Soil erosion
Erosion is the wearing down of the surface of the earth due to the action of wind, water and gravity. It is a natural process and shapes the landscape around us. Use of land by man for agriculture, forestry, and transportation has accelerated these processes. Accelerated erosion has occurred in all the agricultural regions of Canada.
Erosion and runoff from agricultural land affects surface and groundwater quality as well as agricultural productivity. For an overview of these issues and their solutions:
Impacts of erosion
Loss of soil from agricultural land may cause environmental impacts as well as reducing soil productivity.
Dust from wind erosion reduces air quality and in extreme cases may reduce visibility resulting in hazardous driving conditions and potential impacts to human health. On May 14 and 15, 1984 visibilities were reduced to less than one kilometer over 155,400 square kilometres (km2) of the Prairies (Anderson 1984). Farm practices on the Prairies have changed since 1984 reducing the severity of wind erosion but localized dust storms do still occur during periods of drought.
Sediment from water erosion causes turbidity in streams and lakes, and accumulation of sediment over time may reduce the volume of lakes and reservoirs. Plant nutrients and pesticides dissolved in runoff and attached to eroded soil may pollute surface water and have the potential to harm living organisms. Phosphorus is a particular concern since algal growth may stimulated to the extent that much of the oxygen needed by other organisms may be removed from the water by the rapidly growing algae.
Soil productivity loss following erosion may be significant. This may result simply from loss of soil depth over rock reducing the volume available for plant roots. More commonly crop yields are reduced by the loss of plant nutrients, and the degradation of soil physical properties such as soil crusting and in the reduction of water holding capacity. Also soil variability is increased resulting in patchy crops which are difficult to manage and generally do not yield well.
Topsoil, which is the most fertile layer of the soil, is most exposed to erosion and is lost first; also the mechanisms of erosion preferentially remove soil organic matter, clay, and fine silt material. The combination of topsoil loss and the finer soil fractions may have significant impacts on crop yields. Larney et al. (1995) reported that removal of topsoil reduced unfertilized crop yields by 50% for 4 out of 6 soils in Alberta. In most cases extra fertilizer can compensate for the impacts of erosion on soil fertility but this is an extra expense for the farmer and does little to offset the physical impacts of erosion on soil productivity.
Water erosion
Water erosion is a threat to soil productivity on most annual crop land in Canada. It is caused by rain pounding on an inadequately protected soil surface and by the action of runoff when water flows have enough energy to cut into the soil.
A general rule of thumb is that if you can observe erosion then you probably have a problem. That is because soil is at best a slowly renewable resource and once it has been lost the field probably won't be as productive for many years. Over time even small soil losses accumulate to reduce yields.
There are several forms of water erosion. Sheet, sometimes called inter-rill, erosion is caused by the impact of raindrops which remove a thin layer of soil. Rill erosion occurs when enough runoff accumulates to form small channels up to 15 centimetres deep at the soil surface. Rills can either follow channels formed by tillage equipment or form a dendritic pattern, like tree branches. Gullies occur when rills join, resulting in more concentrated flow that cuts deeply into the soil. Rills can be leveled by tillage, whereas gullies require grading and leveling. Gullies tend to form in the same location year after year and if not repaired will disrupt field operations.
Rainstorms causing erosion are most frequent in coastal British Columbia, Central Canada and Atlantic Canada. Rain on thawing soil can cause severe erosion, particularly on Prince Edward Island. Despite the fact that the Prairies receive less precipitation than the rest of Canada, water erosion can occasionally be a problem due to snowmelt and intense summer thunderstorms.
Numerous factors interact to determine the severity of water erosion:
- rainfall intensity, amount, and timing;
- rate of snowmelt;
- soil factors such as texture, aggregate stability, surface roughness, infiltration rates, and moisture content;
- slope length and steepness;
- the area contributing runoff;
- the protection provided by growing crops and residue from previous crops and;
- the effectiveness of any soil conservation practice such as grassed waterways and terraces.
The net result is that water erosion can be very variable within a field and over time. This requires continued vigilance to keep the soil productive.
Water erosion is most effectively controlled by crop residues or growing plants which absorb the energy of raindrops and reduce the cutting energy of runoff. The amount of plant material required depends on the intensity of rain, the amount of runoff, soil conditions, the type and distribution of the residues, and the configuration of the slope. Conditions vary a great deal across Canada, even within individual fields; but generally on slopes of 6 to 9%, about 800 to 1,150 kilograms per hectare (700 to 1,000 pounds per acre) of flat cereal residue are recommended. This is equivalent to having about 35 to 50% of the soil covered with residue, which in many continuous cropping situations would require the adoption of reduced or minimum tillage. Low disturbance direct seeding or zero till is usually needed to maintain the 50 to 70% soil cover recommended on slopes between 10 and 15%. Slopes much above 15% are not recommended for annual crop production. It is recommended that a conservation plan, designed by a local agrologist, conservation technician, or agricultural engineer, be implemented to control water erosion.
When agronomic or other conditions dictate that recommended crop residue levels cannot be maintained, other practices such as cover crops, spreading of residues, contour stripcropping, and terraces will be required to control water erosion. In locations where runoff concentrates it may be possible to prevent the formation of gullies by adding extra residue or double seeding, but usually a grassed waterway is required. Again, advice from a local specialist is recommended.
Wind erosion
Wind erosion most frequently affects soils on the Prairies but wind may also cause problems on sandy soils in Ontario and during the winter on exposed fields in Prince Edward Island. Peat soils may also be affected.
Wind erosion occurs when strong winds blow over a smooth, exposed, loose, and dry soil surface. Depending on conditions, the wind speeds required to initiate erosion of mineral soils vary between 25 and 50 kilometres per hour measured at 30 centimetres above the soil surface. Soil particles between 0.1 and 0.5 millimetre diameter are first to move. The wind pressure causes them to vibrate and, if their resonant frequency is achieved, they are ejected into the wind stream. Gravity quickly brings these particles back to earth but meanwhile they have gained considerable energy from the wind and they collide into the soil surface dislodging other particles. The process is called saltation and is very like an atomic chain reaction. Once a few soil particles are in motion the erosion process spreads very rapidly. Small soil particles and aggregates dislodged by these collisions are carried aloft by eddies in the wind, where they form dust clouds and may be transported for thousands of kilometers. Soil aggregates between 0.5 and 1 millimetre diameter generally does not travel far. They are rolled by impacts of saltating particles and the pressure of the wind. Saltation is just like sandblasting and can be very damaging to the soil and to growing crops.
Wind erosion is curtailed when the wind dies down, when all loose particles have been removed, or when moist, compacted or frozen soil is exposed at the surface. However there have been situations in Canada where the soil to the entire depth of tillage has been lost. When wind speeds die down, loose soil material is deposited on the soil surface. This makes the field especially susceptible to further erosion, unless a rain shower causes soil crusting or a tillage operation roughens the soil surface.
Peat soils, being much less dense than mineral soils, are more easily eroded by wind. Wind erosion of peat soils has not received much attention in Canada and the extent of the problem on agricultural soils has not been determined. Peat soils are frequently wet either at, or close to the surface thus limiting the amount of material which can be lost. Nevertheless, peat soils do blow causing spectacular dust clouds and degradation of this valuable resource.
Winds strong enough to cause wind erosion are a fact of life in many agricultural areas of Canada. In most situations there is enough vegetation and straw to prevent erosion. However, intensive tillage exposes soils, and the production of vegetation may be much reduced during extended periods of drought. In these cases, if the soil is dry and loose it will erode.
All forms of erosion reduce soil productivity due to loss of plant nutrients, soil organic matter loss, a reduction in the availability of water for crop growth, and ultimately a limitation in the volume of soil available for root growth.
Sandblast of growing plants is unique to wind erosion. It causes yield and quality losses. Plants vary in their tolerance with small grains being relatively tolerant of abrasion resulting from an 11 tonnes per hectare wind erosion event. Corn, soybeans and mature alfalfa have a moderate tolerance, vegetables have a low to very low tolerance, and seedling alfalfa and sugar beets have a very low tolerance to abrasion. Seedlings are usually worst affected by sandblast and in some situations re-seeding may be required.
Control of wind erosion usually relies upon the protection afforded by crop residues or growing crop; generally about 1,200 kilograms per hectare (1,000 pounds per acre) of straw is required. Small grain residue is more effective than oilseed residues and corn stover. Standing and anchored residues are about four times more effective than flat residues for erosion control. Growing crops are also effective in reducing wind erosion once there is enough material to absorb the force of the wind. Low disturbance direct seeding and chemical fallow are effective in maintaining enough residue for wind erosion control under most conditions.
In situations where intensive tillage is used, practices such as stripcropping, shelterbelts, and annual barriers must be used to control erosion. Erosion begins first at over-tilled field margins, from pockets of sandy soils, in locations where the wind is funneled, or at the top of ridges or knolls. These areas require special attention to control wind erosion. The assistance of a local agrologist or conservation technician is recommended to design and guide the implementation of a soil conservation plan to control wind erosion.
Tillage
Direct seeding and no-till
Direct seeding, no till and similar low soil disturbance seeding practices provide numerous benefits and yet pose unique management challenges. Discover how farmers have adopted and benefited from these practices and how they are adjusting these practices to adapt to ever changing management challenges.