plant Growth

Salts decrease water potential so plant isdrought stressed at (higher / lower) soilwater content

Effect usually worse on young plant

This one.

Tolerance to varies with species

Percent yield on y-axis and increasing level of salinity on x-axis.

Development of Saline and Sodic Soils

Natural salinity

Rainfall insufficient to leach natural salts

Very high salinity at saline seeps

Irrigation induced salinity

Use of salty water for irrigation adds salt

Evaporation builds up salt at surfaceSaline soil

If Na concentration highSodic soil

Saline-sodic

OK, you can have naturalsalinity, especially where saltsmay concentrate, like in a seeparea, but you can also salinizea soil with long-term irrigation.This has ruined many an acre,around the world.

There are also sodium-affectedsoils and a combination.

Problem is avoided if drainage is good

Say you add a bit of salt with an irrigation. Next time you irrigate, add morewater than just enough to wet the root zone and water percolates deeper, dissolving and leaching the salt from the previous irrigation. Of course, to make this design work, you need good internal drainage.

Measuring Salinity and Sodium

Soluble salt concentration

Exchangeable Na percentage

Na adsorption ratio

For salinity

For sodicity

Soluble salt concentration

Electrical conductivity EC is measured

EC increases with solute concentration

Decisiemens per m (dS /m)

EC increase with concentration of electrolyte, right? Pure water is a poorconductor of electricity. The units of measurement are dS / m.

Work at the USDA Salinity Lab in California showed that conductivity is (ona log-log basis) related to ionic strength, and the latter is something of a measure of electrolyte concentration.

Exchangeable Na percentageESP

ESP = (Exchangeable Na+ / CEC) x 100%

This is an older measure of sodicity that has been replace in soil testinglabs by an easier but indirect measure (sodium adsorption ration, next slide).

Note the similarity of ESP with %BS.

Soils with high ESP have high pH

ESP = 15 % pH = 8.5

Higher ESPs pH > 10

High Na causes particle dispersion and very poor hydraulic conductivityThe above threshold may seem surprising but the basis is that even a 15%saturation of the CEC with Na means that there is little Ca- or MgCO3 in thesoil. Since both Ca2+ and Mg2+ are strongly preferred to Na+ adsorption, if therewas appreciable Ca- or MgCO3 present, the ESP would be lower. Further,in the absence of solid phase Ca- or MgCO3, the pH is not controlled by carbonate equilibria (keeping it about 8.5) so that it increases to plant-bad values.

Sodium adsorption ratio SAR

SAR = [Na+ ] / (0.5[Ca2+] + 0.5[Mg2+])½

Expressed as mmol (+) / L

The basis of this is something called the Gapon model, which is empirical,

NaEx / (CaEx + MgEx) = k [Na+] / ([Ca2+] + [Mg2+])1/2

The left hand side is close to ESP, NaEx / (CaEx + MgEx + NaEx + …) = NaEx / CEC

Types of Saline and Sodic Soils

Type EC SAR

Saline 4.0 < 13Sodic < 4.0 13Saline-sodic 4.0 13

< 4.0 < 13< 4.0 ≥ 13≥ 4.0 ≥ 13

>

SAR = 13ESP = 15

EC = 4

Sodic Saline and Sodic

Not Affected Saline

Saline soils

Salt high enough to hurt growth

But not enough Na+ to disperse colloids

White alkali soil

Salt deposited on surface of soil when water evaporates = white.

Sodic soils

Soluble salts low EC < 4.0 dS / m

But SAR > 13 high

pH 8.5≥ 8.5

High exchangeable Na+ disperses soil

Growth may be hurt by high Na+ and pH

Dispersed organic colloids soil surface

Black alkali soil

Saline-sodic soils

Characteristics of both

But high concentration of soluble saltslimits soil dispersion

Na is a dispersing cation. It isnot tightly held to negativelycharged colloids (figure left is supposed to show 2 parallelcolloidal particles, one pairwith tightly held Ca2+ and oneWith loosely held Na+, off-settingthe particle negative charge.Notice that for the Ca2+, therepulsive negative charges areshielded by a concentratedCa2+ zone but not so for thediffuse Na+ zone. So, Ca2+

is flocculating, Na+ dispersing.

However, if the concentration of cationcharge is sufficiently high, even with Na+

(figure to right), the negative charge ofadjacent particles is effectively shieldedso that particles can closely approach, i.e.,are flocculated. But decrease that high concentrationand they disperse.

Reclamation of Saline Soils

Flush soluble salts from soil profile

Water low in soluble salts and good drainage

Small excess of irrigation water may beapplied to prevent salt accumulation

Can sodic soils be reclaimed by flushingwith good water?

How about saline-sodic soil?

Reclamation of Saline-Sodic and Sodic Soils

No, colloids dispersed and hydraulic conductivity too small.

No, here too. See previous slide.

Add gypsum CaSO4

2Na+ + Ca2+ Ca2+ + 2Na+

Na2CO3 + CaSO4 CaCO3 + Na2SO4

Hydraulic conductivity is preserved orincreased and soluble salts are thenleached

█2Na+ + Ca2+ █Ca2+ + 2Na+

Notice, also, that a solid phase of CaCO3 forms, thereby dropping pH to ~ 8.5.So, there is a use for gypsum besides drywall.