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Les Henry: Soil moisture map for 2021

The situation shows a lot of red ink on the Prairies

Base map courtesy of Andrew Nadler PEAK HydroMet Solutions.

To start with, we must realize this is a very general, high-level picture of the situation. It is meant to do just that, as well as provide a framework for farmers and agronomists to relate their situation to. Having it available early in the new year, means it can be part of the decisions made over winter. With the very rapid adoption of soil moisture sensor probes to depth in farm fields, more cropping and input decisions are made based on known soil moisture reserves.

Let me first acknowledge the assistance of Kendall Gee and Bonnie Manziak of Crop Intelligence in making this map. We had a Zoom meeting and Kendall was able to show on my computer screen the site location map where the soil moisture probes were installed. I was able to direct them to areas where I was in doubt. In many cases, they were able to bring up data from one or more soil moisture probes. I could see on my computer screen, the moisture situation by depth for the entire growing season.

Apart from the information, I also learned some things necessary to make the soil moisture probe data useful. It is obvious the person interpreting the data must know the soil involved and other management practices and environmental conditions that affect moisture use.

In many situations, there were discussions between Kendall and Bonnie about the soil and cropping factors that could affect the result. For example, at one site the moisture content was much higher than nearby sites. The factor was a hail storm that cut off water use by the crop. It is not only the data but also the knowledge and experience of those interpreting the data that makes it useful in guiding farm management decisions.

The starting point November 1, 2019

We now know soil moisture can have a significant memory from year to year. That is particularly true where a near-surface water table is providing great yields with low rain.

At freeze-up in 2019, much of eastern Alberta and western Saskatchewan had very low soil moisture reserves. Good rains in May, June and July were needed. For the most part that happened, and fair to good crops were the result. However, in many areas, the tap turned off suddenly in early to mid-July and stayed like that until freeze-up. On my Dundurn farm, we had no useful rain after July 8.

At freeze-up 2019, much of southeast Saskatchewan and most of Manitoba had soils completely recharged with water and in some areas excess rain had raised the water table. The widespread drought in some of those areas had many talking about a pending disaster. However, those who knew the beginning soil moisture situation realized soil moisture reserves, particularly high water tables, would carry the day.

Base map courtesy of Andrew Nadler PEAK HydroMet Solutions. photo: www.peakhydromet.ca

A lot of red ink

The situation this year shows a lot of red ink (very dry soils) for much of southeast Alberta and a big swath of Saskatchewan. On my Dundurn farm near Saskatoon, we had 2.2 inches of rain from July 9 to November 1, with zippo added to the soil. Any rain was back to the atmosphere within a day or less. The Dry category even covers much of Manitoba. Mapping dry soil in Manitoba is rare. However, fall rains for soil recharge were almost non-existent. Looking back to the 2019 map (see above), we see areas in the Very Wet category with high water tables. In such areas, there could still be some of that in place in lower depths for use by 2021 crops.

When we made the first soil moisture maps for Saskatchewan in 1978 and the 1980s, it was essentially a “top-down” survey. The first step was to make rainfall maps that covered the fall recharge period, starting about mid-August. Most fall rains are system rains rather than thunder showers, so mapping was easy. The rain data came from Saskatchewan Agriculture crop reports with weekly rain data from approximately 300 crop reporters.

We checked the entry of fall rains using a soil probe to determine the wetting front. Super easy — we could do a huge area in a day. The assumption was the crop had sucked the soil dry to four feet, so the only moisture was from fall rains. If the soil probe went in much over two feet, we would try to push through the dry layer to see if there was any residual. But, that was not common.

After the past decade with above-average rains and elevated water table, the residual moisture can be a significant factor.

Table 1 (below) shows some scenarios that could be in place in areas mapped as Dry (yellow) in Manitoba and eastern Saskatchewan.
Scenario A in Table 1 is the situation at my Dundurn farm as of November 1, 2020. We had 2.2 inches of rain from July 9 to November 1, with zippo added to the soil. Any rain was back to the atmosphere within a day or less.

It is mapped in the Very Dry (red) category. If snowmelt and/or early spring rains manage to recharge the top foot we would be OK for crop germination and emergence, but timely rains will be needed to grow a crop. It will take more than five inches of gentle rain with no run-off to completely recharge the soil profile.

Scenario B could be the situation in much of Saskatchewan, that is, in the Dry (yellow) category on the map. It will take nearly four inches to recharge the soil.

Scenario C could be the situation in parts of Manitoba, and particularly in areas mapped in the Super Wet (grey) category in the November 1, 2019 map. In that scenario, three inches from snow and early rains would recharge the soil and you are good to go.

Farmers with soil moisture probes installed, maintained and interpreted by an agronomist, will have exactly that type of data and will be able to make very informed decisions based on facts, not probabilities. Water in the ground is a fact, rainfall is a probability.

This fall, I had a chance to visit the Ag in Motion site near Saskatoon, where there were several probes installed that measured soil moisture every 10 to 120 centimetres (four feet). My interpretation of some probe data is the water table was at a depth of 1.0 to 1.2 metres (four feet or less).

Recent distribution maps of soil moisture probes for 2020 show very rapid adoption of the practice. The first soil probes in the Prairie provinces were in southeast Saskatchewan in 2014. In six years, the practice has spread like wildfire. It has always been my experience that if a new technology works, farmers are all over it like a dirty shirt. Autosteer and GPS was a very good example. It looks as though soil moisture probes will fit into that category.

Managing inputs for a very dry soil

How to manage around a very dry soil depends on the type of farm. I have never been a big fan of pouring gobs of nutrients into very dry ground. However, there are many ways to look at it and many different farm types. We will look at just two.

The first is a multi-generation family farm with much land paid for, considerable financial depth and good operating relationships.

My suggestion for that type of farm is to go ahead and pour in the fertilizer as if there is going to be above-average rain. If it stays dry, nitrogen (N) fertilizer is going nowhere. It will be sitting there ready to go to work when it does rain.

In the dry years of the late 1980s, we saw that in spades at the Saskatchewan Soil Testing Lab. It was easy to see who had poured on the N — it was still sitting there and showed up in the fall soil test.

The second scenario is a young farm, highly leveraged (a polite way of saying lots of debt) with most of the land cash rented at high-per-acre rates.

For that situation, it is probably best to cut the N rate a lot and be prepared to dribble band liquid UAN (28-0-0) mid-season, if the rains suddenly kick in.

There is only one thing wrong with mid-season applications of N. Rain has to come soon after the N is applied to work. Not many have the connections to make that happen.

There you have it. Some ideas to think about as you plan the 2021 crop. I am really going to miss not seeing many of my readers at the Crop Production Show and many other winter meetings that keep one in touch. Stay safe and have a good 2021.

About the author

Columnist

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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