Soil conservation is a combination of all methods of management and land use that safeguard the soil against depletion or deterioration by natural or human-induced factors. It most often attempts to ensure that soil does not erode and wash into streams and lakes or blow away in the wind, but it also involves the protection of the soil from damage by machinery (e.g., compaction) or by harmful changes to its chemistry (e.g., acidification or salinization). (See also Agricultural Soil Practices.)
Most people know that they need clean air and clean water to stay healthy. Fewer people realize that their well-being also depends on the health of the soil. Soil supports the growth of food and fibre, so its productivity is a major factor in the economies of Canada and other nations.
Yet soil also has a much broader, global role. Soil acts as a filter, cleaning air and water. It exchanges gases with the atmosphere and therefore influences the global climate. Soil receives organic wastes and recycles their nutrients back to plants; it also holds and breaks down some toxic wastes. Because soil plays such a key role in world health, economics and environmental stability, it must be conserved and used in a sustainable manner.
The inherent or natural quality of a soil is determined by the geological materials and soil-forming processes (such as chemical and physical weathering) that combine to produce it. The characteristics of a natural soil can be changed by human activities, including land use and farming practices. In agriculture and forestry, decline of inherent soil quality can occur because of erosion, loss of soil organic matter, compaction, desertification and other degrading processes.
Soil conservation practices are commonly used in forestry during harvesting and replanting operations. Likewise, similar practices can maintain or even improve the quality of agricultural soils. Examples of these practices include adding organic (e.g., manure) and inorganic (e.g., limestone) materials, using conservation tillage (reduced tillage or no-tillage systems), reducing the amount and frequency of use of summer fallow, rotating crops and growing legumes (e.g., clover).
The type of farming activity that takes place on an area of land, be it pasture or cultivation of forage or fibre crops, cereals, oilseeds, berry fruits or vegetables, depends on the type of soil, the climate and whether crops are grown under natural rainfall or irrigation. The more any land use disturbs the land's natural ecology, the greater its effect on soil quality.
On pasture lands, agricultural management practices include restricting the density of animal stocking, using rotational grazing (resting fields after they have been grazed), controlling weeds, and protecting vegetation and banks along water courses. On cultivated lands, soil management practices include crop selection and rotation, choice of tillage methods (leaving crop residues on the surface or plowing them into the soil), controlling the traffic patterns of tractors and machinery, determining rates at which to apply fertilizer, controlling pests and managing water.
Crops that provide high-density and year-round ground cover offer greater protection against soil erosion than row-cropping or cropping systems that include extensive use of cultivated fallow. Minimizing the amount of tillage used for weed control or seedbed preparation reduces the breakdown of soil structure (the arrangement of soil particles into granules or clods), and keeps more plant residues on the soil surface compared to more intensive tillage. This helps maintain the soil and control erosion.
Reducing the rate of oxidation of soil organic matter can contribute to increasing the amount of carbon stored in the soil, which is an important factor in reducing the greenhouse gas effect (the accumulation of carbon dioxide and other gases in the atmosphere). Soil management that returns plant nutrients to the soil at the rate of their removal by crops will help maintain soil fertility. Reduced use of pesticides on erosion-prone soils, or use of pesticides accompanied by effective soil conservation measures, reduces the risk of contaminated sediments getting into surface water.
Soil Health by Region
Because of the wide diversity of landscape, inherent soil quality and farming systems in Canada, more specific trends in soil health are best observed and interpreted at the regional level.
The need for soil conservation in British Columbia varies widely, as a result of the diversity of cropping intensity. High-value specialty crops, and their associated intensive tillage and machinery traffic, provide the greatest challenge for soil conservation.
Most of the cultivated land in BC is at a high to severe risk of water erosion when the soils are bare. In the Fraser Valley this is because of high rainfall and some steep cultivated slopes; in the Peace River region it is because of easily eroded silty soils and large fields with long slopes, at the foot of which snowmelt runoff accumulates and washes soil away. However, conservation practices over the past several decades have reduced these risks appreciably.
Many agricultural soils in the prairies are subject to the stresses of a dry climate and are susceptible to wind erosion and salinization. Unprotected soils are also susceptible to water erosion particularly during intense summer storms or spring runoff. Severe wind erosion in the 1930s led to the creation of the Prairie Farm Rehabilitation Administration (PFRA) in 1935, which undertook immediate and drastic measures to control the problem. During the middle decades of the 20th century, better weather conditions led farmers to relax their vigilance. Wind erosion started to occur again more frequently, and efforts were renewed to encourage the adoption of soil conservation measures.
Improvements can be attributed to reduced use of summer fallow, and greater use of conservation tillage and other erosion controls, such as permanent grass cover and shelterbelts (rows of trees or shrubs planted across the prevailing wind direction to reduce wind speeds). The risk of soil salinity has decreased in some areas because of the wider use of permanent vegetation cover (such as hay crops) and less frequent use of summer fallow.
Ontario and Québec
In central Canada farmland is intensively cultivated for crops like corn and soybeans. These crops require the longest possible growing season, so planting is done early and harvesting is late. Soils are often wet when these operations are carried out, and this leads to soil compaction. As well, these crops may result in the soil being inadequately protected by vegetation from rainfall and snowmelt erosion for extended periods of the year.
Soil conservation practices include minimum and no-tillage, which maintain high levels of crop residues on the soil surface and reduce heavy machinery traffic. Crop rotation and periodic use of clover or alfalfa hay crops increase soil organic matter and lead to improved soil structure and fewer compaction problems. The use of manure and adequate fertilizer has a similar effect. Seeding of areas where runoff water collects to create grassed waterways also helps reduce soil erosion.
Wind erosion is seldom a problem, and is mostly limited to places where the soils are very sandy or consist of organic matter (e.g., cultivated marshes). Rows of trees or shrubs can be planted in these areas to create wind breaks to reduce wind speeds, and crop residues can be kept on the soil surface to protect the soils from wind erosion.
None of the four Atlantic Provinces is well-endowed with highly productive soils. The soils tend to be inherently weak many are acidic. The intensive production of potatoes and vegetable crops has further reduced organic matter levels, damaged soil structure and led to severe soil erosion on sloping land.
Soil conservation practices are being adopted by farmers to control these problems. Terraces are becoming common in the potato-growing areas of New Brunswick; these are channels constructed across slopes at regular intervals. The terraces reduce the length of the slopes, which reduces the accumulation of runoff water. They carry off this water to the edge of the field. They also result in farmers planting the crop rows across the slope instead of up and down the slope, further reducing erosion of the soil by runoff.
Other soil conservation practices include crop rotations in which potatoes are grown alternately with cereal grains (e.g., barley) and clover. Grassed waterways are also used in areas where water naturally concentrates, reducing the risk of erosion cutting channels (gullies) into the soil. Soil acidity is often increased by the use of the large amounts of fertilizer needed for the potato crop in this region. To control soil acidity, farmers apply ground limestone to the soil, and then mix it in with tillage implements.