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Les Henry: Soil moisture and rain

One plus one does not always equal two

A soil core showing the dry layer from one- to three-foot depths. In this situation, the only plant-available water is in the top foot. Roots will not penetrate the dry layer to access the moist soil beneath. Sufficient rain must fall to wet up the dry layer before the deeper moisture comes into play.

Soil moisture as a soil management issue has finally come into its own with the advent of tech savvy young folk using soil moisture probes to get real information in real time. Thanks to those who provide me with information to help make the annual, three-province, Soil Moisture Map as of freeze-up.

To make a soil moisture map, one has to understand soil moisture. That is the purpose of this piece. This song has been sung many times, but this is in a slightly different tune.

The November 1, 2020 Soil Moisture Map will appear in the next issue. Fair warning — it will have big areas of red ink (very dry). Our beautiful harvest weather allowed the crop to come off in great shape, but left little for next year.

One plus one equals less than two

The start of “refilling” a soil with moisture after a crop is the fall rains. Fall rains are usually system rains, so are easier to map than the thundershower-type seen in summer.

In dealing with fall rains for moisture recharge, one plus one does not always add up to two. We cannot add up the rain that has fallen since the crop quit using moisture and assume that water is sitting in the soil.

On my Dundurn farm, the rain record this year was May, 1.5 inches; June, 3.0 inches; and July 1-8, 1.2 inches. That totals 5.7 inches, and added to the approximately four inches of available water in the soil at seeding, left us with almost 10 inches of available water. In our hilly land, some low areas still had extra water to contribute to yield, so decent yields were the result.

Since July 8, the rain record was as follows.

It’s all about the dry layer

When we add rain to a dry soil, it proceeds to moisten the soil to its field capacity (FC), to a depth determined by the soil texture. That depth is the wetting front. Soil below the wetting front stays at the wilting point (WP), until more rain comes. The photo to the right shows how it works.

One plus one equals more than two

When it comes to wetting a soil to field capacity, it is all about enough rain to eliminate the dry layer. When the wetting front reaches the deeper moisture (at three feet in the photo at top), the soil is at field capacity and the crop can root to at least four feet.

Table 2 (below) shows some examples.

In Scenario B, if we get 1.5 inches of rain, the total available for a crop will go from 3.0 to 6.0 inches (i.e. 3.0+1.5=6.0 because the 1.5 inches at three to four feet now comes into play). In Scenario C, a three-inch rain will take available water from 1.5 to 6.0 inches (i.e. 1.5+3.0=6 inches. Yes, Virginia, one plus one does not always equal two).

When making the very first Stubble Soil Moisture Map in 1978, we took these facts into consideration and structured the legend accordingly. That legend is still in use with one additional category to describe situations where the water table contributes to crop yield. It is only in recent years we have woken up to the fact that water table contributions can be huge.

A three-pail analogy

Think of it this way — assume you had a four-gallon pail, a six-gallon pail and an eight-gallon pail.

Now add two gallons of water to each pail. Guess what? They all have the same amount of water.

Now add another two gallons to each pail. They will all now have four gallons of water, but the four-gallon pail (i.e. sandy soil) will be full. Now add another two gallons to each pail. Now, the six-gallon pail (medium soil) is full.

Now add another two gallons to each pail. Now, they are all full and hold four, six and eight gallons. Or, in the case of our three soil groups, they will have four, six and eight inches of available water.

In the pail analogy, the pail would simply overflow. In a soil, the water would wet up to a greater depth. When the soil is at field capacity moisture to the water table, an additional inch of water will raise the water table by six inches for a sandy soil, nine inches for a medium soil and 12 inches for a clay soil.

This is a rule of thumb that was not available when Henry’s Handbook was first published. Only in the recent wet years have we learned about the critical role of the water table.

Down on the farm

I know the above facts to be true for a medium-textured glacial till soil, such as my Dundurn farm. With a soil probe, I have confirmed a 1.5-inch rain will bring a dry soil to field capacity to a depth of one foot. And, with my water table wells, I have observed when the soil is at field capacity to the water table, a one-inch rain brings up the water table by nine inches. The theory works in practice!

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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