There are many problems faced by farmers while tilling the soil such as improper pH, erosion, or loss of organic matter and nutrients.
But one of the most serious and persistent problem over the history of agriculture, in so many of the drier regions of the world has been Salinity.
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As a broad way we can say that soils of humid regions tend to become acid, while soils of arid regions tend to become saline.
I’m Tapsendra Patel and in this article we will discuss soil with it’s Salinity issues and how to overcome from this problem.
Farmers in the more arid parts of the nation have a several soil related problem—an accumulation of soluble salts. This accumulation can became a major problem in dry climates where natural amount of rainfall cannot flush the salts out of the soil.
• This problem can also occur with poor drainage in irrigated fields.
Irrigation water imports salts into fields where there is poor drainage.
• Irrigation raises the water table by bringing salt rich water into the root zone of crops.
• The soluble salts of greatest concern in the soil are sulfates, bicarbonates, and chlorides of the bases calcium, magnesium, and sodium.
• These salts may come from parent materials(rock), irrigation with salty water, or even deicing salts.
In salted soils, salts cause a number of problematic conditions
• Primary among these are osmotic effects. In non saline soil, about half the water held at field capacity is available to plants but in saline soil, as little as 10 % may be available because of osmotic potential.
• Other effects include the following :-
• Roots can adjust somewhat to salted soil by raising their own solute levels to lower osmotic potential inside the cell and reestablish a potential gradient. But this takes energy that could go into growth and other functions.
• Specific ions, mainly chlorine and sodium may be taken up by plant roots and accumulate in plant tissue to toxic levels. These are called ion-specific effects.
• Stress on roots from salinity makes them more prone to root rot organisms.
• Nutrient imbalances in the plant can result from excess of some ions at the expense of others.
• For instance, high levels of sodium can induce potassium or calcium deficiencies.
• High soil salt levels inhibit populations of some soil microorganisms this can change the biological nature of the soil.
• In certain cases, extremely high pH occurs.
Saline soils have high level of soluble salts except (Na) sodium.
Measurement of Soil Salinity
Soil salinity can be easily measured by passing an electrical current through a solution extracted from a soil sample—
"the greater the salt content, the greater the electrical current".
The value is called Electrical Conductivity, or EC, and is the means by which we measure salinity.
For many common day-to-day uses, EC is measured as millimhos per centimeter (mmhos/cm).
For scientific publication, the units siemen per meter are preferred.
1 siemen/meter = 10 millimhos/centimeter
Saline Soil
A saline soil is defined as a soil with an EC of 4 or more millimhos / centimeter. However, salinity levels as low as 2 millimhos / centimeter can injure sensitive plants.
• Most salts are chlorides or sulfates.
• Soil pH is 8.5 or less.
• A white crust may be seen on the soil surface due to salts migrating to the surface by capillary rise - this is also commonly observed on the soil surface in potted plants.
Crops such as beans,lettuce, peppers, and carrots are more sensitive.
Sodic soils
Sodic soils are lower in the kinds of salts found in saline soils but high in sodium.
• The exchangeable sodium percentage (or sodium saturation) is 15 or more.
• pH is in the range 8.5–10.
• Sodium is often measured by the Sodium Adsorption Ratio (SAR). This ratio compares concentration of sodium ions with the concentration of calcium and magnesium ions.
Sodic soil has a number of effects on plant growth :-
• Sodium reacts with water to form lye which results in high pH 8.5 or higher which limits growth of most plants.
• For many crops the main effect of sodium is the destruction of soil structure.
• When sodium ions saturate cation exchange sites then the colloids separate from each other and disperse soil aggregates.
• Tilth suffers and crusts hard enough to stop seed germination.
• Plants may take up this much sodium to injure there internal tissues.
• For the most sensitive plants, such as citrus fruits, the nutritional effects of sodium are more important than its effects on structure.
Saline-sodic
Saline-sodic soils contain high levels of both soluble salts and sodium.
• EC is greater than 4.0 millimhos / centimeter.
• SAR is greater than 13.
• pH is less than 8.5.
• The physical structure of these soils is normal. However, after periods of heavy rain or irrigation with low-salt water, soluble calcium and magnesium may leach out of the soil, leaving behind sodium salts only.
• Soil may then become sodic with poor physical structure and drainage.
Reclamation
The basic step to reclaiming soil is to leach out salts, so there must be a source of acceptable water. Very fine-textured soils may not allow sufficient drainage.
If a decision is made to reclaim the soil, the next step is to ensure good drainage to allow salted water to leave the soil profile.
After proper drainage has been installed ⇒ the next steps depend on the type of problem.
Saline soils are most easily reclaimed.
• Farmers flood the soil surface so that percolation leaches salts out of the soil profile.
• High-quality water works best, but larger amounts of fairly saline water will also work.
• Ponding is one way to apply leaching water.
• Sodic soils cannot usually be reclaimed simply by leaching, because the sealed soil surface inhibits drainage.
"It is usually necessary to first remove the sodium. This is typically done by treating the soil with gypsum ⇒ When gypsum enters the soil ⇒ it dissolves and calcium replaces sodium on the cation exchange sites ⇒ The soil slowly begins to aggregate ⇒ sodium sulfate leaches out of the soil.
• Saline-sodic soils must be treated to remove sodium. If they are simply leached with low-salt water calcium and magnesium salts are removed but sodium remains in the soil, forming a sodic soil. Thus, gypsum treatments are useful.
• Saline soils, especially irrigated land in arid climates, may be managed to reduce salt problems. If possible, use high-quality irrigation water⇒ Keep soil moist ⇒ Water dilutes soil salts ⇒ lowering the effect of osmotic potential.
• Salts tend to be most damaging in dry soil, when the salts are concentrated and both osmotic and matric potential are high.
Over irrigate the field enough to leach salts out of the root zones. The amount of extra water needed is called the leaching fraction.
• Avoid over fertilization. Most fertilizers are salts and can increase the salinity problems.
• Maintain a good soil-testing program to monitor salinity and avoid over fertilization.
• Plant on ridge shoulders in furrow-irrigated fields. Salts tend to concentrate on the top of the ridge.
Use drip irrigation—it tends to reduce salt stress because it keeps the soil uniformly moist and moves salts out of the root zone.
• One difficulty with methods for reclaiming and managing salted soils is that they do not eliminate soluble salts but move them to another place. Salty drainage water reappears in rivers downstream of affected farms. Improving irrigation efficiency and practicing minimum leaching are examples of practices that minimize the problem. Where necessary, collection of drainage water in evaporation ponds can be done but it may be expensive.
• Farmers can tightly manage the fertilizer concentrations to avoid fertilizer buildup. If tests shows that the EC has increased too high then the growers can leach the pots with a heavy watering. This is done by watering pots heavily until water runs out of the bottom, allowing more salts to dissolve in it and then repeating the same procedure. Since this treatment leaches out all the fertilizer so this can also cause the nutrients loss the field.
In conclusion
• Salted soils may be Saline, Sodic, or Saline-sodic.
• Saline soils can be treated by flooding to leach out salts.
• Sodic soils are treated with gypsum to displace the sodium.
• Saline-sodic soils contain both soluble salts and sodium.
After a salted soil is treated, it must be managed carefully to reduce salt problems.
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