When I was at the Manitoba meetings in December, Don Flaten, soil science professor at the University of Manitoba, alerted me to a situation in Manitoba.
After many wet years and now a drier spell, salinity is being observed along the edge of some road ditches. There was a good example of road ditch salinity near Saskatoon I showed students on field tours. The extra water sitting in road ditches can be enough to raise the water table so salinity can form in the field next to the ditch.
Don said some farmers are planting alfalfa to “sop up” some of that extra water to arrest the spread of the salty area. I can not quote chapter and verse of an example where the remedy has worked but it makes sense — provided the salinity is not too high.
Alfalfa is not a very salt tolerant plant, but it is a huge “pig” for water and will result in large water table drop after a few years.
The classic diagram illustrating the ‘alfalfa’ effect on soil water is the one by Paul Brown (deceased) of Montana. Paul was our very gracious host in the 1980s when busloads of Saskatchewan farmers toured Montana to look at saline soils.
It is not that alfalfa sucks much more water per day than any other crop, but it takes up water for longer. When the snow goes, alfalfa is soon green and “sucking,” and it continues until the first serious frost in September.
In 2007 I had a good example of that in a new subdivision in eastern Saskatoon. The engineers could not figure out why the water table was so low (greater than 16 feet) in a few test wells. Of course it was the alfalfa. On an August weekend we had four inches of rain and it had no effect on the water table under alfalfa. The wetting front was only a couple of feet below surface. Adjacent summerfallow land would have a water table rise of about four feet.
So, the alfalfa strips on field edges in Manitoba should suck up a lot of water, provided the soils are not too saline. †