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A Soil Moisture Primer

SOILS AND CROPS

This article launches a series of three on the topic of soil moisture. The series includes a background investigation into soil moisture, a discussion on groundwater and water tables and finally a detailed look at existing soil moisture levels in the Prairies heading into the seeding season.

Rainfall in 2010 for areas of Manitoba and Saskatchewan were unprecedented in nearly everyone’s lifetime — even for fossils like me. Wet years like these mean we here a lot about “saturated” soil conditions. Just what exactly does that mean? In farming, we must always remember that Mother Nature is in charge. We can only continue to farm if we understand how natural systems function and how we can best deal with the hand we are dealt.

The soil is a giant sponge made of up of mineral matter, organic matter and air space. How the water functions in that air space is fundamental knowledge needed to understand what we see in the field.

A SOIL MOISTURE PRIMER

1) SATURATION — is the situation that occurs only at the soil surface for a short time after rain (or irrigation) or at the water table.

See Diagram #1

2) FIELD CAPACITY (FC) — is the situation that occurs after the rain has entered the soil. It is a constant value for a given soil and forms a well defined wetting front that can be easily determined with nothing more than a soil probe.

See Diagram #2

3) WILTING POINT (WP) — is the water left in the soil after plants have sucked up all they can.

See Diagram #3

4) AVAILABLE WATER — is the difference between the water present at FC and that left at WP.

How much water soil can hold looks something like this:

Sandy Soil: Available water = 1 inch for each foot of soil at FC

Medium Soil (loam, clay loam): Available water = 1.5 inch for each foot of soil at FC

Heavy Soil (clay, heavy clay): Available water = 2 inches for each foot of soil at FC

THE EFFECT OF RAINFALL

Now we come to what happens when it rains. Diagram #4 shows the refilling of a sandy loam soil after a crop has sucked it dry to four feet.

What happens when the soil is recharged to FC and more rain falls (i. e. the new moisture joins up with the deep moisture)?

The only thing that can happen is a rise in thewater table.And, it happens this way:

Sandy soils: one inch of extra rain mean a five to six-inch rise in water table.

Medium soils: one inch of extra rain means a seven-to eight-inch rise in water table.

Clay soils: one inch of extra rain means a 10-12 inch rise in water table.

Yes, my friends, you read that right. For some soils just one extra inch of rain can bring about a one foot rise in the water table.

If we look back at the November 1, 2009 Stubble Soil Moisture map we see that parts of Manitoba and northeast Saskatchewan were already completely recharged heading into 2010. As well, east of a line from North Battleford to Moose Jaw there was little water storage room left. After a bit of rain in spring all soils in eastern Saskatchewan and all of Manitoba were in a situation where a rise in the water table was only a matter of time.

When farmers were getting combines stuck it was because the water table was too close to the soil surface. It was common to see water in the tracks after the combine was extracted from the mud, usually in one piece.

It should be noted that not all soils have a water table anywhere near the soil surface and in those situations “new” moisture does not join up with the “old” moisture. I learned this in the mid 80s on the Blair and Sheila Bachman farm at Consul, Sask., in the deep southwest, south of the Cypress Hills. I was demonstrating soil moisture probing to Blair and explained that the wetting front would continue the downward journey until it joined up with the other moisture. Blair’s response was “What other moisture?” We had the deep drill along and did a bunch of deep coring and soon learned that Blair was right. There was no “other moisture” anywhere near the soil surface. In that country most excess surface water runs of, and the Battle Creek is the drain for groundwater. Excess rain just makes the creeks run more.

Now we understand how soil moisture works and how it affects the water table. The second article in this series will deal with actual examples of water table rise and the long term trends we must know to interpret the data we receive.

Until next time, keep dry and warm.

J.L.(Les)Henryisaformerprofessorand extensionspecialistattheUniversityof Saskatchewan.HefarmsatDundurn,Sask. Healsorecentlyfinishedasecondprintingof Henry’sHandbookofSoilandWater”,abook thatmixesthebasicsandpracticalaspectsof soil,fertilizerandfarming.Leswillcoverthe shippingandGSTforGrainewsreaders.Simply sendachequefor$50toHenryPerspectives, 143TuckerCres,Saskatoon,SK,S7H3H7,and hewilldispatchasignedbookposte-haste

About the author

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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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