Understanding Sodic versus Saline Soil Management

You say alkali, I say saline. Are we talking about the same thing?

The term "alkali" is frequently used to describe soils that are high in salt. But sometimes people use the term to mean high pH, and at other times, it means high sodium. So we can discuss these problems in the same language, I have avoided the word "alkali" and defined the terms below.

In-Field Diagnosis
Some visual symptoms can be used to diagnose these problems, but ultimately soil testing is the best way for an accurate diagnosis.

High pH soil doesnít usually look any different from soil with neutral pH. However, sometimes the plants growing in that soil give us clues about the problem. High pH reduces the availability of some nutrients (zinc, iron, phosphorus). Therefore, symptoms of yellowing of middle to upper leaves (signs of zinc and iron deficiency) or dark green coloring with purpling of the lower leaves and stems (signs of phosphorus deficiency) can be signs of high soil pH. In particular, looking for symptoms can be useful when growing high pH susceptible plants (dry beans, silver maples).

Plants growing in saline soils may appear water stressed. This because the high salt content of the soil actually prevents water movement from the soil to the plant root. Water will naturally move from areas of low salt content to high salt content. Sometimes a white crust is visible on a saline soil surface. If a soil is both saline and sodic, a brownish-black crust sometimes forms due to dispersion of soil organic matter. By the time these crusts are visible, the problem is severe, and plant growth is usually minimal. Laboratory analysis can be used to diagnose these problems before the plant growth is so severely damaged.

Salt-Affected Soil Classification

Classification	Electrical Conductivity (dS/m) 1	Soil pH	Sodium Adsorption Ratio (SAR) 2	Soil physical condition
Saline	> 4.0	< 8.5	< 13	Normal
Sodic (alkali)	< 4.0	> 8.5	> 13	Poor
Saline-Sodic	> 4.0	< 8.5	> 13	Normal
High pH	< 4.0	> 7.8	< 13	Varies
1dS/m = mmhos/cm 2If reported as Exchangable Sodium Percentage or ESP, use 15% as threshold value.

Laboratory Diagnosis
Soil testing labs typically evaluate pH and EC (electrical conductivity) as part of a routine analysis. If the pH
warrants, analysis of the sodium adsorption ratio will also take place. The lab results can be evaluated with the following table.

Specific Ion Effects
Soil salinity is caused by accumulation of salts. It is a general problem from a combination of salts. However, sometimes a specific salt can cause toxic reactions in plants, which is separate from the general salt effect. Sodic soils are one example of specific ion effects. When sodic conditions are present, soils become dispersed and have low permeability. In addition to very limited water movement, the high sodium levels compete with calcium, magnesium, and potassium for uptake by plant roots.

Therefore, excess sodium can induce deficiencies of other cations (positively charged nutrients). High levels of other cations (calcium, magnesium, potassium) can also cause imbalances and induce nutrient deficiencies.

Anions (negatively charged nutrients) can also have specific ion effects. For example, sulfate and bicarbonate can cause shifts in the cation balance by reducing calcium and magnesium uptake and increasing sodium and potassium uptake. High chloride levels can cause burning in susceptible tree and vine crops and can reduce the
quality of potatoes. Excess boron in soils or water can also cause toxic effects, such as leaf cupping and burning of leaf margins.

The most common of these specific ion effects in Colorado is sodicity.

Types of Salinity Problems

Salinity hazard	affects	plants	can lead to	saline soil condition
Sodium	affects	soils	can lead to	sodic soil condition

After diagnosis, now what do we do?
For any soil-related problem, we have at least three options to correct the problem:

1. change the plant species to a more tolerant species, OR
2. change the variety to a more tolerant variety, OR
3. change the soil. Often, changing the soil is the most difficult of these options.

Salinity Solutions
The only proven soil treatment for high soil salts is leaching the salts out. In order for this treatment to work,
there must be:

1. adequate drainage and
2. acceptable irrigation water quality (sodium adsorption ratio < 10 %).

First of all, drainage must be improved. This can be accomplished with organic soil amendments or physical improvements like drain tiles or French drains.

There are some new products on the market which claim to enhance water infiltration into saline soils. They could possibly be beneficial in the leaching process, but we do not have local data on these products.

Sodicity Solutions
When soils are high in sodium, our goal is to replace the sodium with calcium and then leach the sodium out.
There are two possible approaches:

a) dissolve the limestone (calcium carbonate) or gypsum (calcium sulfate) already present in the soil, OR
b) add calcium to the soil.

If free lime is present in the soil, it can be dissolved by applying sulfur or sulfuric acid. The sulfur products can reduce pH which will dissolve the lime, thus freeing up the calcium. If free lime or gypsum are not present in adequate amounts as determined by soil test, then calcium will have to be added. The most common form of calcium used for this purpose is gypsum, although calcium chloride, which is more expensive, will react more quickly. After broadcasting the calcium source on the soil surface, incorporate it, and be sure adequate moisture is present to dissolve the gypsum.

Make sure drainage is adequate prior to amending the soil, and after application of a sulfur product or a
calcium source, then leach the sodium out as described above.

Remember!!!

1. Adding sulfur products only makes sense when:
   a) a soil is sodic AND has free lime present OR
   b) a soil is basic.
   
2. Adding calcium sources, such as gypsum or calcium chloride to saline soils only increases the salt content further and aggravates the salinity problem.

Jessica Davis
Associate Professor
Extension Soil Specialist