Soil conservation is a combination of all methods of management and land use that safeguard the soil against depletion or deterioration by natural or man-induced factors.
Soil conservation is a combination of all methods of management and land use that safeguard the soil against depletion or deterioration by natural or man-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 (eg, compaction) or by detrimental changes to its chemistry (eg, acidification or salinization).
Importance of Soil Conservation
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 most of our food and fibre, so its productivity is a major factor in the economies of Canada and other nations.
But soil also has a much broader, global role. Soil acts as a filter, cleaning air and water. It exchanges gases with the atmosphere and thus 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, we must conserve it and use it in a sustainable manner.
Effect of Land Use and Management
Causes of Declining Soil Quality
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
Soil conservation practices are commonly used in forestry during harvesting and replanting operations. The quality of agricultural soils can also be maintained (conserved) or even improved by using soil conservation practices. Examples of these practices include adding organic material (eg, manure) and inorganic amendments (eg, LIMESTONE), using conservation tillage (reduced tillage or no-tillage systems), reducing the amount and frequency of use of summer fallow, rotating CROPS and growing LEGUMES (eg, 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 of FERTILIZERS and other soil amendments to apply, 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 soil tilth and control soil 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.
Acceptance of Soil Conservation Practices
Soil conservation practices have been increasingly adopted on Canadian farms since the 1980s. In 1996, the area managed by no-till practices accounted for approximately 16% of cropland. This area has since increased to approximately 56%. Soil-conserving tillage practices (conservation tillage and no-till combined) now represent about 80% of Canadian cropland. Nationally, the amount of farmland under summer fallow, which had decreased by two-thirds between 1981 and 2006, has further decreased by 40.5%. Agriculture and Agri-Food Canada (AAFC) has developed a series of Agri-Environmental Indicators for use in tracking the trends relating to soil conservation and environmental sustainability. AAFC reported on the trends in these indicators for the period 1981 to 2006 (the most recent period for which statistical information is available) in Environmental Sustainability of Canadian Agriculture: Agri-Environmental Indicator Report Series - Report #3 in 2010. Trends in all the soil-quality indicators showed considerable improvement between 1981 and 2006. The majority of the cultivated land in Canada is in the very low risk class for erosion. Canadian cropland has gone from a net source to a net sink of CO2 gas due to increased carbon sequestration and increasing levels of soil organic matter. Moreover, the share of land at risk of salinization has also decreased over this period. This general improvement does not apply to all soils, however, and there remain significant areas of farmland at risk of degradation. Nevertheless, the AAFC provided evidence that the health of our agricultural soils can indeed be maintained and even improved with the right care.
From this data, three general trends have emerged. First, conservation-oriented farming practices are required to maintain soil health, particularly in areas of intensive cropping and where soils are marginal for agriculture. Secondly, soil health can be maintained or is improved in regions where land use and management practices have been tailored to address soil resource and climatic influences that may combine to produce local problems of soil degradation. Finally, declines in soil health occur rapidly, often most dramatically in the first 10 years following conversion of undisturbed land to agriculture, but improvements to soil quality take place slowly and at greater cost than maintaining a good soil in top condition.
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 BC 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. The share of cropland at very low risk of water erosion has increased from 90% to 94% from 1981 to 2006. The share of land at very low risk of erosion from the process of wind and tillage has also increased. These positive trends were largely the result of two significant changes in land management over this 25-year period: summer fallow area declined by 60% and area of cropland under reduced or no-till systems increased to 45% (and to more than 50% according to Statistics Canada 2011).
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.
The risk of soil degradation due to erosion decreased substantially over the 1981 to 2006 period. The share of cropland in the prairies in the very low risk category of water erosion rose from approximately 96% to 98% in this 25-year period. Likewise the share of cropland at very low risk of wind erosion rose from 85% to 96%. These improvements can be attributed to reduced use of summer fallow (land left bare of vegetation throughout the growing season, usually to conserve moisture for the following year) 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. Conservation practices have also resulted in prairie agricultural soils going from a net loss of soil organic matter to a net gain during the mid- to late 1990s. Further improvements in soil quality are expected as more farmers adopt these methods.
Uneroded soils lose 15-30% of the organic matter present in virgin land within a few decades of cultivation, and then levels tend to stabilize. This process can be reversed only slowly by using soil management that reduces the rate of oxidation of soil organic matter (eg, reduced tillage), or that increases the amount of organic matter returned to the soil (eg, by applying fertilizers and eliminating summer fallow). Soil organic matter content declines quickly when soils erode, dropping eventually to less than 20% of original levels. Under current farming practices, most prairie farmland is at a low risk of increasing salinity.
Across the prairies cultivated land is at risk of erosion such that additional soil conservation practices are required to support crop production over the long term. The risk of wind erosion has declined substantially over time, with less than 1% of the cultivated land remaining at moderate to very high risk of wind erosion and requiring further conservation measures.
Ontario and Québec
Central Canada is where much of the farmland is intensively cultivated for crops like corn and soybeans. These crops require the longest possible growing season, so planting is done early in the growing season, 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 (eg, 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. In 2006, about 32% of cultivated soils in Ontario had adequate soil conservation practices that resulted in a very low risk of water erosion. Adoption of soil conserving tillage practices has increased from 22% of cropland in 1991 to 56% in 2006 (and more than 63% in 2011). Water-erosion risk on Québec cultivated soils over the period from 1981 to 2006 was relatively unchanged with approximately 74% at the very low risk and between 1% and 3% at the very high risk level.
None of the four Atlantic provinces is well-endowed with highly productive soils. The soils tend to be inherently weakly structured and 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 (eg, 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.
About 80% of the cultivated soils of Atlantic Canada have a high risk of soil erosion by water if they are left unprotected by soil conservation practices. In 2006, New Brunswick had the lowest share of cropland at very low risk of erosion. This province has a significant proportion of land under potato production and soils that are prone to erosion. The risk of water erosion has not changed significantly in this province over the 1981 to 2006 period, as there has been little change in the types and areas of crops grown and only limited increased adoption of conservation and no-till tillage practices. The erosion risk in Nova Scotia and Prince Edward Island declined since the early 1980s due to increased adoption of conservation tillage in both provinces and, to a lesser extent in Prince Edward Island, increased area of forages that are less erosion-prone than row crops.
Soil conservation is practiced across Canada to preserve our farmland and forests for future generations, and to maximize the benefits to the air and water obtained from these lands. A great deal of progress has been made in recent years towards increasing the area on which soil conservation practices are being applied. However, more is needed because it is essential to the future of all Canadians that our ability to produce food and fibre for ourselves and for export be sustained, and that we continue to enjoy clean air and water.
See also AGRICULTURAL SOIL PRACTICES.