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Controlling salt in the soil

Les Henry says soil salinity is a water problem, not a soil problem. 
Find out how to cope with salt on your farm

Large parts of what are now the Canadian Prairies were once the beds of shallow seas. All that salt water gradually evaporated and left huge deposits of salt clays. Consequently, there are places where groundwater can dissolve those old salts and deliver them to the surface, making farming difficult or even impossible.

Farmers who own land prone to salinity have to learn to cope with it.

“The first thing we have to recognize is that soil salinity is not a soil problem,” explains farmer, Grainews columnist and retired University of Saskatchewan professor Les Henry. “It’s a water problem, so we have to focus on water.”

There are two reasons for this. First, groundwater can move from one place to another, gathering dissolved salts as it goes. Second, it’s about the water table — the measured distance from the surface of the ground down to the level where the oozing water saturates the soil. The closer the water table is to the surface, the more serious the potential problem.

Salts in Prairie water

So what are the salts? Henry says, “In Western Canada, the major problem is magnesium sulfate. There are others of various solubilities but most of our trouble is caused by magnesium sulfate.”

In fact it’s the salts in the sulfate family that you usually find in Prairie water. Calcium sulfate (gypsum) and calcium carbonate (lime) are fairly common in Prairie subsoil but they’re relatively insoluble. Since they won’t dissolve easily into the groundwater they tend to stay where they are.

On the other hand, chloride salts of sodium, calcium and magnesium are soluble and love to travel with the groundwater. Fortunately they’re not common so they’re limited to a few small areas. It’s the sulfate salts that dominate the saline portions of the prairies.

So we know it’s in the water and we know it moves. The next question is: how does it get to the surface? Simply put, it’s carried there by the groundwater but not because a flowing spring brought it there. This is where the water table — that critical distance from the water table to the soil surface — becomes important. The closer it is to the surface, the greater the potential problem.

Moving it to the surface

Water is drawn to the surface through a process called capillarity — the same process plants use to move water from the roots to the leaves. Water can move through microscopic pores, some of which are almost small enough that the individual molecules have to line up and move single file. The combination of surface tension (cohesion between water molecules) and adhesive forces between the water and the surrounding pore material actually causes the water to move upward against gravity. The water takes the salt along for the ride, but when they reach the surface, they part company.

The water evaporates into the air and leaves the salt on the ground where it can form a crust. Generally, if the water table is within two meters of the soil surface the capillary action on the water will bring it all the way to the surface.

“So soil salinity is caused by a high water table, near the surface and close enough that suction lift can bring it up to the surface concentrating the salts there.” Henry explained. “You need to have evaporation greater then precipitation so groundwater movement is required to maintain that water table during the dry periods.”

What keeps that capillary action from working is a ready supply of fresh water coming from the other direction, forcing it the other way. Any form of precipitation, either rain or spring snowmelt, helps to maintain a better quality of soil moisture. As fresh water percolates into the soil it forces the more saline groundwater lower in the profile.

As long as the upper levels of the soil have fresh water, crops will use it quite happily. But if it’s not replenished the salt-rich water from lower down will be drawn to the surface. The crops will take on a bluish tinge and fail.

This has been going on since the last glaciers retreated.

That is the other dimension we have to consider with moving groundwater: time.

“To think about salinity you have to think like a geologist and you have to think in four dimensions.” Henry said. “You’ve got ‘how wide,’ and ‘how broad’ and ‘how deep’ and ‘how long.’ Salinity is the net water movement at the soil surface over ten thousand years.”

So it’s a combination of geology and groundwater movement. There’s nothing new about this, it’s been going on since the last ice age ended.

Controlling salinity

The next question is, “What can we do about it?” Can we control soil salinity?

The answer is an unqualified maybe. These groundwater systems can be fairly complex so controlling soil salinity can be costly and risky. The best hope for reversing salinity is in areas where it’s a recent phenomena or where it may be seen to fluctuate.

The first thing to do is a detailed investigation of the local water quality in nearby wells. It helps to check local soil maps to see what they might tell you.

The next phase, if it’s worth the cost, is to drill into the ground to determine the basic geology and it’s capacity to carry water. At this point either piezometers or a water table well can be installed to monitor the water table and see when it’s nearest the surface. Or Henry says you can get a rough idea of that on the cheap.

“You need a Dutch auger with two extensions.” he says. “You go to the local hardware store and buy 10 feet of thin PVC pipe that they use for central vac, you put a plug on the bottom and you put it in the ground. I’m 72 years old and I can put one of those in in about 20 minutes.”

If you install slotted pipe the level of the water inside will be the actual level of the water table. If the pipe isn’t slotted the level of the water inside the pipe will indicate the “piezometric head,” — the level maintained by the groundwater pressure. The next step is reclamation.

“Reclaiming saline soils is draining plus leeching. Drainage or leeching alone won’t do it, so do the two of them together.” Henry suggests. “If it’s calcium and magnesium salts, you drain it and leech it, as simple as that. It’s very easily reclaimed. If you have saline sodium salts then you may need to add gypsum and then drain and leech, but for most of the situation that we have all you need to do is drain them and leech them and they reclaim very quickly.”

There are two other possibilities. Some native plants cope with salt better than crops so it may even be best to seed to grassland and use the land for grazing.

“The best way to manage saline soils is to sell it in the winter time.” Henry laughs. On a more serious note, he says, “To understand soil salinity you must understand the ground water movement and there’s no spray or spread solution. If you’ve got a serious problem the only thing you can do is to plant it down.” †

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