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Irrigation
Salinity – out of sight isn’t enough

White crusts may have disappeared from around sloughs...


November 29, 2007
By Helen McMenamin

White crusts may have disappeared from around sloughs, and crops may have survived
closer to the sloughs than they usually do, but the situation is likely to be
short-lived, according to salinity experts.

"All the rain we had in 2005 has leached salts down through the soil profile,
so plants can germinate and grow," says Jack Payne, agronomy instructor
at Olds College in Alberta. "But, ground water pushing up from below overwhelms
salt-free rain sooner or later. The salinity will be back stronger than ever,
probably in spring."

Rob Dunn, conservation cropping specialist with Alberta Agriculture, Food and
Rural Development, agrees because he sees increasing problems with salinity.
"Annual cropping, especially systems with fallow, aggravate salinity because
they don't use enough water – their water use intensity is too low,"
he says. "Differing soil types, depth to ground water and other things
affect the time before salinity problems develop – it can be years or even
decades."

 88a
Jack Payne uses mini-fields to demonstrate the effects of salinity
for his crop diagnostics students. Barley has emerged well, but peas shows
patchy emergence that could be mistaken for herbicide damage.

Salinity is caused by ground water carrying salts that have been dissolved
out of subsoil layers, and then seeping up to the soil surface and evaporating.
The salts are concentrated at the soil surface when the water evaporates. Moderate
salinity signs can be more subtle than the classic white-ringed slough, but
worsening salinity problems are a sign ground water is building.

"Look for poor crops in low spots that should be more productive,"
says Payne. "A yield map is ideal, but sometimes you can just see the crop
is thinner there than on mid-slopes. Poor stands of peas can be a red flag too.
Peas are very sensitive to salinity and emerge poorly in salt-affected areas."

Soil test results that include EC values as low as '2' likely indicate salinity
problems, because salinity usually varies considerably over a field and most
soil tests are an average for the whole field. If you suspect salinity, soil
test the affected areas. An EC value of 2 to 4 in affected areas shows slightly
saline soils that can affect sensitive crops. EC of 4 to 8 reflects moderately
saline soils and 8 to 16 as severely saline where only saline tolerant plants
can survive. An EC greater than 16 means few plants can survive.

A salinity meter can give you a good idea of salinity right in the field. Check
that the salt-causing problem is the more common calcium sulphate and not sodium
sulphate. Sodium problems are much more difficult to handle.

Deal with mildly saline areas by growing barley rather than wheat. However,
that just solves a cropping problem and is not really a long-term salinity solution.
And, applying manure definitely does not help, says Payne. "Manure is high
in salt, so it won't help soil with high salt levels grow a crop," he says.
"Nothing you add to saline soil really helps."

Anatomy of a saline seep
"Salinity problems have two parts: a source and a seep," explains
Payne. "Water entering the source as rain or snow seeps through the soil
profile to an impermeable layer, then travels along it until it's close enough
to the soil surface to form a slough or a saline area. Construction can force
ground water to the surface – that's why saline seeps are often beside
a road."

"Eliminating fallow is the first step in managing saline land," says
Dunn. "Most annual crops only use water in the top one to 1.5 metres. Sunflowers
are the exception, rooting as deep as two metres. Diverse rotations with cereals,
pulses and oilseeds like mustard and canola can use a lot of water compared
to crop-fallow, but deep-rooted perennial forages increase water use intensity
even more.

"Salt tolerant grasses may be better in very saline areas. They can tolerate
salinity and lower water tables to promote deeper leaching of salts. Don't just
focus on the seep area, though: Surrounding cropland is the most likely ground
water source," says Dunn.

"To really solve a salinity problem, you have to stop the water at the
source," says Payne. "Finding the source can be a challenge because
the water follows an underground grade, which may or may not match the surface
contours. Once you've identified the likely source, the goal is to intercept
the moisture that's recharging the saline seep."

Alfalfa, which can extract water from a depth of five to six metres and even
intercept water from nearby upland areas before it enters the seep, is generally
the best way to do this. As the crop uses moisture the seep gradually dries
out, allowing rain and snow to flush salts deeper into the soil profile, so
plants can grow.

Farmers are often reluctant to 'give up' their best land to alfalfa. It is
true, the source area is often some of the best land. That is because salts
in it are leached down through the profile and carried to other parts of the
field.

"You don't need livestock to get a good return from alfalfa," says
Payne. "You may be able to sell it as a standing crop or have it custom
baled."

As the saline area below the seep improves, farmers are often tempted to take
out the alfalfa. "Resist that urge," says Payne. "If you put
the source area back into annual crops, the salinity will return."

Managing salinity is not always straightforward. Sometimes, salinity does not
improve after the alfalfa is growing well. The soil surface may conceal unexpected
gradients for ground water. A neighbour may own the recharge area, or drainage
patterns may have been altered by roads or other structures.

Some landowners opt for draining the seep rather than diverting water at the
source. However, this does not prevent salts coming to the soil surface with
ground water and salinization may continue.

Working with salinity experts to develop strategies to control the recharge
areas is the best approach to limiting salinity problems. Often, a landscape
approach involving many farmers is required but can be difficult to achieve.
What is important, though, is a long-term attitude to managing water because
in most saline areas, farmers are really farming water as their most valuable
and, paradoxically, their most damaging resource. 


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