True man-made hardpan — a compacted layer of soil just below the soil surface — is rare in the Prairies, but it does happen. Far more common, but equally troubling, is hardpan caused by soil composition, such as is the case when dealing with Solonetzic soils. In Saskatchewan and Alberta, soil specialists tend to refer to hardpan as a soil condition specific to Solonetzic soils. These soils have a dense soil layer, high in clay or sodium, which inhibits air and water infiltration and the movement of plant nutrients and roots through the soil.
This hard, dry Solonetzic layer can extend anywhere from 10 cm to 75 cm below the surface. The layer restricts rooting volume and water movement, resulting in poor topsoil quality, crusting problems and soil erosion. It’s a pretty large problem — there are 4.5 million acres of Solonetzic soils in Saskatchewan, occurring primarily in the Brown and Dark Brown soil zones.
Solonetzic soils often occur in patches in fields that contain otherwise non-Solonetzic soils. They are a localized problem, occurring in areas of the field where a high amount of sodium is present. These areas are much harder than surrounding soils, even if the non-Solonetzic areas in the field are also compacted. Topsoil depth will vary across the field, deeper in better growth areas, which contributes to a wavy growth pattern of crops in these fields. In some of these soils, leaching forms an acidic, white colored soil horizon, which can be seen above the hardpan layer.
In other areas with heavier, clay soils, such as the Red River Valley region of Manitoba, hardpan or “plow pan” is used to refer to subsurface compaction and is generally caused by in-season field operations such as seeding, spraying and harvesting, and can become especially pronounced when traffic on the fields coincides with wet conditions.
“When soil has significant amounts of clay, even small amounts of clay, it will become rigid when dried, like this year. But that is not necessarily compaction, nor is anything that you can manage without adding water,” says David A. Lobb, senior research chair of the Watershed Systems Research Program at the University of Manitoba. “Any true hardpan that may develop over the course of a year is likely to be effectively broken up by cracking and/or heaving. The only place where we see significant, prolonged compaction at the surface is where severe erosion has exposed the subsoil and there is excessive traffic.”
There are various forms of soil compaction. Crusting at or slightly below the soil surface is largely caused by tillage and precipitation or irrigation droplets. Deeper, traffic-induced compaction by farm equipment is much more common in Prairie soils than true hardpan. These types of compaction can usually be remediated by changes in management practices or by Mother Nature.
“In most of Western Canada we experience regular wetting-drying cycles as well as freeze-thaw cycles, which results in fairly mellow soils,” says Ken Panchuk, provincial soil specialist with the Saskatchewan Ministry of Agriculture. “Also we are adding organic matter back into our soils, so we rely on our natural systems to get the soil structure back in place by the next cropping season. If you use good flotation on your equipment, manage your time in the seeding and harvesting windows and keep heavy equipment off the field if you don’t need to be there, everything will be fine.”
How to identify hardpan
Hardpan can be identified through visual and mechanical clues. Water ponding in the field following rainfall or snowmelt, uneven crop growth, poor penetration of tillage equipment and plant roots growing horizontally at a certain depth in the soil are all indications of subsurface compaction. Under normal circumstances the repeated wetting-drying and freeze-thaw cycles of a prairie winter and spring will mitigate most surface or near surface compaction issues, but they will be ineffective in breaking up true hardpans, which will persist over many growing seasons.
Measuring resistance with a device called a soil penetrometer is one way to determine the degree of soil compaction or hardpan. A soil penetrometer consists of a 30 steel cone at the end of a steel shaft with a pressure gauge on the other end which reads in pounds per square inch (p.s.i.). The penetrometer is pushed into the soil at a rate of one inch per second. Readings of 400 to 500 p.s.i. indicate potential soil compaction; the higher the reading the more the soil is compacted.
For farmers who don’t have access to equipment like this, a simple shovel is a lower tech solution. It’s important for farmers to dig out a cross section of the compacted area and inspect the roots, says Dr. Ying Chen, professor of biosystems engineering at the University of Manitoba. “Especially in clay soils, if the roots are growing horizontally and there is standing water in the area, I would suspect they have compaction problems,” she says.
Other methods involve digging a small hole (the width of a shovel extending down to 60 cm) and pushing a knife blade into the side of the pit at various depths. A hardpan layer will make it difficult to push the knife blade into the soil. The blade will stick when removed. Soil samples can be sent to a testing laboratory to confirm whether or not the soil is Solonetzic.
A good homemade probe to check for compaction is the subsoil moisture probe used by many farmers to determine subsoil moisture recharge in the fall before freeze-up or in the spring just prior to seeding. This probe is a 3/4-inch steel ball welded on a 4.5 ft. rod with a handle welded across the top. The moisture probe is pushed into moist soil (after fall rains or spring snowmelt), and stops when the ball end hits dry soil or a compacted layer. Dry soil or a true hardpan will stop the probe, Panchuk says.
Soils naturally become denser at lower depths because there is less organic matter present, and more minerals. Dense soils can be maintained through careful crop rotation and management practices that avoid using heavy equipment when soils are most vulnerable to compaction, such as during very wet conditions.
“If soils are maintained in a mix of forages and crops and we retain root channels, cracks or earthworm channels, we can have quite a dense soil and still get good movement of air, water and roots and that’s really all we care about. And that’s what the plant cares about too,” says John Heard, provincial soil specialist with Manitoba Agriculture Food and Rural Initiatives.
It’s important that the rotation includes a mix of taproots (from crops such as canola, chickpeas or sunflowers, and forages like alfalfa and sweet clover) as well as fibrous roots (from grass forage crops and cereal grains). This ensures that roots penetrate into different levels of the soil profile, and facilitates sufficient water and air infiltration to maintain healthy, productive soils.
Management practices play another huge part in avoiding serious soil compaction problems. “Many of our cropping systems are harvested in the dry time of year so we don’t do the damage that is seen in some other areas,” says Heard. “So we avoid a lot of the problems if we already have good management.”
In a wet year, however, heavy equipment (such as tractors, seed carts, combines, trucks and manure spreaders) compresses soil particles into smaller volumes, which provides less space for air and water to move through the soil. With larger equipment, wheel traffic compaction can occur to a considerable depth within the root zone, and becomes deeper as soil moisture content increases. In compacted soils, plant emergence can be reduced because of surface crusting. Plant roots are limited in their ability to take up required moisture and nutrients, and yields can suffer as a result.
Soil tillage is often seen as a fix for compaction problems, but in some cases it can exacerbate the problem. Tillage can remove the protective residue from the soil surface, leaving it vulnerable to erosion and environmental factors like excess moisture or heat. Excessive tillage can lead to soil crusting at the surface and tillage implements can cause soil compaction below the depth of tillage, especially when soils are wet.
Most farmers see deep tilling, subsoiling or deep ripping as a last resort to deal with heavily compacted soils, says Chen, because it is expensive, energy intensive and can cause damage to the soil. When it is necessary in clay soils, Chen advises using deep rippers, which have edge-on shanks for lower draft requirement. They loosen subsoil, but cause little soil mixing. Under most conditions deep rippers will break out a slot of soil that is slightly wider than the tool point, causing less soil disturbance and allowing more residue to remain on the surface to conserve moisture. Chen’s research has shown that it is only necessary to subsoil once every three years. “We measured the soil resistance in soils that had been subsoiled annually and every two and three years we found the results were the same,” she says. “So subsoiling is still having an effect after three years.”
Tillage every three years also seems to fit the economic realities researched by Alberta Agriculture, which estimates that an average pay-back period of four years is needed to recover the costs involved with deep tillage, assuming a yield increase of 8.5 bushels per acre of wheat valued at $4 per bushel. The pay-back period for subsoiling was 2.1 years based on a yield increase of five bu./ac. at the same value.
A lot of research into Solonetzic soils has been done in Alberta, where there are more Solonetzic soils that any other province, and these soils affect 30 per cent of the arable land. Improving surface drainage, trying to prepare the seedbed during dry and cool conditions, avoiding too much spring tillage and fertilizing only at rates that correspond to lower yield potential are all important first steps. Also, including forage crops in the rotation and returning crop residues to the soil helps build organic matter and improves soil structure and tilth.
While not all Solonetzic soils are suitable for deep tilling, in some cases deep tillage becomes necessary to try and open up soil channels so that moisture can penetrate into the soil layers. But there are many drawbacks. Deep tillage or subsoiling can seriously disturb the soil and make the surface uneven and lumpy, meaning additional tillage may be necessary to prepare for seeding. If excessive moisture is received immediately after deep tillage the broken soil can quickly become waterlogged and unworkable. In areas where salts have accumulated on the soil surface, deep tillage can mix soil layers and speed up salinization by bringing more salts to the surface.
Crop response to deep tillage seems to vary depending on location, soil type and environmental conditions. Research in Alberta has shown that yield responses are greater in areas of higher precipitation than in drier regions and depend on crop type, rotation and management.
Meanwhile ongoing research at the University of Saskatchewan is looking at the effects of using a subsoiling implement called a para-plow, which loosens the hardpan layer by lifting up the soil and lowering it again in a wave-like motion, resulting in very little soil mixing. The research has mainly been done on irrigated land, which often tends to have more problems with soil compaction. The effect on yields of wheat, canola and flax across different soil types, has been inconclusive. “From our work we have observed that the kind of soil loosening that we get with the para-plow unit has produced measurable decreases in the density of the soil,” says Dr. Jeff Schoenau, professor of soil science at the University of Saskatchewan. “And it has increased the infiltration rate of water. But we really haven’t seen it translate into much of a yield benefit.”
Many soil specialists fear the issue of compaction may be oversold, and that farmers may be tempted to take a step backwards in terms of soil health. “Saying that tillage is an immediate solution is not correct,” says Panchuk. “Mitigation of soil compaction is a slow process and with reduced tillage systems we are building organic matter and soils are becoming more productive and healthier than when we were under major tillage. Compaction generally isn’t a Prairie-wide issue.” †