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How Much Water Can Your Soil Hold?

Table 1. Rise in water table for 1 inch of rain added to soil at full capacity

Soil Texture

Loamy Sand Loam

Clay Inches of water table rise per inch of water added.

5 7

12

Soil is a porous medium. About half the volume is air space (pores) and the other half is solid — mineral and organic. Soil holds water in its pores. It’s the size of the pores in a soil type that determines its water-holding capacity.

Let us describe soil moisture situations and what they mean:

Air Dry. This happens only at the soil surface — usually only a few inches or deeper if we cultivate deeper. (We used to cultivate a lot and each time we did, the soil dried to “air dry” to depth of cultivation.) At air dry, almost all the pores are filled with air.

Wilting Point (WP). This is moisture left when the crop has sucked up all the water it can. There are still fine pores that have some water — and in clay soils that can be a fair bit of water — but the plant cannot access that water.

Field Capacity (FC). This is the amount of water after a rain or irrigation and the pores have drained as much as they can. At this moisture content, about half the pores are filled with water and half with air — the ideal situation for plant growth.

The difference between the water at FC and at WP is the amount available for plant growth. That equates to about one inch per foot of sandy soil, 1.5 inches per foot of medium (loam and clay loam) soil, and two inches per foot of clay soil.

Saturation. When all pores are full of water the soil is at saturation. That is the definition of the water table. When we dig below the water table of a sandy soil, it is immediately evident as the water comes pouring into the hole. If we dig below the water table in a clay soil, it can be a very long time before we see any water.

Let us take a look at Figure 1 on page 27. It shows what happens when we start adding water to a sandy loam soil after a crop has sucked up all the available water to four feet. Each inch of water moves the wetting front down one foot and for the first four inches there is no impact on the water table — the WP soil is just converted to FC soil.

But after the fourth inch of rain, any additional water will start to bring the water table closer to the soil surface. When a soil is at FC throughout, the impact of an additional inch of rain for various soil textures is a bit different than you may think. See Table 1.

The rise in water table for each inch of rain is much greater for a clay soil than for a sandy soil. That is because a clay soil has a much larger percentage of fine pores that hold water against drainage so at FC there is not as much room left for more water. Guess what? That is why clay soils are much more prone to soil salinity. It is actually quite difficult to salinize a very sandy soil.

Depth of the water table varies greatly over parts of the landscape and varies greatly over time. Many so called environmental studies make reference to the water table as if it were written in stone and was the same everywhere over the farm and stays the same over time.

In spring, the water level in a slough is the water table at that point in time and space. As the water drains below the slough, the water table gradually lowers until it slips below the soil surface where we cannot see it — but it’s not too far below ground.

YOU CAN INFLUENCE THE WATER TABLE

How we crop the land can have a great impact on the

When a soil is at field capacity throughout, an inch of rain lifts the water table a different rates for various soil textures. The water table rises faster in clay soils because clay soil has a much larger percentage of fine pores that hold water against drainage. At field capacity (FC) there is not as much room left for more water.

water table. A recent example in city planning near Saskatoon demonstrates it well. The summerfallow photo shows a drill rig on a summerfallow field. As we brought in the water truck to set up, it immediately went down and had to be pulled out. My trusty soil probe showed that the water table was at less than three feet below the surface. The rig had to be supported on ties to allow drilling to proceed.

A high water table is one reason why combines get stuck in the fall. When the water table is only a few feet down, the soil loses its capability to carry loads — and down you go.

Now take a peak at the photo of the alfalfa field. That field had been in alfalfa for seven years and the water table was below 20 feet. In August 2008, Saskatoon received four inches of rain over a weekend. That rain had zero impact on the water table beneath the alfalfa. I probed the soil after the rain and it had penetrated about three feet — but the alfalfa took off and grew a great second cut

and used all that water up. The impact of alfalfa on the water table is not just because it is deep rooted. The main effect is that alfalfa starts using water almost as soon as the snow melts and continues to do so until a hard frost stops it in September — or October, as was the case this year.

CAPILLIARY FRINGE

Take a good look at the bottom of Figure 1. If the capillary fringe is high enough that annual crop roots can get at it, then the soil is essentially sub-irrigated. A good deal of our highly productive Thick Black soils provide that situation. When the crop seems to hang on for weeks without rain, it might be getting water from the water table.

There is a fine balance however. If the capillary fringe is high enough to bring water near enough to the soil surface, then soil salinity will be the result.

RECHARGING THE WATER TABLE

When doing soil moisture maps in central and northern Saskatchewan I often encountered “residual” moisture about three feet down. When the wetting front from fall rains meets up with that residual moisture, the soil profile is “recharged” and roots of the next crop have a good chance of finding enough water for a long time.

I often explained it such that the wetting front joined up with the “other moisture” at depth.

On one occasion I was explaining this phenomenon to Blair Bachman, an AgEcon Grad from U. of S. and farmer from Consul, Sask., south of the Cypress Hills. When I mentioned “the other moisture,” he quickly retorted “what other moisture.” As we continued our soil salinity tests with deep auger drilling, I quickly learned that he was right— there was no other moisture until many feet below ground surface.

WATER TABLE AND AQUIFERS

We can have a water table within 15 or so feet from the surface, but our well may be 100 feet deep. A search for water to drill a well is a search for sand. In a clay soil, water only seeps slowly in. Many old bored or dug wells were essentially seepage wells that were not in any significant aquifer.

So you see when we talk of the water table, it is not a static thing and is affected by many natural and manmade activities. It is always nice to know how it acts in our situation, so we know how we might be affecting it and how it may be affecting us — as per how many times you need the tow rope at harvest.

J. L. (Les) Henry is a former professor and extension specialist at the University of Saskatchewan. He farms near Dundurn, Sask.

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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