Weather, soil moisture and crops

Your definition of drought will depend on the amount of subsoil in your fields

Weather, soil moisture and crops

My 2016 barley crop was seeded May 8 into excellent moisture at a depth of 1.0 to 1.5 inches — a bit deeper in some soft sloughs. Up to the date of seeding there had been essentially no spring rains. A May 4 burn off with 2,4-D + Glyphosate took care of the volunteer canola and pigweed etc. to conserve surface moisture.

After a very dry 2015 spring and early summer, rains starting July 28 (3.6”) had the soil fully recharged with moisture by canola swathing time (mid August). Planning for the 2016 crop started at that point. Mother Nature had declared the savings account was fully topped up. Any further rain would just raise the water table, which was already too high in parts of the rolling landscape.

Once fully recharged we know that the subsoil moisture is in place and will be there until a plant root comes and sucks it out. On the other hand surface moisture is fickle — here today, gone tomorrow. Planning is all about preserving surface moisture.

A September 20, 2015 application of 2,4-D plus one liter/acre glyphosate took out the volunteer canola and annual weeds. On November 6 anhydrous ammonia was knifed into excellent moisture at 3.0 to 4.0 inches. A warm day, November 8, was the opportunity to tine harrow at 6.5 mph to lay the canola stubble as a mat over the surface to reduce evaporation losses in spring.

Winter snow was little and gone by April 10 followed by essentially no rain and much hot weather until seeding. The surface moisture was conserved, the crop will germinate and if it does not get too hot, no rain is needed until about mid-June.

The drought

I get very frustrated by some of the media reports about the “big drought” and how badly off we all were. In Western Saskatchewan and parts of Alberta it was (and is) a bad scene. But in today’s age with all the instant communication why can we not effectively communicate something as simple as the geography of soil moisture.

The only three-province soil moisture map as of freezeup each fall is that published right here in Grainews. This old fossil makes the map so I know it is not perfect, but it does show that in most of Manitoba and Eastern Saskatchewan a little dry spell in the spring is no big deal. With the soil full of water, as long as seed is placed in moisture, the crop will be fine for a long time. If surface moisture is not managed the situation is much different.

With the early and very warm spring I have seen several fields with very healthy growth of winter annuals like flixweed that have almost completely sucked the surface moisture. The winter annuals got a great start last fall with a well-developed root system all ready to suck away starting almost as soon as the snow is gone. The hot days (some 30 C) and lots of winter annuals set the stage to lose the all important surface moisture.

There is a very cheap and very old solution to the winter annual problem: a simple application of 2,4-D in fall, usually October. With all the new, exotic, and expensive herbicides we sometimes forget about the old, cheap and effective solutions.

We had one inch of rain May 10-11 so even if surface moisture was gone all crops will now germinate. It is now all about how long our soil full of water will keep us if rains fail.

Once the crop is up, Job 1 is to establish an effective root system to suck the moisture from the soil to maintain the plant. Serious soil water depletion does not happen until mid June in most years.

When fully recharged:

  • Light, sandy loam soils hold one inch of available water per foot, for a total of four inches per four feet.
  • Medium, loam/clay loam soils hold 1.5 inches of available water per foot, for a total of six inches per four feet.
  • Heavy, clay/heavy clay soils hold two inches of available water per foot, for a total of eight inches per four feet.

The rate at which a crop uses water depends on temperature. Table 1 tells the tale. Table 1 is based on wheat, but other crops use about the same amount of water, once a full plant canopy is established. Different rooting depths will change the depth from which water can be extracted.

From Henry's Handbook, Table 7.1, page 133. Chapter 7 tells the full story.
From Henry's Handbook, Table 7.1, page 133. Chapter 7 tells the full story.

So, with a simple calculation it is easy to see know how long a crop will last if the rains stay away. A clay soil full of water should maintain a crop for about 40 days at moderate temperatures (in the mid 20s).– From Henry’s Handbook, Table 7.1, page 133. Chapter 7 tells the full story.

But, if the soil is not full the situation is much different. For example, if a soil is full of water only to two feet, once that water has been used up it is all over in a few hot days in July. We have not seen that in recent years so some young folks have not experienced it. But they will.

On the other hand, when the water table is near the surface, capillary rise from the water table can maintain a crop indefinitely. We saw that in spades in 2015.

It is my hope that weather types will get better informed about soil moisture and provide more precise comment on a regional basis. Hollering “drought” after a few of weeks of sun and warm leaves the wrong impression in the industry and the rest of society.

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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