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Natural sub-irrigation

In December it was my good fortune to attend the Manitoba Agronomists Conference in Winnipeg. My duty was to deliver a lecture on saline soils. It has been dry the past two summers in Manitoba, and black soils have turned white — as predicted in this column a year or two ago.

It was my pleasure to listen and learn from many other speakers. A special presentation was that made by Hans Kendel of North Dakota State University, Fargo. The topic was soybean management but it included unique information on soil water management. Many parts of the Red River Valley have poorly drained soils that can be wet in spring. Tile drainage is taking place on more acres each year.

Installing “check” valves in drain tiles is a new technology to control the water tables. Check valves maintain the water table close enough to surface to provide additional water for crop but deep enough to avoid problems. Thus the crop receives sub-irrigation — that is to say irrigation from below by “sucking up” water from the capillary fringe above the water table

On my rolling land at Dundurn we have had three irrigation years in a row — 2010, 2011 and 2012. In 2010 we had an unprecedented 20 inches of rain from May to September. That is much above crop requirement and the excess went towards raising the water table.

In 2010 and 2011, while soil probing to follow water use by the crop, I noticed that the crop kept growing but the soil stayed moist. I have been soil probing long enough to know when I am close to the water table and that is what soil probing showed.

When the same thing was happening in 2012 I decided to install a couple of shallow observation wells by hand to see where the water table was and how it was changing — and to convince the neighbours that I was right about the high water table. Nothing like a good splash to settle an argument!

All it takes is a dutch auger with a couple of extensions and a 10 foot length of the two-inch PVC pipe used to install Central Vac in homes. Solvent weld (glue) a cap on the bottom and hacksaw slits for about the bottom half of the pipe.

It takes a bit of technique — turn the auger only one and a half turns and then extract. If you pile it full expect back problems when trying to pull it out.

In medium or heavy (clay) soils you can auger well below the water table without any water filling the hole. In sand your augering is over when the water table is reached. Installing in sand can be a problem as it is hard to get the pipe very far below the water table.

Once it’s installed, leave it sit a day or more to reach equilibrium (static) and then measure with a well sounder or device to get a measure to the nearest centimeter.

Be careful not to tramp down the crop — it has to use up the water!

Results from the well

On July 9 the wheat crop still looked pretty good — should have doused it with fungicide — by July 20 it was not as pretty.

The water table depth from ground surface at that site is shown in the table. The well was pulled on August 27, as harvest was near.

As you can see, when the crop quit using water, the water table stayed put. It will gradually drop over winter but spring snowmelt could revive it again. The soil never did dry much below field capacity, and the earthworms stayed in the topsoil until well into September.

It is clear that the crop was living off the water table, as the soil stayed moist even though there was little rain in July and August:

July 4: 0.4”

July 16: 0.9”

August 3: 0.4”

August 25: 0.9”

The August rain was just enough to bring the soil back to field capacity and not raise the water table.

At that elevation and lower the crop was being sub-irrigated by Mother Nature. On my farm, natural sub-irrigation is not a common occurrence. Usually the crop uses up the water as fast or faster than rain replenishes it and the soil dries out from the top down.

But in many thick black soil areas on level topography sub-irrigation is a common occurrence. The soils are thick black in part because of the higher rainfall regime they are in but in part because of sub-irrigation.

That is why they are thick black. The grass grew like crazy even with dry weeks in summer.

So, if you are curious about where the water table is on your farm — get your friendly ag advisor to install a water table or 10 to check it out. If this old fossil can install a well in 20 minutes surely you kids can do it! †

About the author


Les Henry

J.L.(Les) Henry is a former professor and extension specialist at the University of Saskatchewan. He farms at Dundurn, Sask. He recently finished a second printing of “Henry’s Handbook of Soil and Water,” a book that mixes the basics and practical aspects of soil, fertilizer and farming. Les will cover the shipping and GST for “Grainews” readers. Simply send a cheque for $50 to Henry Perspectives, 143 Tucker Cres., Saskatoon, Sask., S7H 3H7, and he will dispatch a signed book.



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