Soil organic matter

Table of contents

Summary

  • Soil organic matter is critical to soil health and fertility. When soil organic matter levels drop, agricultural productivity and long-term sustainability are put at risk.
  • The Soil Organic Carbon Change Indicator measures the amount of soil organic matter in Canada’s agricultural soils and how this changes over time and space. Combined with the Relative Soil Organic Carbon Indicator, this can help estimate the risk of soil degradation across Canada.
  • Overall, soil organic carbon levels have greatly improved over time.
  • However, in 2021, more than half of the cropland in Central Canada, and Atlantic Canada, was at a high risk of soil degradation because of loss of soil organic carbon.
  • Soil organic carbon can be increased by management practices that add carbon to soils (such as reduced summerfallow, reduced tillage, planting high-residue crops, and spreading manure) and maintaining natural areas that accumulate and store carbon.

Soil organic matter in Canada: why does it matter?

Soil organic matter is critical to soil health and fertility. Along with plant roots, it holds soil particles together. This stabilizes soil structure, reduces soil erosion and improves the ability of soil to store and transport air, water and nutrients. Good soil structure also improves soil workability (or tilth).

Loss or degradation of soil organic matter can lead to degraded soil structure, increased soil vulnerability to erosion and lower fertility. These can lead to reduced long-term sustainability of the soil, resulting in reduced soil productivity (crop yield and quality) or higher costs to producers through fertilizer or irrigation.

Soil organic matter also provides environmental benefits. It binds potentially harmful substances (e.g., heavy metals and pesticides), reducing their adverse environmental effects. It also acts as a storage reservoir (sink) for carbon dioxide captured from the atmosphere.

Monitoring and studying Canada’s soil organic matter helps us identify factors that impact soil health across Canada. This knowledge is important for maintaining the land’s productive capacity and ensuring the agriculture sector’s continuing profitability and sustainability.

Improving and increasing soil carbon stores is also a valuable part of helping achieve Canada’s greenhouse gas reduction targets and contribute to a net-zero economy. The Soil Organic Carbon Change and Relative Soil Organic Carbon indicators are important tools for understanding agricultural greenhouse gas emissions in Canada and support Canada’s annual reporting efforts under the United Nations Framework Convention on Climate Change.

Where does soil organic matter come from?

About 60% of soil organic matter comes from carbon. Plants capture carbon dioxide gas from the atmosphere during photosynthesis. This carbon is converted to a solid form in plant tissues. Animals and micro-organisms consume plants and this carbon becomes part of the food web. When plants and animals die, their tissues decompose. During this process, most carbon returns to the atmosphere. However, a small amount of this organic carbon is transformed into soil organic materials that do not decompose as easily.

Over time, soil organic matter builds up until it reaches a “steady state.” At this point, the amount of carbon in the soil remains stable since new carbon added to the soil equals the amount of decomposed carbon returned to the atmosphere as carbon dioxide.

Soil Organic Matter Indicator

The Soil Organic Carbon Change Indicator quantifies the amount of organic carbon in Canadian agricultural soils and how this varies over space and time.

The indicator considers land management changes, such as tillage methods and shifts in management between annual crops and perennial hay or pasture. Soil carbon storage is estimated using the Intergovernmental Panel on Climate Change (IPCC) Tier 2 Steady State method. This method considers the type of crop production and the resulting crop residue adding carbon to soils, the frequency of summerfallow, and the impact of livestock manure application. The indicator also considers how land-use changes – especially the conversion of native grassland to cropland and the clearing of forests for agriculture – impact soil organic carbon stocks.

The amount of soil organic carbon change is reported in five classes: large increase (more than 90 kg per hectare per year), moderate increase (25 to 90 kg per hectare per year), negligible to small change (-25 to 25 kg per hectare per year), moderate decrease (-25 to -90 kg per hectare per year) and large decrease (more than -90 kg per hectare per year). Preferred values for the indicator range from no loss of organic soil carbon from soils that have high organic matter, to accumulation of organic carbon in soils that are low in organic carbon.

A second indicator – the Relative Soil Organic Carbon Indicator – is used to compare the current soil organic carbon level across different climates and soil types in Canada. Relative soil organic carbon is reported in five classes: very low (<0.55), low (0.55 to 0.7), moderate (0.7 to 0.85), high (0.85 to 1.0) and very high (>1.0). Areas with low or very low values represent areas where soil organic carbon should be increased using management practices.

The Relative Soil Organic Carbon and Soil Organic Carbon Change indicators can be used together to determine the risk of soil organic carbon degradation. Areas that are losing soil organic carbon that have low or very low relative soil organic carbon values are at the greatest risk of soil organic carbon degradation and loss of soil quality.

The Soil Organic Carbon Change and Relative Soil Organic Carbon indicators are calculated every five years. They help the Government evaluate the status and trend of soil organic carbon across Canada’s agricultural lands and where farming practice change should be encouraged.

Trends in soil organic carbon change, relative soil organic carbon and soil degradation

Soil organic carbon change

Since the mid-1980s, improved management practices led to large, positive increases in soil organic carbon levels. In 2021, 76% of agricultural land was in the large increase or moderate increase class. This occurred because of improved management practices and because increases in crop yield and crop residue added more carbon to soils. Over this time, Canadian agricultural soils moved from a net carbon source (losing 4.2 megatonnes [Mt] of carbon dioxide per year in 1981) to a net carbon sink (capturing 22.4 Mt of carbon dioxide per year in 2021).

Soil Organic Carbon Change, 2021

Despite overall improvements, changes in soil organic carbon vary, considerably, across Canada.

Eastern Canada

From Ontario eastward, many hectares changed from producing hay to producing annual crops, leading to an overall loss of soil organic carbon between 1981 and 2021. This land use change was driven, in part, by declining cattle populations and an associated reduction in the demand for feed. These losses have been partially offset because of gains in soil organic carbon from higher levels of crop residue, application of livestock manure, and adoption of conservation tillage. Carbon gains from conservation tillage have been highest in Ontario and Quebec. These increases have not occurred in Atlantic Canada where the cooler, wetter climate discourages the use of conservation tillage.

Prairies and British Columbia

In the Prairies and British Columbia, a decrease in tillage intensity, a decrease in the amount of land under summerfallow and an increase in land under perennial hay crops have led to large increases in soil organic carbon over time. However, since 2006, declining cattle populations have led to a loss of hay and pastureland and an increase of land converted to annual cropland. This led to a decline in the amount of land in the large increase and moderate increase soil organic carbon classes in 2021.

Relative soil organic carbon

In 2021, most of the land in the Prairies, Quebec and Atlantic Canada had moderate to high relative soil organic carbon. However, many areas with low or very low relative soil organic carbon were present in southwestern Ontario, the south-central Prairies, large portions of the Peace River region of Alberta and British Columbia, and parts of the Atlantic provinces.

Relative Soil Organic Carbon, 2021

Risk of soil organic carbon degradation

Areas with low to very low relative soil organic carbon and declining soil organic carbon are at high risk of soil degradation. These areas raise the greatest concern about soil quality in Canada.

In 2021, more than half the cropland in Central and Atlantic Canada was at a high risk of soil organic carbon degradation. A total of 45% of the land in Central Canada and 35% of the land in Atlantic Canada had low or very low relative soil organic carbon values and decreasing soil organic carbon levels.

The majority of land with high and very high relative soil organic carbon is also losing soil organic carbon. This has been the result of shifts in farming from cattle- and forage-based systems to grains and oilseeds. This is not as worrisome, from a soil-health standpoint, as is the loss of soil organic carbon combined with low relative soil organic carbon.

In the Prairies, the majority of cropland was at very low to low risk of soil organic carbon degradation. This is because of increases in soil organic carbon levels since 1981. A total of 28% of land was in the very low and low classes in 2021 and the majority of this land had increasing soil organic carbon levels. A total of 14% of the land with low or very low relative soil organic carbon had decreasing soil organic carbon levels.

In British Columbia, almost half of the cropland has very low or low risk of soil degradation with the remainder at a moderate or high risk.

Across Canada, the effects of degradation are most noticeable on sandy and clayey soils. Soils with low relative soil organic carbon have the greatest potential for improvement through the adoption of management practices that increase soil organic carbon levels.

Soil Organic Carbon Degradation Risk, 2021

How can soil organic matter be improved?

Despite overall improvements in soil organic carbon, there have been significant declines in some areas. Practices that can increase soil organic carbon accumulation include:

  • Reducing summerfallow and tillage which will return crop residues to the soil.
  • Including high-residue crops, cover crops, short-term forage crops, or green manure crops in rotation with low-residue root crops (such as carrot, potato and sweet potato).
  • Spreading composted manure to increase soil health and productivity.
  • Maintaining areas that naturally accumulate and store carbon. This can be achieved by limiting wetland drainage and avoiding clearing – or actively planting – trees, shrubs in natural areas.

In areas of concern, a more structured approach may be needed to understand and reverse soil organic carbon loss. This is especially important in soils with low relative soil organic carbon levels. The first step is to adopt practices that limit soil erosion. In some cases, highly-eroded marginal lands could be retired from crop production altogether.

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An infographic showing an agricultural landscape with crops, a tractor, soil and grazing livestock adjacent to a natural landscape with a watercourse, forest and wild animals. Text boxes are placed next to specific landscape elements to identify their connection to the agricultural sustainability indicators. One text box identifies each of the following indicators: Soil cover, particulate matter, soil organic matter, soil erosion, soil salinization, nitrogen, pesticides, phosphorus, ammonia, greenhouse gases, coliforms and wildlife habitat. Arrows are used to connect text boxes to show interrelationships.

Agriculture and Agri-Food Canada's agri-environmental indicators (AEIs) provide a science-based snapshot of the current state and trend of Canada’s agri-environmental performance in terms of soil quality (soil organic matter, soil erosion, soil salinization), water quality (nitrogen, pesticides, phosphorus, coliforms), air quality (particulate matter, ammonia, greenhouse gas emissions) and farmland management (agricultural land use, soil cover, wildlife habitat). While indicator results are presented individually, agro-ecosystems are complex, so many of the indicators are interrelated. This means that changes in one indicator may be associated with changes in other indicators as well.

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