Soil Salinization

 Introduction to Soil Salinization:

What is salinization?

Salinization is the level of salts, or the salt content,  in water or soil. All water on earth contains salt, even rainwater can contain salts, especially in the open ocean or in coastal areas. Although you should realize that in rain water the salinity is very much diluted. Even the water you drink contains salt you can check that yourself by looking at the label of a bottle of mineral water.

So what do we do with these salts? Are they harmful? I don’t think so because you and I are still alive! So the question remains: What do we do with these salts? Do we use them?

Only in very small quantities. The majority of the salts we drink each day, are leached, or washed out of our body, either by sweating or by going to the toilet. You can easily check that by tasting your own sweat. How do those salts come into the soil and in the water? Now we are talking about the process called ‘Salinization’, or how salt content increases in water and soil.

There are two types of salinization:

The first one is primary or natural salinization, which is caused by the weathering of rocks. Rain that falls on the ground, enters the soil and picks up the salts of weathering rocks. The second one is when the water, in which the salts are dissolved,  is used to irrigate a crop. This crop only uses the water and, unless there is adequate drainage, the salts are left behind in the root zone. This is called secondary or human-induced salinization.

Now ‘Soil salinization’ is defined as the presence of salts in the soil profile. This is again further classified into two groups:

Saline soils: These are soils that contain sufficient, neutral soluble salts that can adversely affect the growth of most crops. These soluble salts mainly consist out of sodium, chloride, sulphate, calcium or magnesium. High concentrations of these salts affect plant growth because they restrict the uptake of water by the roots of the plant. Soil salinity is expressed in electrical conductivity, or EC.

Sodic soils: Sodic soils contain sodium salts that can cause alkaline hydrolysis which means that the high level of exchangeable sodium will affect the physical characteristics of the soil like stability, water availability, and permeability, but it can also create toxicity to the crops. Sodicity is expressed as exchangeable sodium percentage, or ESP.

Salt-affected soils occur worldwide, in more than 100 countries, under almost every climatological conditions they occur. In total more than 1 billion hectares are affected. In irrigated areas this amounts to something like 10-16% of the total irrigated areas in the world.

Causes of Salinization:

Natural causes of salinity and sources of salt:

Rock Weathering: Significant quantities of sodium (Na), and to a lesser extent chloride (Cl), occur widely in the parent rocks from which soils form. Over time, rock weathering can lead to appreciable salt accumulation in soils if leaching is restricted. Did you know that rock weathering is the primary source of salt in seawater.

Sea water and accession of salt in marine sediments: Significant quantities of sodium (Na), and to a lesser extent chloride (Cl), occur widely in the parent rocks from which soils form. Over time, rock weathering can lead to appreciable salt accumulation in soils if leaching is restricted. Did you know that rock weathering is the primary source of salt in seawater.

Atmospheric Deposition: Salt derived from the sea, either deposited via rainfall or dry fallout, is the primary source of salt across large areas, e.g., many millions of hectares in southern Australia. In arid areas, salt can also be derived from dry lake beds and then be blown considerable distances by wind (e.g., Eurasia and parts of Australia).

Human-induced Causes:

The management of land and water resources is responsible for the development of human-induced saline and sodic soils. The main causes are:

1. Poor drainage infrastructure which induces a rise of the groundwater table. This is a major cause

    of soil salinization in India, Pakistan, China, Kenya and the Central Asian countries.

2. Use of brackish groundwater for irrigation. This is a major cause of secondary salinization in 

    parts of Asia, Europe and Africa.

3. Intrusion of seawater in coastal areas, for example in Bangladesh.

4. Poor on-farm water management and cultural practices in irrigated agriculture.

5. Continuous irrigation over very long periods, particularly in the Middle East.

6. Replacement of deep rooted perennial vegetation with shallower rooted annual crops and 

    pastures that use less water leading to the rise of saline groundwater, for example in southern 

    Australia.

 Measuring Salinity:

Soil salinization can proceed to the point that there are solid salt crystals on or in the soil. However, when that stage is reached, generally no plants are able to grow at all. Therefore, in this course which focuses on soil for life, we emphasize salinization before salt crystals form. In this stage, salt accumulates only in the soil water. So let's first find out how to measure that salt accumulation. The salt in the water, or the salinity of the water, is easy to measure. A century ago, when modern techniques and infrastructure were not always available, for example in desert regions, salt concentrations could be measured by simply evaporating water until only salt crystals were left. From the weight of the salt crystals and the original volume of water, the salt concentration of the water could be determined - in grams/dm3 or grams per Liter. Nowadays, lab facilities are often available. Therefore, we can use a more high tech approach to determine both how much salt, and which salts are in soil water. The units used for this are mol/dm3 or mol per Liter.

About the fastest way to measure salinity is to determine the electrical conductivity EC, because this conductivity increases almost linearly with salt concentration. The EC, which is the measure of how easily a solution can conduct electricity, has the unit of milli Siemens per centimeter. You may be familiar with EC in a slightly different way: the electrical resistance is one divided by EC. The electrical conductivity can be measured by just sticking a conductivity meter into the solution or into soil.

The EC may be very easy to measure, but sometimes it does not give enough information. The reason is, that plants can be more sensitive to some salts than to others. An example is that sodium can be quite toxic to roots of some plants, but not to others. If that may be important, more advanced chemical analysis in the laboratory is needed. Once you have the EC, it can be changed into other units, just as you can change money.

For example, an EC of 1 mS/cm represents a concentration of about 0.01 mol per liter of Sodium chloride, NaCl, or what we commonly know as table salt. For other salts, this conversion is a bit different, but can be looked up in hand books. If you know the name of the salt, you can find the molecular weight, and then calculate the concentration in grams per liter.

This degree of salinization depends on two major factors: one is the amount of water and the other, of course, the amount of salt. Consider that we have two beakers. In both beakers, we have the same amount of salt. Which of the two beakers has the highest concentration of salt, or the higher salinity?

The answer, which I bet you already figured out, is that the small beaker has the highest concentration. This is because concentration is the amount of salt divided by the volume of water.

For this reason, the small beaker with less water is more concentrated. The comparison is at the core of determining salt dynamics in soil. It's the kind of information you use to make a water and salt balance – which is what we need to do with the soil water or soil solution, to know what's going on in there.

Suppose the concentration in the small beaker is 1 mS/cm and in the large beaker it is 0.5 mS/cm

What is the concentration if half of the water in the large beaker evaporates? Yes, it goes up, from 0.5 to 1.0. We got that answer by dividing the original salt concentration (0.5) by the fraction of remaining water, which is also 0.5. Now, what would it be if 90% of the original amount of water evaporates?

Well, using the same approach – we take the original salinity and divide it by the fraction of remaining water – which now is only 0.1, and that takes it all the way up to 5 mS/cm which could create problems for plants! Finally, what would happen to the salinity of the smaller beaker if we doubled the amount of water? Yes! The salinity drops from 1 to 0.5 – because now there is the same amount of salt in twice as much water. With a basic understanding of the water and salt balance we can manage soil salinity, so that the soil can continue to support life.