Rain and roots: not always simple

When we experience a wet May you often hear someone say, “the crop will be shallow rooted because the moisture is near the surface and the crop doesn’t have to root down.”

With a soil probe, I’ve seen for many years that a crop roots down very nicely in soil at the field capacity moisture content. In 2010 we had 11 inches of rain by July 1 and another three inches in July. Probing showed wheat roots down three feet or more. At the end of July the moisture was half gone in the top foot, but still at field capacity below that.

In 2010, folks at the Agriculture and Agri-Food Canada Scott Research Farm used a much more sophisticated technique to study rooting of several crops. They installed plastic tubes where a soil core had been removed and got actual pictures of the root. Despite 14 inches of rain, they found very deep rooting in July — sometimes deeper than the one metre soil depth they were studying.

So, roots grow down quite well in field capacity soil despite good rain early in the growing season. However, crop roots do not grow down through dry soil.

Roots in dry soil

If a soil is only moist to two feet or less at seeding and bone dry below that, early season rain can keep the crop growing nicely. But if July rains fail and the crop has depleted the soil moisture, then it is all over but the shouting in a few hot days. That circumstance led to the “old wives tale” of early rains resulting in shallow rooted crops.

If a crop hangs on despite July drought, it is because of lots of deep soil moisture. The roots find it just fine, thank you very much!

Often an inch of rain in late July is credited with “saving” a crop. I remember a good rain many years ago on the weekend of the Craven Jamboree in late July. (If you have not heard of Craven jamboree you must live on the moon. Google it.).

That fall, my soil moisture probing for the Stubble Soil Moisture Map showed that the July rain was still resting comfortably in the top six inches or so. By the time the rain came the crop was past the high water use stage.

All this talk of dry soil may have you thinking that this old fossil must be on the moon. Doesn’t he know we’ve had a string of wet years?

That brings me to another scenario: The soil is at field capacity moisture at seeding time and the water table is within six feet or so of the soil surface. Then there are buckets and buckets more rain. When a soil is at field capacity moisture and more rain falls, the result is a rise in the water table. In a clay soil, an extra inch of rain will bring the water table up a foot. Less in medium or sandy soils.

That might mean that the water table is now two feet or less from the soil surface. Now, roots do say whoa. They will not root into the water table. Now you have a shallow rooted crop that will do very poorly.

If you see water gathering in the ruts of the sprayer, you know the water table is right there and crops will indeed be waterlogged.

The bottom line

The take home message is this: A crop does not need a dry spell to “force” the roots down.

A crop will root down very nicely in a moist soil. The roots are then in place to deliver the water to the plant in a dry spell. †

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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