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Soil moisture: the old and new stories

Know your subsoil moisture and your chance of rain to make seeding decisions

Weekly rainfall from September 26 to October 2, 2017.

The first freeze-up stubble soil moisture map was made in Saskatchewan in 1978. Readers with Henry’s Handbook of Soil and Water can see it on Page 109. It showed a lot of “very dry” and “dry” space. Red ink was common in the 1980s.

The maps below show the situation in fall 1987 and fall 1988. There was a lot of red ink on the map and also a lot of red ink on farm balance sheets.

In fall 1987 the west central area of Saskatchewan (Rosetown-Kindersley) not only had no stubble soil moisture but the summerfallow of the day was not fully recharged either. In those areas the 1988 rural municipal average wheat yields were under 10 bushels/acre. The 1988 rainfall for Rosetown in 1988 was four inches for May through July. Kinderley’s rainfall was six inches for the same period. That is as much rain as those areas had in 2017 when crop yields were great.

Saskatchewan stubble soil moisture at November 1, 2017. photo: Government of Saskatchewan

The old soil moisture story

The early soil moisture maps as of freeze-up each year were made on the assumption that the year’s crop had sucked the soil moisture completely to a depth of four feet. As the fall rains came (or not), maps of that rainfall were made using the excellent crop reporting and rainfall data service of Saskatchewan Agriculture.

Based on the rainfall maps, field verification was done by soil probing to determine the wetting front. The wetting front is the depth to which the rain wets the soil. It is very easy to determine.

Fall rains are usually system rains rather than local showers, so you can determine the wetting front with very few probes over a large area. I can map a large area in a one-day field trip. If no significant fall rains had occurred there was no need to do much field checking.

If the wetting front was down to 30 inches or more we would sometimes try to probe through the dry layer to determine the depth that residual moisture occurred — ie, “the other moisture.” Packrats can check out the April 26, 2016 issue of Grainews to read all about “the other moisture.”

The old moisture story was mostly written in a weather cycle of a 30-year (1975 to 2005) period of cumulative drought. We were living hand to mouth where many crops had to live off the rainfall of that growing season with scant reserves in the soil. The old moisture story did not take in to account that there might be some interaction between groundwater and available water in the soil profile.

The new soil moisture story

Starting with the big snow melt of 2005 and the many years of abundant rainfall since that time, the water excess to plant needs has raised the water table.

The water table rise is huge. For each inch of excess rain the water table will come up eight inches in a medium textured soil. Thus, the 10 inches of extra rain in 2010 would raise water tables 80 inches, 6.6 feet, two metres. Whatever unit you choose, it is huge.

In recent years the three province soil moisture maps in my columns in Grainews have been made like this:

The Alberta portion of our map uses the soil moisture map that Alberta Agriculture and Forestry produces and is available on their website. All I do is tinker the lines a little to fit into my legend and use the fantastic daily rainfall data for 300+ stations to help make the lines meet at the Saskatchewan border. Amazing what the glory days of high priced oil did for Alberta. Thanks to the folks that used some of that money to do things like their weather website.

For Saskatchewan the excellent rainfall data collected with the Saskatchewan Ag Crop Reporting service is the main resource. Each week 200+ crop reporters check a rain gauge and send in reports. Sask Ag also has a very good GPS arm that can make good maps of the rainfall data and those maps are recorded each week in the Crop Report.

I’ve included an example of the excellent rain maps made in Saskatchewan. Note the big blue band that carved a path from Swift Current to Hudson Bay. That was not a typical fall rain as there were thundershowers embedded in the system. At my farm I have two rain gauges about 150 m apart and they usually read very nearly the same. On October 2 one had 0.8 inches and the other had 1.25 inches. Soil probing on a very dry knoll on the far corner of the quarter suggested that even more rain fell there. That is why each farm should be determining what their situation is on various parts of the farm. That should be the starting point of any precision management.

For Manitoba I use the three-province rain maps made by Agriculture and Agri-Food Canada where the Manitoba data includes the excellent weather station network maintained by Manitoba Agriculture. Manitoba also has easy-to-access daily rain records that can be used to check out a point that is in doubt.

With the current wet cycle and much excess rain, it has been recognized that water tables are rising. A fifth category has been established to deal with areas where there is just too much of a good thing.

Managing for various soil moisture scenarios

Scenario 1:
Topsoil Moisture: None
Subsoil Moisture: None

Put the seed in the ground and pray for rain. No point in waiting for rain. If it does not rain soon and lots of it there will be no crop anyhow.

If regular and much-above-average rain happens a big crop is still possible. But, what are the odds?

If you are in a very stable financial situation put on enough nitrogen for a good crop. If the rains fail the N will still be there for next year.

If you have a big mortgage and few cash resources lay off the N and be ready to move quick with N if Mother Nature declares with a nice soaking rain of a few inches.

Now, that is a lot of “ifs.”

Scenario 2:
Topsoil Moisture: Good to ~six inches
Subsoil Moisture: None

Seed and hope that rain comes within about three weeks.

Scenario 3:
From three to five inches: moist
Above and below that: dry
Crop is wheat

This is a tricky one and is the exact situation we had in 2001. With a modern zero-till drill with narrow openers and on-row packing it is possible to seed right in to the moisture and the crop will come up real nice. If the rain is a long time coming then the yield potential will be set and good rain after will not recover the crop.

In 2001 my next door neighbour seeded wheat with just such a seeder and the crop came up real nice but a low yield potential was soon set. I seeded with the old discer and no N fertilizer. It came up very poorly and crop was at many stages all year. Good rain came June 11. The June through July total rain was six inches.

My neighbour’s yield was eight bu./acre; my yield was 20 bu./ac. Young folk will break out with laughter as both are a crop failure in light of our last eight big yield years. Hey kids, beware the other side of the average!

Scenario 4:
Topsoil Moisture: Excellent
Subsoil Moisture: Excellent

That is the exact situation we had in 2015. I seeded canola early in May and with the good moisture seeded very shallow. But, a very cold May with no rain made for very poor germination. The surface moisture was gone by the time the seed germinated.

A couple of mid-June showers germinated a bit more but it was a mess and made me sick. My neighbour that had seeded canola deeper had a nice looking crop and the good subsoil moisture carried it nicely through a huge drought that lasted until July 28 when 3.6 inches fell. I checked his fields and the canola that came up was all seeded at least an inch deep and some 1.5 inches. Not ideal in some years, but a game changer in that kind of year.

The lesson is this: if you seed too shallow you are assuming the crop will pop up nicely before the surface moisture is gone. Surface moisture can be literally Gone with the Wind in a few short days.

Scenario 5:
Topsoil Moisture: Top inch very dry but good below that.
Subsoil Moisture: Excellent

Seed to the moisture. If you lay it in the dust you are expecting Mother Nature to come along soon with a decent rain. What are the odds of that?

In summary

It all comes down to this: Moisture in the ground is a certainty, rainfall is a probability.

How can you make a good decision if you do not know what is certain and do not consult the rainfall probability for your area?

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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