Publications

Health of our Soils | Index

Chapter 5 - Changes in Soil Organic Matter

E.G. Gregorich, D.A. Angers, C.A. Campbell, M.R. Carter, C.F. Drury, B.H. Ellert, P.H. Groenevelt, D.A. Holmstrom, C.M. Monreal, H.W. Rees, R.P. Voroney, and T.J. Vyn

Highlights

• Organic matter is an essential component of soil that stores and supplies plant nutrients, aids water infiltration into the soil, retains carbon, stabilizes the soil and reduces erosion, and controls the effectiveness of pesticides.
• Levels of organic matter in Canada's uneroded agricultural soils have declined by 15-30% since cultivation began.
• Soil organic matter levels are now being maintained or increased in many Canadian croplands.
• Optimal levels of organic matter for crop production vary with soil type and at present are not well-defined for Canadian soils.
• Soil organic matter increases under farming systems that use both conservation tillage and adequate fertilization and that reduce summerfallow.
• Inputs of crop residues, animal manure, green manure, sewage slidge, wood chips, or peat into the soil increase the organic matter.

Introduction

Organic matter is anything that is living or was once alive. Soil organic matter includes plant and animal remains in various stages of decomposition, cells and tissues of soil organisms, and substances produced by the soil microbes (Fig. 5-1). Well-decomposed organic matter forms humus-a dark brown, porous, spongy material with a pleasnat, earthy smell.

Organic matter is an essential component of soil because it:

• holds soil particles together and stabilizes the soil, thus reducing the risk of erosion
• aids crop growth by improving the soil's ability to store and transmit air and water
• stores and supplies many nutrients needed the the growth of plants and soil organisms
• maintains soil in an uncompacted condition, making it easier to work
• retains carbon from the atmosphere
• reduces the negative environmental effects of pesticides, heavy metals, and many other pollutants.

In this chapter we look at the condition of organic matter in Canada's agricultural soils. We also evaluate the effects of farming on soil organic matter and identify practices that degrade or improve soil organic matter.

Land, then, is not merely soil; it is a fountain of energy flowing through a circuit of soils, plants, and animals. Food chains are the living channels which conduct energy upward; death and decay return it to the soil. The circuit is not closed; some energy is dissipated in decay, some is added by absorption from the air, some is stored in soils, peats, and long-lived forests....

Aldo Leopold
Sand Country Almanac

Composition of soil organic matter

Soil organic matter is affected by climate, vegetation, parent material (materials from which soils are formed), and topography (landforms), and by land use and farming practices. Carbon is cycled from carbon dioxide in the atmosphere to organic compounds in plants, to organic matter in soil, and back to atmospheric carbon dioxide (Fig. 5-2). Soil organic matter, like the plants and animals from which it derives, is composed of carbon chains and rings to which other atoms are attached. The terms soil organic matter and soil organic carbon are often used interchangeably, because carbon, the key component of organic matter, is readily measured in the laboratory. Although dry, undecayed plant organic matter contains about 40% carbon, soil organic matter typically contains about 50% carbon. Soil organic matter also contains about 40% oxygen, 5% hydrogen, 4% nitrogen, and 1% sulfur.

Levels of soil organic matter

The amount of organic matter added to the soil and the amount released or removed from the soil control the content of organic matter in soil. The biological processes of plant growht and organic matter decomposition are the main factors in whether the level of soil organic matter increases, decreases, or remains the same.

The amount of organic carbon in the top 30 centimetres of the soil in nonforest ecosystems is usually much greater than the amount contained in the roots, leaves, and stems of the vegetation (Fig. 5-3). Agricultural soils may contain more than 10 times the amount of carbon found in the crop growing on the soil.

The amount of organic matter in soils varies widely, from 1 to 10% (total dry weight) in most agricultural soils to more than 90% in wetlans where peat has accumulated (Fig. 5-4). Cherenozemic soils, formed in grassland ecosystems found in the prairies, contain large amounts of organic matter. They occupy the largest area of improved cropland in Canada (see map). Soils common to eastern Canada, such as Gleysols (soils formed under conditions of poor drainage) and Podzols (acidic soils formed under trees), also have relatively large amounts of organic matter but do not make up a large share of agricultural land. Organic soils (those found in bogs, fens, and swamps) have the greatest amount of organic matter but occupy only a small area of Canada's cropland.

The optimum organic matter content of a soil depends on local climate, the amount and type of clay material present in the soil, and the soil's intended use. Ideally, in a good-quality soil, all the functions of organic matter listed earlier operate fully. However, soil organic matter alone cannot supply the quantity and balance of nutrients required by crops under intensive production; fertilizer must be added to meet the crop's needs. On the other hand, soil needs optimum amounts of organic matter to maintain its structur and keep it in a tillable condition. (The decomposition machine)

Decline of soil organic matter

When soil is cultivated to produce crops, especially annual crops, the nature plant-soil system is altered. Harvesting removes much plant material, leaving little to be worked bakc into the soil to replenish the organic matter. Some plants are removed almost completely, leaving little residue. Commercial fertilizers can be added to replace nutrients removed from the soil but they do not directly build up soil organic matter. Rather, they promote plant growth, which in turn results in more residue being returned to the soil. Other materials, such as crop residues, animal manure, green manure (leguminous plants, such as alfalfa and red clover, and grasses), compost, peat, and wood chips must be added to maintain or increase soil organic matter.

When an undisturbed forest or grassland soil is converted to agriculture, soil organic matter levels usually decline (Fig. 5-5). This decliine results from:

• increased decomposition of existing soil organic matter because of tillage and changes in moisture, aeration, and temperature conditions
• reduced replenishment of soil organic matter by crop residues.

Studies in the 1970s and 1980s focussed on the decline of soil organic matter; some scientists suggested that losses of up to 50-70% had occurred since virgin land was first cultivated. A recent survey in eastern Canada, carried out under the National Soil Conservation Program (NSCP), suggests these estimates are high; actual losses of soil organic matter since converting virgin land to agriculture are typically only 15-30%.

Most of the loss in soil organic matter occurs within 10 years of clearing forest or native grassland; the size of the loss varies according to the type of soil. Losing soil nitrogen may not be as serious as losing soil carbon, because agricultural production usually involves adding nitrogen fertilizers. Adding nitrogen may result in soil-nitrogen levels equal to or greater than those found in natural, undisturbed soils. (What is living in your soil?)

Monitoring soil organic matter

A nationwide system for monitoring soil organic matter was not in place until Agriculture and Agri-Food Canada's benchmark program to monitor soil quality was set up in 1989 (see Chapter 4). This program has delivered baseline data on soil organic matter for 23 benchmark sites across the country and is designed to provide comparative data over 10 years.

Although we lack comprehensive data on soil organic matter across Caanda, current information from specific regions of the country suggests general trends for the effects of land use and farming practices on soil organic matter. Evidence shows that soil organic matter levels are being maintained or increased in many Canadian croplands because of improved management. For example, long-term cropping experiments in the Prairie Provinces suggest that levels of soil organic matter are holding steady meaning that the amount of organic matter taken out of the soil in the form of crops is replenished by adding crop residues, manure, and commercial fertilizers. As another exmaple, many soils under potato production in Prince Edward Island now have higher levels of soil organic matter than 10 years ago, in part becasue longer crop rotations have been introduced. These rotations include forages that are plowed under. (Organic matter affects soil structure)

Factors affecting soil organic matter

Tillage

Conservation tillage is a method designed to keep most crop residue on the soil's surface, there it protects against soil loss caused by erosion and reduces water loss by runoff and evaporation. This type of tillage concentrates organic matter at the soil's surface because crop residues are not mixed into the soil.

A study in Ontario showed that no-till increased organic matter at the soil's surface and throughout the soil profile, compared to soil tilled conventionally with a moldboard plow (Table 5-1). A study in southwestern Saskatchewan looked at changes in soil organic matter on conventionally tilled land that had been in a fallow-wheat rotation with minimal fertilization for 70-80 years and was then converted to a system that grew wheat every year using conservation tillage and fertilization. Soil organic matter increased steadily during the period of conservation tillage but appears to approach a steady state (Fig. 5-6).

The total amount and the rate of increase in soil organic matter varies throughout Canada according to soil, climate, and farming system used. But systems that use conservation tillage and fertilization help to conserve organic matter in all agricultural soils.

We used a computer simulation model to predict changs in the level of organic matter in the top 15 centimetres of virgin soil subjected to 50 years of conventional tillage, no-till, or no-till plus fertilization with nitrogen at an annual rate of 50 kilograms per hectare. Results show that soil organic matter levels would continue to decline to some steady state under conventional tillage without fertilization; they would decline at a slowe rate under no-till without fertilization. In contrast, no-till with adequat efertilization would significantly increase soil organic matter over present-day levels, according to the model (Fig. 5-7).

Crop rotations

Crop rotation affects the amount of organic matter in the soil. Forages and legumes have extensive rooting systems that leave large amounts of organic matter in the soil when they die. Fallow periods in rotations have been used in semiarid regions of western Canada to conserve moisture for the succeeding crop. However, fallow (especially when conventionally tilled) exposes the soil to erosion, which reduces the levels of organic matter and creates temperature and moisture conditions that speed up the rate at which soil organic matter decomposes.

Studies in the Prairies Provinces have shown that soil cropped to wheat every year, especially when fertilized, maintain levels of organic matter better than soil under rotations that include fallow (Fig. 5-8). The improvement results from:

• more crop residues being returned to the soil each year
• the soil being protected against erosion
• organic matter decomposing slower in cropped soils.

A long-term study of corn grown in Ontario showed the positive effect of fertilization on soil organic matter levels and crop yields, especially when corn was rotated with other crops (Table 5-2). Using forages in rotations works well in Ontario and Quebec, where there is a large livestock industry, but is not as practical where livestock production is limited. (Atomic differences in carbon)

Soil amendments

Organic matter added to the soil besides crop residues includes ocmpost, manure, sewage sludge, wood chips, and peat. In a long-term study in Alberta, levels of soil organic matter increased steadily over a 50-year perod when manure was applied to the soil. Without manure, soil organic matter levels declined over the same period (Fig. 5-9). A field study in Ontario showed that adding manure helps prevent the loss of organic matter on severely eroded soils by promoting the formation of aggregates that resist erosion.

To enrich the earth I have sowed clover and grass to grow and die. I have plowed in the seeds of winter grains and of various legumes, their growth to be plowed in to enrich the earth. I have stirred into the ground the offal and the decay of the growth of past seasons and so mended the earth and made its yield increase.

Wendell Berry
Enriching the Earth

Erosion

Erosion selectively detaches and transports those soil particles richest in organic matter. The organic matter content of a soil decreases over the years with cultivation (Fig. 5-10). Where no erosion occurs, soil organic matter declines quickly and then levels off at about 80% of the original value (for a loss of about 20%). Where severe erosion occurs, levels of soil organic matter may drop much faster and, even after 70 years of cultivation, may not level off.

Soil-quality monitoring at two New Brunswick benchmark sites under intensive potato production (using a potato-potato-grain rotation) demonstrates the value of conservation farming practices to control erosion and maintain levels of soil organic matter. One site is cultivated up- and down-slope, and the other is under a conservation system (cropped along the contour of the land, with variable grade diversions and a grassed waterway). Under potato production, soil lost from the contour-cropped site was less than 10% of that lost from the site cultivated up- and down-slope. After 3 years, the soil cultivated up- and down-slope had significantly less organic matter than that under conservation management (see Fig. 4-5 in Chapter 4).

Highly erodible soils benefit from conservation tillag emethods that disturb the soil as little as possible; no-till disturbs the soil the least. No-till (direct seeding) leaves crop residues on the soil surface, where they protect the soil from erosion by wind, rains, and the runoff of melted snow. (Underseeding increases organic inputs)

Conclusions

rganic matter is essential for maintaining a soil's ability to produce crops economically and to resist degradation. Systematic monitoring of soil organic matter is now under way in Canada. Considerable reserach in the past 10-20 years has resulted in a good understanding of the effects of land use and management practices on types and levels of organic matter.

Although some organic matter has been lost from soils since they were first converted to agriculture, the loss is not nearly as high as was thought in the early 1980s. Research shows that certain farming practices stabilize and may even increase levels of soil organic matter by increasing inputs of organic matter into the soil and reducing the loss of organic matter from the soil.

Since the 1980s, government programs have encouraged farmers to adopt conservation farming practices. Practices that increase organic matte rinputs into the soil include:

• using permanenet crop covers
• growing crops that contribute large amounts of organic matter (such as those with large fibrous roots)
• using legumes and forages in crop rotations
• adding manure and other organic amendments to the soil
• keeping the crop robust and productive by fertilizing it adequately.

Practices that reduce the loss of organic matter from the soil include conservation tillage, crop residue management, and erosion control (including contour cultivation, crop underseeding, grassing waterways, terracing, strip-cropping, establishing shelterbelts, and building structures for erosion control). Farming systems must be chosen to suit specific soils and locations.

The action begins when roots decay and above ground residues break down, and the released nutrients begin their downward tumble through soil catacombs to start all over again.

Wes Jackson
Altars of Unhewn Stone