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Health of our Soils | Index

Chapter 6 - Changes in Soil Structure

G.C. Topp, K.C. Wires, D.A. Angers, M.R. Carter, J.L.B. Culley, D.A. Holmstrom, B.D. Kay, G.P. Lafond, D.R. Langille, R.A. McBride, G.T. Patterson, E. Perfect, V. Rasiah, A.V. Rodd, K.T. Webb

Highlights

• Weakened aggregates and soil compaction are the most-recognized forms of structural degradation in Canada's agricultural soils.
• Degradation of soil structure reduces seedling emergence, restricts entry and movement of air and water into and through soil, increases the risk of erosion, and reduces crop yields.
• Soils are particularly vulnerable to structural degradation when they are fine-textured, wet, low in organic matter content, or eroded; intensive tillage, row-cropping, and insufficient crop rotation promote structural degradation.
• A study of soil degradation using the nonlimiting water range method showed that prairie soils are less degraded structurally than eastern soils in Canada.
• Methods that enhance soil structure include conservation tillage, growing forages in crop rotations, crop residue management, continuous cropping (reducing summerfallow), erosion controls (such as cover cropping, underseeding, contour cultivation, terraces, and grassed waterways), and installing subsurface drainage systems.

Introduction

Soil structure may be thought of in terms of "architecture" and "stability". The size, shape, and arrangement of rooms determines how a building is used and how it stands up under adverse conditions (such as snow loading and earthquakes). In the same way, the size, shape, and arrangement of the pore spaces and solids (clumps composed of sand, silt, and clay) in soil are key factors in soil quality.

Sand, silt, and clay particles in soil are bound together mainly by organic matter to form clumps or aggregates. Soil organic matter and other binding agents stabilize the arrangement of pore spaces and particles. Soils with good structure allow air, water, and nutrients to move through the spaces within and between aggregates. They also retain their arrangements of solids and pore spaces when exposed to the stresses of cultivation, harvesting a crop, and the impact of raindrops, among others.

The thirsty earth soaks up the rain
And drinks, and gapes for drink again
The plants suck in the earth, and are
With constant drinking fresh and fair.

Abraham Cowley
Drinking

Development and effects of soil structure

Soil structure develops naturally but slowly under the influence of environmental processes, such as wetting-drying and freezing-thawing, and as organic matter (plant and animal residues) is added. Maintaining good soil structure is essential to sustaining long-term agricultural productivity. But structural quality is difficult both to define and to measure.

Good structure implies that the state and stability of aggregates do not limit a crop's yield potential, that the soil is suitable for maximum root growth and penetration, and that the soil is stable against forces causing soil degradation. Soil structure affects crop growth and yield by influencing many soil and landscape processes, including:

• storage of water and nutrients in the soil and their availability to crops
• movement of water and nutrients in and through the soil during infiltration, drainage, and leaching
• aeration for roots and soil microbes
• the soil's resistance to erosion by wind and water
• the soil's resistance to compaction and crusting.

Degradation of soil structure

Deteriorating soil structure is hard to detect and measure because this property is tied so closely to other factors, such as water flow and aeration in soil. The most-recognized forms of degradation in soil structure are weakened aggregates and compacted soil (Fig. 6-1).

Soils that have weak and unstable aggregates are the most susceptible to accelerated structural damage. An unstable soil may appear to have a good structure, but its aggregates may break down easily when exposed to rain, or cultivation, or both. The breakdown of aggregates at the surface of fine-textured soils can result in the formation of a hard surface crust. This crust can prevent seedlings from emerging and can block larger soil pores, thereby restricting the entry of water and air into the soil.

Soil compaction happens when soil particles are pressed together, reducing the pore space between them (which increases the soil's bulk density). Soil can be compacted in many ways; the main cause is repeated passes of heavy machinery over wet soil during tillage and harvesting. Fine-textured soils (clayey and silty) in humid climates are the most susceptible to compaction. The risk increases by growing row crops, such as corn, soybeans, potatoes, and vegetables, because intensive tillage and harvesting often take place when the soil is wet.

The loss of a stable soil structure generally reduces crop yield through its detrimental effects on a wide variety of crop growth factors. For example,

• reduced seedling emergence reduces the crop stand
• the loss of large pores decreases the oxygen supply to crop roots, restricting their growth and increasing the probability of root diseases
• reduced entry of water into the soil limits the water available for crop growth.

Deteriorating soil structure also increases the risk of soil erosion. Fine soil particles created by the breakdown of aggregates at the soil surface are especially vulnerable to being carried away (eroded) by wind and surface runoff. Furthermore, because less water soaks into a compacted soil, surface runoff increases and compounds the effects of erosion. Degraded soils are prone to further degradation at a quicker rate (Fig. 6-2).

Current issues of structural degradation

Compacted soil

A 1986 Science Council of Canada study estimated that soil compaction reduces crop yields in Canada by 10% and costs producers more than $130 million each year. The estimated loss is greatest in Quebec, where 20% of land under monoculture row-cropping (continuous cultivation of only one type of row crop) is compacted. The fertile and productive St. Lawrence lowlands are especially affected by compaction.

Soil compaction is also a problem in other parts of the country. It is widespread in the medium- and fine-textured soils of the Fraser Valley of British Columbia as a result of relatively high rainfall, high water tables, and intensive cultivation of row crops.

In southwestern Ontario, an estimated 50-70% of clayey soils have been adversely affected by soil compaction: three-quarters of this land is rated as moderately compacted and one-quarter as severely compacted. In a recent survey of corn growers in Ontario, soil compaction was the problem most frequently identified in soil and water conservation.

The Atlantic Provinces are plagued by naturally compacted subsoils and hardpans (hardened soil layers with greatly reduced porosity), in addition to compaction from farming activities on moist soils. In this region almost one-third of the good cropland has poor soil structure. Soils with poor structure are common in Nova Scotia along the Northumberland shore and in the central part of the province, as well as in most of New Brunswick.

The Prairie Provinces do not have a serious problem with compaction. (Loss estimates for this region were not made by the Science Council study.)

Weakened aggregates

Weakened aggregates produce some of the same visible effects on soil as compacted soils. Although weakened aggregates are widely recognized as a problem of soil structural degradation, estimating their effect is more difficult than for compaction. Separate estimates for these two forms of soil degradation are not yet possible.

Soil clods continually hit by metal spikes and blades lose the weak bonds of aggregates, and the soil becomes fluffy and dusty.

Hans Jenny
Meeting the Expectations of the Land

Assessing soil structural quality

British Columbia

The Fraser Valley is extremely wet, and many soils are susceptible to compaction. However, the suspicion of agricultural workers that compaction is a serious problem in this region has not been confirmed by research. Forage-grass production, for which a five-cut system is common (five crops taken off in a season), is likely the main cause of soil compaction, because machinery traffic in the field is heavy throughout the long growing season, often when soil is wet. Production of vegetables, particularly carrots and potatoes, is also thought to cause soil compaction.

A favoured method to reduce soil com-paction is growing a cover crop after the main crop is harvested. In this way, a forage crop can be harvested and then plowed into the soil before planting the next year's main crop.

The Prairie Provinces

Differences in levels of soil organic matter and soil moisture have resulted in the best natural development of soil structure and stability in the Black soil zone of the prairies. Cooler temperatures and water deficits contribute to poorer soil structure in the Brown soil zone of the southern prairies. Decreases in organic matter associated with the agricultural development since the late 19th century have resulted in these natural soil structures deteriorating. However, it is widely believed that degraded soil structure does not limit prairie agriculture.

In the semiarid climate of the prairies, most cultivation and seeding takes place when the soil is dry enough to resist compaction by farm implements, except in some poorly drained fine-textured soils. Summerfallowing, used to conserve soil moisture in the driest areas, increases the risk of soil loss, salinization, and structural degradation. Although studies in east-central Saskatchewan have shown that crop yields in fallow systems are 12% higher than in continuous-cropping systems, this short-term benefit is insignificant compared to the damage to soil caused by summerfallow over the long term. Adopting conservation farming practices has markedly decreased summerfallow on cultivated land over the past 20 years (see Figure 8-7), but further reductions are needed.

Studies of soil structure based on measuring the amount of water in soil that is available to plants--the nonlimiting water range (NLWR) method--confirmed that prairie soils are less structurally degraded than Ontario soils (Fig. 6-3). All prairie soils in this study were potentially able to supply at least 55% of available water to crops, compared to much lower values for Ontario soils. Under continuous wheat and fallow-wheat rotations, the clay loam in Alberta showed similar potential to supply available water as did soil under grass. However, soil under grass generally had a higher potential to supply available water in its top layer (10 cm) than soils tilled conventionally for 81 years.

Ontario

Few measurements of compaction are available for Ontario's agricultural soils. However, most medium- to fine-textured soils and all low-lying soils in southern Ontario naturally have imperfect to poor internal drainage, making them vulnerable to compaction during cultivation. Relatively high precipitation and cool soil temperatures produce wet soils during spring tillage and seeding operations. Fall precipitation often makes soil wet during harvest, particularly for corn. Tile drainage is used in most of southern Ontario's loamy and clayey soils to improve soil moisture conditions.

Soil-compression studies show that soil's friability (crumbly texture) and load-bearing capacity are best when the soil water content is at or below the plastic limit (the soil water content at which the soil changes from being plastic, or pliant, to being brittle). Field traffic and tillage of agricultural soils should be restricted to these times (Fig. 6-4).

A study that identified areas in southwestern Ontario that are susceptible to traffic and tillage-induced soil compaction found that the risk of compaction is greatest for soils that are medium-textured (loamy) and imperfectly to poorly drained. Fine-textured soils were often sufficiently compacted to begin with (probably as a result of natural processes, such as intense drying cycles and glacial pressure) that corn production did not cause further compaction.

Conventional tillage often causes soil structure to deteriorate, especially when used in the monoculture of corn and other row crops. Soil structure studies at Clinton, Ont., using the NLWR method, showed that a sandy loam under a corn-soybean rotation could potentially supply at least 40% of available water to crops; no-till and grass improved this potential by 20-30% over conventional tillage (Fig. 6-3). The conventionally tilled soil had almost no potential to supply water at a depth of 30 cm and a relatively low potential at a depth of 20 cm, because of subsoil compaction. The clay loam at the same site was so structurally degraded that even no-till failed to improve the soil's ability to supply available water, but the roots made compensation by growing primarily in the soil cracks and macropores (Ontario producer tries no-till).

Another study has shown that including forages in corn rotations can improve soil structure. However, under conventional tillage, the structural improvement resulting from forages is quickly lost upon return to corn production (Rebuilding soil structure under corn production). The time needed for structural improvement under forages decreases as the clay content of the soil increases (Table 6-1). This result suggests that clay soils are more responsive than coarser-textured soils to conservation tillage and cropping practices and will show improvements in soil quality more quickly.

Quebec

Many soils in the St. Lawrence lowlands are poorly drained and are often continuously cropped to corn for many years. The soil is often worked early in the spring and late in the fall when moisture conditions make it particularly susceptible to compaction by farm machinery. More than 80% of soils under monoculture of annual crops in this region are structurally degraded; compac-tion affects about 20% of these soils.

A 1990 survey showed that cultivated clay soils had a higher bulk density than soils under grass, and two independent studies estimated that yield reduction caused by soil compaction was 10% for corn and 25% for hay. Light-textured soils that are continuously cropped to potatoes or cereals are also susceptible to structural degrada-tion and surface crusting.

Conservation tillage, along with proper crop rotations and additions of organic matter, increases levels of organic matter in the surface layer and improves soil structure. For example, applying solid cattle manure every 2 years to a poorly structured soil cropped to corn for silage improved its content of organic matter, structural stability, and porosity (Table 6-2).

Despite the importance of conservation farming techniques to maintaining good soil structure, a recent survey indicated that fewer than 10% of Quebec farmers have adopted these practices. However, some farmers use ridge tillage, and many dairy farmers rotate suitable crops and regularly apply manure to maintain levels of soil organic matter.

And is thy earth so marred,
Shattered in shard on shard?

Francis Thompson
The Hound of Heaven

New Brunswick

Soil structural degradation is a serious problem in land under potato and vegetable production in northeastern New Brunswick, where its damaging effects are second only to those of water erosion. Compaction, the most common type of soil degradation in this region, is the result of machinery traffic on soils that have been pulverized by tillage and harvesting operations. Because of the cool, humid climate and short growing season, producers often work the soil in early spring and late fall, when soils are wet.

Potato-grain and potato-potato-grain rotations are common, which return small quantities of organic matter to the soil. Erosion on land under up- and down-slope cultivation has also accelerated loss of soil organic matter. Benchmark studies have shown that soil under potatoes typically has a very weakly structured or structureless top layer (0-15 cm deep) over a more-compacted, platy subsurface layer (15-25 cm deep). The compacted subsurface layer restricts both root growth and the down-ward movement of water. Earthworms, which improve soil structure, are scarce in soil under potatoes because of disturbances by intensive tillage, low returns of organic matter to the soil, and heavy application of pesticides.

Nova Scotia

Soils in Nova Scotia are inherently weakly structured, low in soil organic matter and nutrients, and acidic. Poor soil structure may be evident as compacted subsoils or the presence of naturally occurring hardpans or hard-set layers, or both. The use of heavy machinery on moist, fine-textured soils during the wet conditions of early spring and late fall accelerates structural degradation.

The risk of structural degradation is highest in soils under row crops, such as corn, pea, bean, carrot, and other late-maturing vegetables. Medium risk of compaction is associated with spring cereals and tree fruits. Crops with a low risk of inducing compaction include winter cereals, forages, and pasture. The Annapolis Valley, predominantly in Kings County and to a lesser extent in Annapolis County, are the areas most at risk of structural degradation caused by the cropping systems combined with the inherent structural quality of the soil.

In 1990, about 6% of Nova Scotia's farmland was planted to row crops. This amount has remained essentially unchanged since 1981, suggesting that the area at high risk of severe structural deterioration is relatively small.

However, there is increasing interest in improving soil structure, using methods such as planting forages instead of cereals, installing subsurface drainage systems, and modifying machinery to reduce the effects of field traffic. (In areas of low and medium risk for soil structural degradation, there has been a 2% shift toward planting forages during the past 10 years.)

Prince Edward Island

The loam to loamy sand soils of Prince Edward Island are all somewhat limited for crop production, mainly because of inherently low fertility and poor soil structure. The large amount of silt and fine sand in the soil results in weak aggregation, which in turn makes it difficult for soil structure to be regenerated and maintained. Also, poor soil structure in the subsoil causes low permeability, which inhibits drainage.

Most of Prince Edward Island's agricultural soils currently have organic matter contents of 2-3%; only about 20% of potato fields have organic matter levels above 3% (the level commonly thought needed for good soil quality). Studies have shown that direct drilling of cereals increases organic matter in surface soil (0-5 cm deep) to well over 3% and increases the mean weight diameter of water-stable aggregates to more than 3 mm. However, direct drilling cannot be used for tuber and root crops, such as potato. Increased organic matter in soil under potatoes must be achieved by rotating crops and returning residues (Table 6-3), practices to which soils in this province respond well.

Yields of spring cereals in humid climates generally decline when the soil's macro-porosity (the volume of large pores in the soil) falls below 12% and the severity of root rot (a disease common where soil aeration is inadequate) increases, as is the case in compacted soils. Excessive compaction in the crop-root zone can usually be corrected by using conservation tillage methods. However, excessive compaction in the subsoil is considered permanent.

Improving the structure of Canada's soils

Evidence continues to accumulate in all regions of the country showing that conservation farming practices aid in maintaining and improving soil structure. Appropriate tillage practices are those that disturb the soil least. These include:

• using no-till and seed-drilling, where suitable for the crop
• replacing conventional deep tillage (more than 25 cm) with shallow tillage (10 cm)
• eliminating secondary tillage by using one-pass tillage systems
• using rotational tillage or crop-specific tillage practices.

Practices that increase the amount of organic matter returned to the soil and improve soil structure include:

• continuous cropping (reducing summerfallow)
• managing crop residue to reduce fall tillage and provide soil cover
• green manuring
• growing forages in crop rotations
• planting cover crops to protect the soil after the main crop is harvested
• underseeding grain crops that are grown in rotation with potatoes.

In hilly locations, especially those planted to row crops, contour cultivation and landscape alterations (such as terracing and constructing grassed waterways) reduce soil losses resulting from erosion, which contribute to deteriorating soil structure. The risk of compaction can also be lowered by reducing the pressure of field traffic (including restricting axle loads and using flotation tires and tandem wheels) and by installing subsurface drainage systems, especially in loamy soils overlying slowly permeable subsoils that retard internal drainage. (New technique to describe soil structure)

Conclusions

Although too few data are available to characterize the structural condition of Canada's soils, the following generalizations apply:

• in humid regions, soil quality is degraded mainly by compaction
• soils under conventional tillage and monoculture cropping systems are degraded structurally by loss of soil organic matter
• soils under tillage methods that incorporate surface residue are more likely to suffer a decline in structural quality
• residue incorporation combined with monoculture of crops produces the most serious degradation of soil structure.

Halting the degradation of soil structure requires a shift to conservation farming practices. Until these practices are more widely used, structural quality is expected to decline.

Management practices that will enhance or at least maintain soil structure will vary by region and cropping system. For example, suitable practices might include direct drilling for cereals in Prince Edward Island, no-till for corn in Ontario, and conservation tillage combined with continuous cropping in the prairies. Procedures can be developed to indicate optimal periods for cultivation and harvest to prevent additional soil compaction from agricultural machinery.

While the earth remaineth, seedtime and harvest, and cold and heat, and summer and winter, and clay and night shall not cease.

Genesis 8:22